Organic Chemistry MCAT Flashcards
What is the general formula of an alkane, alkene and cycloalkanes, and alkyne?
CnH2n + 2, CnH2n for alkene and cycloalkane, and CnH2n− 2 for alkyne
each double bond and cyclical molecule leads to two fewer hydrogens on the molecule, or one degree of unsaturation.
alkyne has two degrees of unsaturation because it has two pi bonds.
Describe the roots of the different structures.
CH4=methane, C2H6=ethane, C3H8=propane, C4H10=butane, C5H12 = pentane, C6H14 = hexane, C7H16 = heptane, C8H18 = octane, C9H20 = nonane, C10H22 = decane, C11H24 = undecane, C12H26 = dodecane
What are the different names used in organic chemistry?
common names ethylene and propylene instead of ethane and propene.
ethanol may be named ethyl alcohol, or 2-propanol may be named isopropyl alcohol.
The common names formaldehyde, acetaldehyde, and propionaldehyde are used almost exclusively instead of their respective IUPAC names methanal, ethanal, and propanal
the common names formic acid, acetic acid, and propionic acid
Describe the parts of the molecule that have higher priority?
when there are both double and triple bonds in a molecule, the molecule’s name ends in “ y-root-en-x-yne,” where the first number y describes the position of the double bond, the second number x describes the position of the triple bond, and root is the prefix representing the length of the principal carbon chain. These numbers must be chosen so that the sum of x and y is as small as possible, and (as stated before) the double bond is given the lowest number where there is a choice and the higher precedence
Describe the process of naming a compound.
- Find longest chain parent chain, if two same length most substituted (more stuff) gets priority, When counting out the longest chain of carbons, it MUST include the highest-priority functional group, this group must receive the lowest possible number, and the compound’s name must end with the suffix of this group, molecules just need to be more oxidized than all of their neighbors carbon with the most bonds to oxygen tends to be the most oxidized 2. The ring is numbered starting at the point of greatest substitution. Once again, this means that the carbon with the most stuff attached to it will be assigned the number 1. Number the chain , so that the most substituents get the lowest numbers possible 3. Name the substituents, methyl, propyl, general formula for both cycloalkanes and straight-chain alkenes is CnH2n, multiple identical substituents, then we use the roots di–, tri–, tetra–, ring itself is the longest carbon chain, substituted cycloalkanes will be named as derivatives of the parent ring. The ring is numbered starting at the point of greatest substitution, if the ring structure is not part of the largest carbon chain, it will be listed as a substituent. Figure 1.6 shows two examples. 4. Assign a Number to Each Substituent. Rings are numbered starting at the point of greatest substitution, and as always, try to get the lowest possible number for every ring two fewer hydrogens than straight chain=degree of unsaturation. 5 Names will always begin with the substituents listed as prefixes in alphabetical order, with each substituent name preceded by its assigned numerical prefixes such as di–, tri–, and so on, as well as the hyphenated prefixes (tert– [or t–], sec–, and n–) are ignored in alphabetizing. In contrast to this, non-hyphenated roots that are part of the name, such as iso–, neo–, or cyclo–, are alphabetized. We then separate numbers from numbers with commas, and we separate numbers from words with hyphens. Remember that the end of every name — the suffix— will be the name – backbone chain number.
- The suffix of the highest priority functional group is used as the ending, and this functional group gets the lowest possible number in the C-skeleton.• Any other functional groups become substituents. Acetylene= ethyne
- Prefixes such as di-, tri-, etc., as well as the hyphenated prefixes (tert-sec-, n-) are ignored in alphabetizing. By contrast, cyclo-, iso-, and neo- are considered part of the group name and are alphabetized. Noncyclic alkenes have the general formula CnH2n
In the case of double bonds the priorities consist of…
If there are multiple double bonds, select the chain that contains the greatest number of double bonds, and give the carbons the lowest numbers possible. if there are multiple double bonds, they must be named using the numerical prefixes (di– , tri– , etc.) and each bond must receive a number. Also, you may need to name the configurational isomer (cis/trans, Z/E). Vinyl derivatives are monosubstituted ethylene’s (ethenyl–), which is actually just a carbon– carbon double bond as a substituent. Propylene attached to a backbone at the C– 3 position of the propylene (2-propenyl–), meaning the double bond is at the end of the chain and the single-bonded carbon is attached to the rest of the chain. Methylene– refers to the =CH2 group, where the substituent is only one carbon that is double bonded to the rest of the molecule. Cycloalkenes (rings containing one or more double bonds). Conjugation gives the molecule notable stability because its electrons can be delocalized
In the case of triple bonds the priorities and rules consists of…
number indicates the position of the triple bond. No matter how the triple bonds are depicted, they are actually linear. acetylene knows two triple-bonded carbons. The IUPAC name ethyne is almost never used
In the case of substituted alkanes or haloalkanes
numbered and listed alphabetically in the compound name so that substituents have lowest possible numbers. presence of the halide does not dramatically affect the numbering of the chain; we still proceed so that substituents receive the lowest possible numbers. Chloromethane is called ethyl chloride using the alkyl halide naming convention
In the case of alcohols…
. chain is numbered so that the carbon attached to the hydroxyl group (– OH) receives the lowest number possible. Even when there is a double bond in the molecule, the – OH group still takes precedence and is given the lowest number because of its higher oxidation state. OH has priority over double and triple bonds when numbering the chain. The hydroxyl groups of vicinal diols are in the vicinity of each other— that is, from adjacent carbons diols (or glycols). Diols with hydroxyl groups on adjacent carbons are referred to as vicinal, and diols with hydroxyl groups on the same carbon are geminal. Geminal diols (also called hydrates) are not commonly observed because they spontaneously lose water (dehydrate) to produce carbonyl compounds (containing C=O).
