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Alkanes
simplest organic molecules, consisting only of carbon and hydrogen atoms held together by single bonds
Methane
CH4
Ethane
CH3CH3
Propane
CH3CH2CH3
Butane
CH3CH2CH2CH3
Pentane
C5H12
Hexane
C6H14
Heptane
C7H16
Octane
C8H18
Nonane
C9H20
Decane
C10H22
Undecane
C11H24
Dodecane
C12H26
General formula for a Alkane?
CnH(n+2)
Rules for naming branched molecules
- find the longest chain in the compound
- Number the chain in a way that the lowest set of numbers is obtained for the substituents
- Name the substituents with the ending -yl. If two or more equivalent groups are present, the prefixed di, tri, and tetra are used
4.Assign a number to each substituent
5Complete the name- the substituents are listed in alphabetical order with their corresponding numbers. (prefixes are ignored with alphabetizing)
CYCLO, ISO, and NEO are considered part of the group name and should be used to alphabetize
Cycloalkanes Nomenclature
when an alkane forms a ring
- they are named according to the number of carbon atoms in the ring with the prefix CYCLO-
- The substituents are names, and the carbon atoms are numbered around the ring starting from the point of creates substitution (GOAL is to provide the lowest series of numbers)
Alkenes
also called Olefins
- are compounds that contain carbon-carbon double bonds.
- the nomenclature is the same but the ending is -ENE
-when numbering the parent chain make sure the double bond receives the lowest number possible
Vinyl derivatives
are monosubstitued ethylenes
Allyl Derivatives
are propylene’s substituted at the 3rd carbon position
Cycloalkenes
are named like cycloalkanes, but with the suffix -ene
-if the molecule has only one double bond and no other substituent then a number is not necessary
Alkynes
are compounds that possess a carbon-carbon triple bond
- the suffix is -yne
- the posit of the triple bond is indicated by a number when necesarry
What is the common name for ethyne
ACETYLENE
Haloalkanes Nomenclature
compounds containing a halogen substituent
-if the halogen is the highest priority substituent, ensure it has the lower number when deciding from which end of the carbon chain to start counting
-ALSO haloalkane may be named as a alkyl Halide
EX: chloroethane is called ethyl chloride
Alcohols Nomenclature
are named by replacing the -e of the corresponding alkane with -OL
- the chain is numbered so that the carbon attached to the hydroxyl group (OH) receives the lowest number possible
- In compounds that possess a multiple bond and a hydroxyl group, numerical priority is given to the carbon attached to the -OH
Diols Nomenclature
molecules wit two hydroxyl groups, they are also called glycols
-are named with the suffix -diol
Vicinal Diols
diols with the hydroxyl group on adjacent carbons
Geminal Diols
diols with hydroxyl groups on the same carbon
-are not commonly observed because they spontaneously lose water (DEHYDRATE) to produce carbonyl compounds
What is another name for geminal DIOLS
carbonyl hydrates
Ethers Nomenclature
are named as derivatives of alkanes and the larger alkyl group is chosen as the backbone
- has a ALKOXY- prefix
- the chain is numbered to give the ether the lowest position
- COMMON NAMES for ethers is when the two alkyl groups are name in alphabetical order and add the word ether
Cyclic ethers Nomenclature
numbering of the ring begins at the oxygen and proceeds to provide the lowest numbers for substituents
Three membered rings of a cyclic ether is called?
oxiranes
Aldehydes Nomenclature
are named according to the longest chain containing the functional group.
- the suffix -AL replaces the -e of the corresponding alkane
- the carbonyl carbon receives the lowest number, although numbers are not always necessary
KNOW THE COMMON NAMES FOR ALDEHYDES
FORMALDEHYDE
ACETALDEHYDE
PROPIONALDHYDE
AND STRUCTURE
Ketones Nomenclature
- named just like aldehydes but with -ONE as the suffix
- if highest priority the carbonyl croon must be assigned the lowest possible number
- in complex molecules the carbonyl group can be names as a prefix OXO-
- also the individual alkyl groups may be listed in alphabetical order and followed by the word ketone
Carboxylic Acids Nomenclature
are names with the ending -oic acid
-are terminal functional groups and are numbered one
KNOW THE COMMON NAMES FOR CARBOXYLIC ACIDS
METHANOIC ACID
ETHANOIC ACID
PROPANOIC ACID
Amines Nomenclature
- the longest chain attached to the nitrogen is taken as the backbone.
- for simple compounds name the alkane and replace with the final -e with -AMINE
- more complex often use the prefix AMINO
- the prefix N is used to specify the location of an additional alkyl group attached to the nitrogen
Isomers
are chemical compounds that have the same molecular formula but differ in structure (their atom connectivity or the spatial orientation of their atoms)
-Isomers may be similar or different when it comes to their physical and chemical properties
Structural Isomers
Also known as constitutional isomers
- are compounds that share only a molecular formula
- differ in where and how atoms are connected to each other and thus often have very different chemical and physical properties (MP, BP, solubility)
Stereoisomers
are compounds that have the same connectivity between their atoms and differ from each other only in the way their atoms are oriented in space
- Different TYPES:
1) Cis-Trans Isomers
2) Enantiomers
3) Diastereromers
4) Meso Compounds
5) Conformational Isomers
Cis-trans isomers
also known as geometric isomers
- are compounds that differ in the position of substituents attachéd to the two carbons that from a double bond
- since the double bonds cannot rotate the substituents are fixed
- If the substituents on the carbon are both on the same side of the double bond then they are called CIS
- If the substituents on the carbon are on opposite sides of the double bond then they are called TRANS
When it comes to naming Cis-Trans isomers, How do you name when there is more than one substituent on either carbon?
The Highest Priority substituent has to be determined:
1) From the carbon of interest, determine the molecular weight of the first atom encountered along each bond. The group with the highest atomic weight atom has the highest priority
2) If two atoms are the same look at the next atom attached to each; the group that has the second atom with the highest molecular weight is higher priority
3) If two atoms are the same, a double bond takes priority over a single bond. This is a tie breaker ONLY
- The alkene is called Z if the two highest priority subsistent on each carbon are on the SAME SIDE
- the alkene is called E if the two highest priority substituents are on opposite sides
Chirality
- a molecule that is not superimposable upon its mirror image
- carbon atoms are chiral ONLY if they have four different substituents– a carbon like this is called ASYMMETRIC since it lacks a plane or point of symmetry
Enantiomers
pair of chiral molecules that are non-superimposable mirror images
Fischer Projections
a three dimensional molecule can be represented in 2D by fischer projections
- horizontal lines indicate bonds that project out from the plane of the page
- vertical lines indicate bonds behind the plane of the page
How do you obtain the mirror image of a fischer projection
If only one substituent is interchanged or if the molecule is rotated by 90 degrees
– the mirror image and thus the opposite enantiomer of the original compound is obtained
Configuration
describes the spatial arrangements of the atoms or functional groups of a sterioisomer
Relative configuration of a chiral molecule