What are ethers and why are the rules associated with them?
oxygen stuck in middle of carbons, the backbone chain is numbered to give the carbon bound to the oxygen the lowest position. The ether functionality is specified by an alkoxy– prefix, indicating the presence of an ether (– oxy– ) and the corresponding smaller alkyl group (alk– ), methoxy, and the ethane is simply named (because it is the larger group): hence, methoxyethane
For cyclic ethers, the numbering of the ring begins at the oxygen and proceeds to provide the lowest numbers for the substituents. Three-membered rings are called oxiranes by IUPAC, but they are almost always referred to by their common name, epoxides
Describe the structure of the carbon atom configurations for molecules with various groups attached.
carbon atom adjacent to the carbonyl alpha(α ), and the carbon atoms successively along the chain are named beta (β ), gamma (γ ), delta (δ ),
What are aldehydes and the rules associated with them?
carbon double-bonded to an oxygen. with the carbonyl is located at the end of the chain. Because the functional group is terminal, it will always receive the number 1 use –al.
What are ketones and the rules associated with them?
carbonyl is located somewhere in the middle– carbon chain, number must be assigned to the carbonyl, and the suffix – one with the carbonyl lowest possible, list the alkyl groups in alphabetical order followed by the word ketone. Group with higher priority, carbonyl named as substituent –Oxo.
What are carboxylic acids and the rules associated with them?
carboxylic acids are terminal functional groups, and the carbonyl will always receive the number 1 when the chain is numbered. Carboxylic acids contain a carbonyl (C=O) and a hydroxyl (OH) group, making them quite oxidized. In fact, they are the most oxidized functional group, with three bonds to oxygen. Carboxylic acids are the highest-priority functional group, so every other functional group on a carboxylic acid will be named as a substituent
What are amines and the rules associated with them?
nitrogen containing compounds, longest chain attached to nitrogen is backbone, -amine, higher-priority functional groups, are named using the prefix amino–, additional groups attached to nitrogen use N-
What are isomers and what are the various kinds of isomers?
isomers have the same molecular formula but have different structures. The type of isomers include structural/constitutional isomers (have the same molecular formula and weights, they can have different functional groups and are the most different), stereoisomers (same connectivity and chemical formula with different locations of substituents in 3D space including geometric isomers, enantiomers, diasteromers, mess compounds, and conformational isomers) and conformational isomers (they are the most similar with the same molecules only different at points in natural rotation, with a different conformation of a compound, with different positions the single bonds can take as they freely rotating such as staggered or anti, described by eclipsed, and gauche)
What are the differences between physical and chemical properties?
Physical properties are characteristics of processes that don’t change the composition of matter, such as melting point, boiling point, and solubility. Chemical properties determine how the molecule reacts with other molecule same
Describe the structure of staggered, gauche, and eclipsed.
no overlap of atoms along the line of sight (besides C– 2 and C– 3), the molecule is said to be in a staggered conformation. Specifically, it is called the anti conformation (the most favorable type of staggered conformation) because the two-methyl groups are antiperiplanar to each other and exhibit minimal steric hindrance. It’s gauche (or inappropriate) for one methyl group to stand too close to another group! Groups are eclipsed when they are completely in line with one another. occurs when the two methyl groups are 60° apart. To convert from the anti to the gauche conformation, the molecule must pass through an eclipsed conformation in which the two methyl groups are 120° apart and overlap with the H atoms on the adjacent carbon. When the two methyl groups overlap with each other, with 0° separation, the molecule is said to be totally eclipsed and is in its highest energy state
in an attempt to accomplish the lowest energy state possible every molecule wants to be in the lowest energy state possible, so the higher the relative energy, the less time the molecule will spend in that energetically unfavorable state. conformational interconversion barriers are small (3– 4 kcal/mol) and are easily overcome at room temperature. do not possess sufficient energy to cross the energy barrier, they may not rotate at all (as happens to all molecules at 0 K).
What is the lowest energy state for cyclic compounds and some examples of why they would adopt this conformation?
low energy, you sit down in a chair to rest. Boats can be tippy, so they are less stable.
bulky groups will prefer to be equatorial to minimize the steric repulsion. This results from ring strain, which arises from three factors: angle strain, torsional strain, and non-bonded strain (sometimes referred to as steric strain). Angle strain results when bond angles deviate from their ideal values. Torsional strain results when cyclic molecules must assume conformations that have eclipsed interactions. Non-bonded strain (van der Waals repulsion) results when nonadjacent atoms or groups compete for the same space. Non-bonded, or steric, strain is the dominant source of energy in the flagpole interactions of the boat conformation. To alleviate this strain, cycloalkanes attempt to adopt various non-planar conformations.
Cyclobutane puckers into a slight V shape, cyclopentane adopts what is called the envelope conformation, and cyclohexane (the one you will undoubtedly see the most) exists mainly in three conformations called the chair, the boat, and the twist or skew-boat
Describe the conformation of the cyclohexane.
cyclohexane/ unsubstituted- the most stable conformation of cyclohexane is the chair conformation. In this conformation, all three types of strain are eliminated. The hydrogen atoms that are perpendicular to the plane of the ring (sticking up or down) are axial, and those parallel (sticking out) are called equatorial. The axial– equatorial orientations alternate around the ring. The boat conformation is adopted as an intermediate state when the chair flips and converts to another chair (known as a ring flip). In such a process, hydrogen atoms in the equatorial position become axial, and vice versa, in the new chair. In the boat conformation, all of the atoms are eclipsed, creating a high-energy state. To avoid this strain, the boat can twist into a slightly more stable form called the twist or skew-boat conformation.
Describe the structure of a mono substituted and a disubstitued cyclohexane…
Monosubstituted- For a bulky group, the equatorial position is favored over the axial position, because in the axial position, there is steric repulsion with the other axial substituents slowing conversion
• Disubstituted- the molecule will put the biggest or bulkiest group into the equatorial position. If both substituents are located on the same side of the ring, the molecule is called cis; if the two groups are on opposite sides of the ring, it is called trans doesn’t change with a ring flip; it simply moves them from equatorial to axial
Of the stereoisomers, What are configurational isomers?
configurational isomers can only change from one form to another by breaking and reforming covalent bonds. Two common categories of configurational isomers are geometric isomers (differential arrangement of substituents about a double bond) and optical isomers (differential spatial arrangement affects the rotation of plane polarized light).
Of the stereoisomers, What are geometric isomers?
compounds that differ in the position of substituents attached to a double bond or a cycloalkane. If two substituents are on the same side, the double bond is called cis. If they are on opposite sides, it is a trans double bond. Complicated compounds with polysubstituted double bonds the highest-priority substituent attached to each double-bonded carbon has to be determined. Using the nomenclature convention, the higher the atomic number, the higher the priority, and if the atomic numbers are equal, priority is determined by the substituents attached to these atoms. The alkene is named (Z ) (from German zusammen, meaning “ together” ) if the two highest-priority substituents on each carbon are on the same side of the double bond, and (E ) (from German entgegen, meaning “ opposite” ) if they are on opposite sides
What is chirality?
an object that is not superimposable upon its mirror image or lacks an internal plane of symmetry is chiral. Achiral objects have mirror images that can be superimposed; for example, a fork is identical to its mirror image and therefore achiral. atom with four different substituents. Most commonly, carbon will be the asymmetric core of optical activity. These carbons, known as chiral centers, lack a plane of symmetry nonsuperimposable mirror images of a chiral molecule are enantiomers. Some molecules, called diasteromers, are chiral even though they are not mirror images of each other. This is because they differ at some (but not all) of their multiple chiral centers. Look at first atom attached the higher the atomic number of this first atom, the higher the priority— this same system is used to determine Z and E isomers. Clockwise is like turning the steering wheel clockwise, which makes the car turn right— so the chirality at that center is R.
• Or, think of the way you write an R and an S. An R is drawn with a clockwise movement, whereas an S is drawn with a counterclockwise movement. Assign priority by atomic number.
• Orient the molecule with the lowest priority substituent in the back.
• Move around the molecule from highest to lowest priority (1→ 2 → 3).
• Clockwise = R; counterclockwise = S.
What is the difference between relative and absolute configuration and how are they determined?
configuration spatial arrangement of atoms or groups of a stereoisomer, relative configuration- configuration in relation to another chiral molecule through chemical interconversion to determine enantio/diasteromers. Absolute configuration- exact spatial arrangement of atoms or groups, independent of other molecules. Step 1) prioritize four substituents looking at atoms directly attached to chiral center relative to highest atomic number if equality priority of combination of atoms attached double bonds count as two bonds. Step 2) atoms with lowest priority/hydrogen back of molecule, and switch the other neighboring groups. 3) If the circle is drawn clockwise, the asymmetric atom is called R (from Latin rectus, meaning “ right”). If it is counterclockwise, it is called S (from Latin sinister, meaning “ left”), 4) Name the compound S&R in parentheses separated by a dash, if more than one location is specified by preceding number with R or S within parentheses without dash. Differences in chirality (handedness) can produce isomers such as enantiomers (non-superimposable mirror images) as well as diasteromers (non- mirror image stereoisomers). Meso compounds contain chiral centers but are not optically active because they have an internal mirror plane of symmetry.
What are Fischer Projections?
- horizontal lines indicate bonds that project out from the plane of the page, whereas vertical lines indicate bonds going into (or behind) the plane of the page. The point of intersection of the lines represents a carbon atom. With lowest priority projected into the page can be on the top or bottom, another advantage is that, we can manipulate Fischer projections without changing the compound. By interchanging any two pairs of substituents, or by rotating the projection on the plane of the page by 180°, we still have the same compound, if only one pair of substituents is interchanged, or if the molecule is rotated by 90°, the mirror image of the original compound is obtained(R instead of S, or S instead of R). Lowest priority pointed into the page Option 1: Go ahead and determine the order of substituents as normal, number 1 → number 2-→ number 3. Remember, number 4 doesn’t’ t count, so just skip right over it when determining the order. Then obtain the designation(R or S). The true designation will be the opposite of what you just obtained. Option 2: Make a single switch: Just swap the lowest priority group with one of the groups on the vertical axis. Obtain the designation (R or S), and once again, the true designation will be the opposite of what you just found. Option 3: The final option is to make two switches. Basically, just use option 2, moving the lowest priority group into the correct position. Then, switch the other two groups as well. Because we made two switches, this molecule will have the same designation as the initial molecule. This is the same, as holding one substituent in place and rotating the other three in order— like the rotation of a methyl group. Meso compounds are made up of two halves that are mirror images. Thus, they are not optically active. M eS o compounds have a Mirror plane of Symmetry.
Describe the rotation of light by enantiomers.
• . At the molecular level, one enantiomer will rotate plane-polarized light to the same extent but in the opposite direction of its mirror image (if the concentration and path lengths are equal). Returning to the molecular level, if plane-polarized light is passed through an optically active compound, the orientation of the plane is rotated by an angle α. The enantiomer of this compound will rotate light by the same amount but in the opposite direction. A compound that rotates the plane of polarized light to the right, or clockwise (from the point of view of an observer watching the light approach), is dextrorotatory and is labeled (+). A compound that rotates light toward the left, or counterclockwise, is levorotatory and is labeled (−). The direction of rotation cannot be determined from the structure of a molecule and must be determined experimentally. That is, it is not related to the absolute configuration of the molecule. Beer’s law- α = [α] • concentration • length
What are enantiomers?
- (nonsuperimposable mirror images) are molecules that have the opposite configuration at their one chiral center. Or, if there are multiple chiral centers, they must have the opposite configuration at every one of their chiral centers to be enantiomers. Enantiomers have identical physical properties and most of the same chemical properties. However, the exceptions are optical activity and how they react in chiral environments. A compound is optically active if it has the ability to rotate plane-polarized light. Ordinary light is unpolarized, which means it consists of waves vibrating in all possible planes perpendicular to its direction of motion. A polarizer allows light waves oscillating only in a particular direction to pass through, producing plane-polarized light. Since optical activity occurs when plane-polarized light is shifted in one direction by a chiral molecule. The amount of rotation depends on the number of molecules that a light wave encounters. This depends on two factors: the concentration of the optically active compound and the length of the tube through which the light passes. 1 g/mL for concentration and 1 dm for length. Rotations measured at different concentrations and tube lengths can be converted to a standardized specific rotation ([α])
- When both (+) and (-) enantiomers are present in equal concentrations, we call the mixture a racemic mixture. In these mixtures, the rotations cancel each other out and no optical activity is observed. A racemic mixture displays no optical activity. Because they have different arrangements in space, they will have different physical properties.
What are diasteromers?
has different chemical properties, but because they have the same functional groups, they might behave in a similar fashion. Diastereomers are non-mirror-image configurational isomers. For example, cis and trans-alkenes are diasteromers. Diasteromers are also possible when a molecule has two or more stereogenic centers and differ at some, but not all, of these centers. This means that diasteromers are required to have multiple chiral centers. For any molecule with chiral centers, there are 2n possible stereoisomers. Thus, if a compound has two chiral carbon atoms, it has a maximum of four possible stereoisomers
What are meso compounds?
the criterion for optical activity of a molecule containing any number of chiral centers is that it has no plane of symmetry. If an internal plane of symmetry exists, the molecule is not optically active, even though it possesses chiral centers. This plane of symmetry can occur either through the chiral center or between chiral centers. As long as there is a plane of symmetry, the compound is not chiral. A molecule with an internal plane of symmetry is called a meso compound
Describe the different kind of bonds possible, and the bonding capability of carbon?
Carbon is tetravalent, which means that it can form bonds with up to four other atoms, allowing for the massive versatility required to form the foundation of life. This versatility is compounded by the fact that carbon, located near the center of the periodic table, can form bonds with many different elements. In addition, because carbon atoms are fairly small, the bonds that they form are strong and stable. Two types- first is ionic, in which electrons are transferred from one atom to another; the second is covalent, in which electrons are shared between atoms.
Describe the structure and function of orbitals.
Within each electron shell, there can be several types of orbitals (s, p, d, f, etc., corresponding to the azimuthal quantum numbers l = 0, 1, 2, 3, and so forth). Each type of atomic orbital has a specific shape. An s-orbital is spherical and symmetrical, centered on the nucleus. A p-orbital is composed of two lobes located symmetrically about the nucleus and contains a node (an area where the probability of finding an electron is zero) in the nucleus. A d-orbital is composed of four symmetrical lobes and contains two nodes. Four of the d-orbitals are clover-shaped, and the fifth looks like a donut wrapped around the center of a p-orbital
What are the quantum numbers and what do they represent?
•The first three quantum numbers, n, l, and m, describe the size, shape, and number of atomic orbitals an element possesses. The principal quantum number n, which can equal 1, 2, 3, etc., corresponds to the energy levels in an atom and is essentially a measure of size. The smaller the number, the closer the orbital is to the nucleus, and the lower its energy. n, the principal quantum number, describes the size of the orbital and can be any integer value from 1 to ∞. l, the azimuthal quantum number, describes the shape of the orbital and is calculated as n− 1 (l = 0 is an s-orbital, l = 1 is a p-orbital, etc.). m, the magnetic quantum number, denotes the orientation of the orbital in space and can be any integer value from +l to – l. s, the spin quantum number, distinguishes between the two electrons in an orbital by arbitrarily assigning one of the electrons a spin of +1/2 and the other a spin of − ½
What is the structure of a single bond?
two atomic orbitals combine forming a molecular orbital, obtained by adding or subtracting the wave functions of the atomic orbitals. If the signs of the wave functions are the same, a lower-energy (more stable) bonding orbital is produced. If the signs are different, a higher-energy (less stable) antibonding orbital is produced. Molecular orbital formed by head-to-head or tail-to-tail overlap resulting sigma bond, all sigma bonds are sigma bonds accommodating two electrons with the bond holding atoms more closely together and stronger than longer bonds.
What is the structure of a double bond?
consists of both a σ bond and a π bond (and a triple bond consists of a σ bond and two π bonds). Pi bonds are weaker than σ bonds in isolation, but the strength is additive, making double and triple bonds stronger than single bonds. Two orbitals line up in a parallel (side-by-side) fashion, with their electron clouds overlapping and molecular orbital, a pi (π) bond, one pi bond on top of an existing single bond, to form a double bond, single bond and two pi bonds form a triple bond, with double and triple bonds hindering rotation locking the atoms into position. A pi bond cannot exist independently of a sigma bond, formation of a sigma bond will p-orbitals of adjacent carbons be parallel and in position to form pi bond. The more bonds formed between atoms, the shorter the overall bond length. Double bond is shorter than single bond, triple bond shorter than double bond, shorter the bond length, the greater the strength of bond. Individual pi bonds are weaker than sigma bonds; geometric isomers are converted between cis and trans conformations. Breaking a single sigma bond requires far more energy, so it happens much less often.
What type of bonds are found in carbons?
Carbon has the electron configuration 1s22s22p2 and, therefore, needs four electrons to complete its octet. Although 1 sigma and 3 pi available how are bonds in methane equal? Mixing different types of orbitals forms hybrid orbitals. Just as with molecular orbitals, we can use math to combine three p-orbitals and one s-orbital. The result? Four identical sp3 orbitals with a brand-new, hybrid shape. Hybridization is a way of making all of the bonds to a central atom equivalent to each other. The sp3 orbitals are the reason for the tetrahedral shape that is a hallmark of carbon-containing compounds. All four of these orbitals point toward the vertices of a tetrahedron to minimize repulsion, which explains why carbon prefers a tetrahedral geometry. The hybridization is accomplished by promoting one of the 2s electrons into the 2pz orbital. Produces four valence orbitals, each with one electron, which can be mathematically mixed to provide the hybrids.
What is the sp3 orbital?
we have one s and three p, so the bond therefore has 25% s character and 75% p character. 109.5o apart. Saturated hydrocarbons- they have no double bonds, and thus each carbon is saturated with the maximum number of hydrogens it can hold. This is reflected in the general formula for alkanes, CnH2n + 2.
What is the sp2 orbital?
• one s-orbital is mixed with two p-orbitals (33% s, 66% p), three sp2 hybrid orbitals are formed, alkenes, third p-orbital of each carbon is left un-hybridized— this is the orbital that participates in the pi bond. The three sp2 orbitals are oriented 120° apart, which allows for maximum separation. Hybridized p-orbital is tied up in the pi bond of the double bond. Two of the sp2 hybrids will participate in C– H single bonds, and the other hybrid orbital will line up with the pi bond and form the C=C double bond.
What is the sp orbital?
• triple bond two of p-orbitals to form pi bonds and the third p-orbital combine to from two sp-hybrid orbitals 50% s character, 50% p character. These orbitals are oriented 180° apart, which explains the linear structure of molecules such as acetylene.
Describe the structure of various carbon constituents.
•a primary carbon atom (written as 1°) is bonded to only one other carbon atom. A secondary (2°) carbon is bonded to two, a tertiary (3°) to three, and a quaternary (4°) to four other carbon atoms. Hydrogen atoms attached to 1°, 2°, or 3° carbon atoms as 1°, 2°, and 3°. Hydrogens attached to carbons will always carry the same name as their parent carbon atom.
What happens as chain length is increased?
- ↑ Chain length: ↑ boiling point, ↑ melting point, ↑ density. ↑ branching: ↓ boiling point, ↓ density.
- Physical properties for higher molecular weight as it increases, so do the melting point, boiling point, and density. This is logical; the heavier the molecule is, the harder it should be for the molecule to break away from others and enter the higher-energy gas phase. That’s why, at room temperature, straight-chain compounds of up to 4 carbons are found in the gaseous state, chains of 5 to 16 carbons exist as liquids, and the longer-chain compounds are waxes and harder solids.
What is the general trend of physical properties for increased branching?
slightly lower boiling points than their straight-chain isomers. This is because greater branching reduces the surface area of the molecule available for interactions with neighboring molecules. The weakened intermolecular attractive forces (van der Waals forces) result in a decreased boiling point. Melting point- the melting point does follow the same trend— after all, with greater branching, the molecules cannot stack up as close to each other as they would prefer. But for a given number of carbons, what really matters is how symmetrical the molecule is: the more symmetrical a molecule, the higher the melting point.
What occurs in the combustion reactions?
reaction of alkanes with molecular oxygen to form CO2, water and heat, proceed through radical and is often incomplete with a lot of carbon monoxide present as well Alkane + O2→ CO2 + H2O + heat
What occurs in Free radical halogenation and what are the steps of the processes?
R + X2→ RX. Halogenation, in which one or more hydrogen atoms are replaced with a halogen atom (Cl, Br, or I) via a free-radical substitution mechanism
•Initiation- diatomic halogens homolytically cleaved (two electrons of sigma bond split equally by heat or ultraviolet light, formation of two free radicals, neutral species with unpaired electrons, extremely reactive and readily attack anything near them,
•Propagation- radical produces another radical to continue the reaction, radical reacts with alkane removing hydrogen atom to form HX and create an alkyl radical, reacting with X2 to form alkyl halide generating another radical halogen, begins and ends with a radical product of first step is starting material of second, and product of the starting material for the first step of the chain reaction
•Termination- Propagation will continue until two free radicals combine with one another to form a stable molecule, ending the reaction. The more stable the intermediate is, the more likely the reaction is to occur. Larger alkanes have many hydrogens available for the free radical to attack. Bromine radicals react fairly slowly and primarily attack the hydrogens on the carbon atom that can form the most stable free radical. Always think of carbocations and free radicals as electron-deficient because both species have fewer than the eight electrons needed to be complete. In this way, we can remember the trends of stability of both of these species: 3° > 2° > 1° > methyl.
What is pyrolysis?
molecule broken down by heat (pyro-), cracking, reduce average molecular weight of heavy oils and increase production of more desirable volatile compounds, C-C bonds cleaved, producing smaller-chain alkyl radicals recombine to form variety of alkanes.
What is disproportionation?
a radical transfers a hydrogen atom to another radical, producing an alkane and an alkene
What are substitution reactions?
can take part in nucleophilic substitution reactions, nucleophiles electron-rich species attracted to positively charged/positively polarized atoms, electrophiles. Alkyl halides most common undergoing nucleophilic substitution reactions
What are the correlations between nucleophilicity and basicity?
nucleophilicty roughly correlated to basicity if same attacking atom, stronger the base more likely to attract positively charged proton bronstead-lowry definition, nucleophilic strength measures how much atom wants to find positive charge with a strong correlation.
nucleophilicity strength decreases in the order of RO->HO->RCO2->ROH>H2O
What are the correlations between nucleophilicity and size and shape?
if the attacking atoms differ, or if the conditions differ, nucleophilic ability doesn’t necessarily correlate with basicity. For example, in a protic solvent (solvents with protons in solution, such as water or alcohols), large atoms tend to be better nucleophiles because they can shed the solvating protons surrounding them and are more polarizable. When it comes to nucleophilicity, size matters in protic solvents (those capable of hydrogen bonding or donating protons). The solvent, which decreases its ability to act as a nucleophile, can easily surround the smaller atoms and a larger atom thus becomes more nucleophilic in comparison. Big that it can be polarized, meaning the electrons can shift around, making some areas more negative than others, giving it a much better chance to make it to the electrophile while it is still negative enough to attack. Basicity and nucleophilicity are directly related because both imply that the compound wants to donate electrons, but the solvent may stabilize these species and alter that trend. In aprotic solvents, the trend stays intact.
With the nucleophilicty strength decreasing in the order
CN->I->RO->HO->Br->Cl->F->H2O
What are the properties of an aprotic solvent?
without protons, nucleophiles don’t have proton coats, not solvated, nucleophilic strength related to opposite to protic order of nucleophilic strength same as base strength.
F->CL->Br->I-
What determines the ease of nucleophilic substitution?
depends on how good a leaving group is, leaving group are weak bases, stable anions or neutral species, easily accommodate electron pair Good leaving groups, weak bases, conjugate bases of strong acids, opposite of base strength in cases of halogens. Weak bases make good leaving groups because they are able to spread out electron density, making the species more neutral or more stable.
I->Br->Cl->F-
What is the SN1 designation?
designation for unimolecular nucleophilic substitution reaction, rate of reaction depends only on substrate, original molecule, rate-determining step dissociation of species to form stable, positively charged species of carbocation. k [RX]
the dissociation of a molecule into a carbocation and a good leaving group, followed by the combination of the carbocation with a nucleophile, understanding of the intermediate will be essential in determining all of the facts surrounding the reaction, including the rate and the products
To get carbon into a carbocation- polar protic solvents with lone electron pairs, because the electron-rich groups can solvate the carbocation and help stabilize it. Carbocations are also stabilized by charge Rate = delocalization. The more highly substituted the cation is, the more stable it will be.
What is required to get a particular product substient from the SN1 reaction?
• To get particular product original substituent better leaving group than nucleophile, reverse reaction outcompete forward reaction, carbocation such strong electrophile, pick up anything comes near it with lone electrons, does not require strong nucleophiles, the more reactive the nucleophile, more SN2 reaction will result. Rate of reaction depends upon slowest step, rate limiting or rate=determining step, limiting speed of reaction, depends only on concentration of original molecule, rate increased by anything accelerating formation of carbocation. Structural factors- highly substituted alkyl halides allow for distribution of positive charge over greater number of carbon atoms, forming most stable carbocation. Solvent effect- highly polar solvents better at surrounding and isolating ions than less polar solvents, polar protic solvents water, and solvation stabilizes intermediate state. Nature of leaving group- weak bases dissociate more easily from alkyl chain making better leaving group increasing rate of carbocation formation
What is the requirement for an intermediate of an SN1 reaction to be more stable?
• An intermediate is distinct from a transition state. An intermediate is a well-defined species with a finite lifetime and must be at a relative minimum energy for this to occur. On the other hand, a transition state is a theoretical structure used to define a mechanism. The transition state represents a maximum (in energy) between two minima on a reaction coordinate.
Describe the SN2 pathway
• SN2The penta-coordinate (five-coordinate) transition state, where one bond is forming and the other is breaking, is the hallmark of the SN2 reaction. The increased crowding around that central atom will guide the rate and products of the reaction. Rate = k [Nu][RX]. Certain conditions, the formation of a carbocation is unlikely, if not downright impossible. Even under such conditions, substitution reactions can still proceed, but they must occur by a different mechanism that avoids the carbocation altogether. Enter SN2: An SN2 (bimolecular nucleophilic substitution) reaction involves a strong nucleophile pushing its way into a compound while simultaneously displacing the leaving group, in one concerted step. Only one-step, it must be the rate-determining step. The reaction is called bimolecular because the rate-determining step involves two molecules. SN2 reactions, the nucleophile actively displaces the leaving group in a backside attack. For this to occur, the nucleophile must be strong, and the substrate cannot be sterically hindered. Primary substrates are the most likely to undergo SN2 reactions, followed by secondary, whereas tertiary substrates are just too crowded to participate in this mechanism. Nucleophile attacks the reactant from the backside of the leaving group, forming a trigonal bi-pyramidal transition state (sp2). As the reaction progresses, the bond to the nucleophile strengthens, while the bond to the leaving group weakens. The leaving group is displaced as the bond to the nucleophile becomes complete. Single step two reacting species so concentrations of both have a role in determining rate meeting in solution, second order kinetics. The molecule with a leaving group, often an alkyl halide or a tosylate) and the nucleophile.
What are the stereochemistry changes for the SN1 and SN2?
- Stereochemistry- SN1 mechanism involves a carbocation intermediate in which the carbon only has three groups bound to it. With only three substituents, the molecule takes on a planar shape, with 120° between each of the bonds. That means that the carbons are sp2 hybridized. Because the molecule is planar (and therefore achiral), the nucleophile can attack either the top or the bottom of the compound. This means that as long as the end product has four different groups, we can have two different products,
- Depending on whether the nucleophile attacks from the top or the bottom. SN1 leads to loss of stereochemistry; SN2 leads to a relative inversion of stereochemistry owing to backside attack. Be careful, though, because the absolute configuration may remain the same if the leaving group and the nucleophile do not maintain the same priority. If the original compound was optically active, the product will be a racemic mixture and, thus, no longer optically active.
- SN2 stereochemistry- nucleophiles must attack backside of molecule, as leaving group leaves from other side, molecule will flip and inversion of configuration. One important thing to be aware of is that this inversion of stereochemistry will lead to an inversion of the absolute configuration only if the leaving group and the nucleophile have the same priority (R will be changed to S, and vice versa). If the nucleophile and the leaving group have different priorities, even though the molecule will still flip, the designation will not be changed.
What are the physical properties of alkenes and alkynes?
Physical Properties- melting and boiling points increase with increasing molecular weight and similar to corresponding alkanes, Trans-alkenes generally have higher melting points than cis-alkenes because they are more symmetric, which allows for better packing in the solid state. However, trans-alkenes tend to have lower boiling points than cis-alkenes because they are less polar.
Describe the polarity of alkenes and alkynes?
Polarity- result of an asymmetrical distribution of electrons in a molecule. This causes the molecule to have one partially negative region and another that is partially positive. In alkenes, unequal electron distribution creates dipole moments that point from the electropositive alkyl (single-bonded) groups toward the electronegative alkene (double bond). That is, the sp3 carbons donate electrons to the sp2 carbons. Sp3 carbons have less s-character (25%) than sp2 carbons (33%), and s-electrons can be found at the positive nucleus, making them more stable.
What is the synthesis of an elimination reaction and the unimolecular elimination?
elimination reactions alcohols or alkyl halides, elimination carbon backbones kicks off/eliminates hydrogen and halide (dehydrohalogenation), molecule of water (dehydration) forming double bond,
• water (dehydration) forming double bond,
Elimination- E1- unimolecular rate of reaction depends on concentration of only one species, substrate, departure of leaving group and formation of carbocation intermediate, proton of adjacent carbon (β-carbon) removed by weak base double bond form from electrons from now broken carbon-hydrogen bond. E1 favored by polar protic solvents, ability to form stable carbocation, highly branched carbon chains, good leaving groups, absence of good nucleophile, mechanisms occur under similar conditions, competitive and occur simultaneously with higher temp favoring E1.
How is E2 controlled over SN2>
• Controlling E2 over SN2- 1. Steric hindrance, highly substituted carbon chains forming most stable alkenes, E2 easily and SN2 rarely, bulk of base larges impact on dominant mechanism hard time getting to backside easier to pluck off hydrogen from neighboring chain. 2. Stronger base favors E2 over SN2, SN2 favored over E2 by weak Lewis bases or strong nucleophiles, cyanide or iodide, strong base pull of hydrogen before reaching carbon atom resulting in E2 or higher temperatures. SN2 depends on properties of substrate and base. Oxidized, it loses electrons, and if it is reduced, it gains electrons, if species is reduced, it will be getting more hydrogen; if it is oxidized, it will be losing hydrogen and, thus, gaining double bonds or oxygen atoms.
Describe bimolecular elimination, E2.
Bimolecular elimination- rate of an E2 reaction depends on two species, the substrate and the base. As a strong base (such as the ethoxide ion, C2H5O– removes a proton, and then a halide ion
• Anti to the proton leaves, resulting in the formation of a double bond. Two possible products, double bond can form on either side of departing halide, more substituted double bond more stable and constitute a larger percentage of products. Double bond form either side of departing halide, more substituted double bond more stable and constitute larger percentage of products, if molecule can form either geometric isomer, trans isomer predominate, more stable than strained cis.
What is catalytic hydrogenation?
reducing alkene by adding hydrogen to double bond with aid of metal catalyst, platinum (Pt), palladium (Pd), and nickel (Ni). Occasionally rhodium, iridium, or ruthenium is used, Takes place on surface of metal, face of the π -bond becomes coordinated to the metal surface where molecular hydrogen is bound. That’s why the two hydrogen atoms are added to the same face of the double bond, reaction takes place where the two touch. This type of addition is called syn addition.
What are the electrophilic additions?
π -bond is higher in energy, and thus, more reactive than the σ -bond. Therefore, it can be broken without breaking the σ -bond. As a result, we can add compounds to double bonds while leaving the carbon skeleton intact. Although many different addition reactions are possible, most operate via the same essential mechanism. Because the electrons of the π -bond are reactive, they are attracted to molecules seeking to accept an electron pair (Lewis acids). Recall that these seekers (or “ lovers”) of electrons are called electrophiles. Lewis Acids Accept electron pairs, Lewis bases donate electron pairs.
What occurs in the addition of HX?
electrons in the double bond act as a Lewis base and bond with the partially positive hydrogen of HX molecules. This first step yields a carbocation intermediate. In cases where the alkene is asymmetrical, the initial protonation produces the most stable carbocation. Be careful here; this means that the proton will actually add to the least substituted carbon atom (the carbon atom with the most protons). This addition will form the carbocation on the adjacent carbon (that is more substituted), because alkyls substituents help stabilize carbocations.
What occurs at markovnikov’s addition?
• and the goal are to produce the most stable carbocation. In the second step, the halide ion combines with the carbocation to yield an alkyl halide; Markovnikov’s rule refers to the addition of a group to the more substituted carbon of the double bond. It does so because the more stable carbocation intermediate (the more highly substituted) will form in the slow first step and the nucleophile will then attack that positive charge in the fast second step.
What happens in the addition of X2?
addition of diatomic halogens to a double bond is a rapid process. In fact, it is frequently used as a diagnostic tool to test for the presence of double bonds. The double bond acts as a nucleophile again in this reaction, attacking one half of an X2 molecule and displacing X– from the other side. The second halogen is just acting as the leaving group. The first step results in a cyclic halonium ion intermediate (bromonium and chloronium ions are known to exist). X– then attacks the ion on the opposite face because the bromine is blocking the other side (anti-addition!), resulting in the dihalo compound. If this reaction is carried out in a nucleophilic solvent, the cyclic halonium ion can be attacked by solvent molecules before the second halogen ion gets a chance to do so.
What occurs in the addition of H2O?
Water can be added to alkenes under acidic conditions (most commonly H2SO4), double bond is protonated according to Markovnikov’s rule, forming the most stable carbocation. This carbocation then reacts with water, yielding a protonated alcohol, which then loses a proton to become an alcohol. Perform this reaction at low temperatures because at high temperatures, the reverse reaction, acid-catalyzed dehydration, is heavily favored. Remember, if heat is part of the reaction, look for the formation of a double bond. Hydration of the double bond can also be achieved under slightly milder conditions with oxymercuration-reduction. As with the harsher acid-catalyzed hydration, Markovnikov radiochemistry is observed.
What occurs in free radical additions?
When peroxides or UV light are present, expect free radical reactions that do not follow Markovnikov’s rule. HX to alkenes: through a mechanism that uses free-radical intermediates. This reaction occurs in the presence of peroxides, oxygen, or ultraviolet light. Free-radical additions disobey the Markovnikov rule because X• adds first to the double bond, producing the most stable free radical. Halogen will end up on the least substituted carbon, quite similar to standard electrophilic additions, where H+ adds first to produce the most stable carbocation. The important thing to realize here is that both of these mechanisms are in place to create the most stable intermediate. This reaction is useful for HBr, but it is not practical for HCl or HI, because they are energetically unfavorable. In anti-Markovnikov reactions, the most stable radical forms on the most substituted carbon (just as the most stable carbocation formed before), but because the halogen adds first, it ends up on the least substituted carbon. Remember, the most stable intermediate and least energetic transition state will always determine the favored products.
What occurs in hydroboration?
Diborane (B2H6; often written as borane: BH3) adds readily to double bonds. The boron atom (owing to its incomplete octet) is a Lewis acid and attaches to the less sterically hindered carbon atom. At the same time, a hydride is transferred to the adjacent carbon (a concerted mechanism). The second step is an oxidation-hydrolysis with peroxide and aqueous base that directly transfers water to the bond with boron, producing an alcohol with overall anti-Markovnikov, syn orientation
What does the reagent potassium permanganate use?
Alkenes can be oxidized with KMnO4 (potassium permanganate), although depending on the reaction conditions, we can end up with drastically different products, If we make our conditions as mild as possible, using cold, dilute KMnO4, the product simply has – OH groups added to each side of the double bond. Such products are called 1,2 diols (vicinal diols), or glycols, and they have syn orientation.
• Cold, and dilute conditions should always make you think of a mild or weak reaction (adding alcohols to a double bond). Hot, acidic conditions should make you think of rigorous or strong reactions (breaking the double bond altogether and forming carboxylic acids).
• Kick it up a notch and use a hot, basic solution of potassium permanganate, followed by an acid wash, nonterminal alkenes are cleaved to form two molar equivalents of carboxylic acid, and terminal alkenes are cleaved to form a carboxylic acid and carbon dioxide. If the nonterminal double-bonded carbon is disubstituted, a ketone will be formed. Under these intense conditions, we simply chop the double bond in half and make those cleaved carbons as oxidized as possible.
What occurs in ozonolysis?
more selective cleaves the double bond in half, it only oxidizes the carbon to an aldehyde (or a ketone if the starting molecule is disubstituted) under reducing conditions (Zn/H+ or (CH3) 2S, Ozonolysis under oxidizing conditions (H2O2) yields
also cleaves double bonds but will only oxidize primary carbons to aldehydes or secondary carbons to ketones.
What occurs in the peroxycarboxylic acids?
Alkenes can also be oxidized with peroxycarboxylic acids, which are strong oxidizing agents. Peroxyacetic acid (CH3CO3H) and m-chloroperoxybenzoic acid (mCPBA) are commonly used. The unique thing about this reaction is that the products are epoxides (also called oxiranes). This reaction is an example of syn addition
What occurs in polymerization?
Polymerization is the creation of long, high-molecular-weight chains (polymers) composed of repeating subunits (called monomers). Polymerization usually occurs through a radical mechanism, although anionic and even cationic polymerizations are commonly observed.
What are alkynes and what are their physical properties?
- hydrocarbons that possess one or more carbon– carbon triple bonds. All triple bonds form straight lines with 180° between carbons as a result of the SP hybridization. Nomenclature: -yne and specify the position of the triple bond when it is necessary ethyne, which is almost exclusively called acetylene. Frequently, compounds are named as derivatives of acetylene.
• Physical properties- similar to those of analogous alkenes and alkanes. In general, similar to alkanes and alkenes, the shorter-chain compounds are gases, but alkynes boil at somewhat higher temperatures than their corresponding alkenes. Internal alkynes, like alkenes, boil at higher temperatures than terminal alkynes. Asymmetrical distribution of electron density causes alkynes to have dipole moments larger than those of alkenes but still small in magnitude. Thus, we can assume that solutions of alkynes will be slightly polar, or at least more polar than a solution of alkenes. The acidity of the hydrogen on a terminal alkyne is the one major difference from all other hydrocarbon molecules. If anything about alkynes. Terminal alkynes can stabilize a negative charge fairly well, something that is uncommon for carbon atoms. As stated earlier, this stabilization stems from the 50 percent s-character. Recall that s-electrons have some probability of being found near the carbon nucleus (negative electrons are happier by the positive nucleus). This property is exploited in some of the reactions of alkynes
What are the synthesis strategies for alkynes?
triple bonds are through two rounds of elimination of a geminal (remember twins, on the same carbon) or vicinal (neighbors in the vicinity, on neighboring carbons) dihalide. Reaction requires high temperatures and strong base, already existing triple bond into a new carbon skeleton. To do this, a terminal triple bond is converted into a nucleophile by removing its acidic proton with a strong base (NaNH2 or n-BuLi), producing an acetylide ion. Remember that terminal alkynes are fairly acidic, so this is a reasonable process. Once formed, the ion will perform nucleophilic displacements on primary alkyl halides at room temperature,
What is the reduction reaction for alkynes?
Reduction- reduction as adding bonds to hydrogen and oxidation as adding bonds to oxygen. Alkynes, just like alkenes, can be hydrogenated (reduced) with a catalyst to produce alkanes. If we want alkenes as our final product, we need to stop the reduction after addition of just one equivalent of H2. First uses Lindlar’s catalyst, which is palladium on barium sulfate (BaSO4) with quinoline, a heterocyclic aromatic poison that stops the reaction at the alkene stage. Because the reaction occurs on a metal surface, the alkene product is the cis-isomer. The second method uses sodium in liquid ammonia at temperatures below − 33° C (the boiling point of ammonia) and produces the trans-isomer of the alkene via a free-radical mechanism.
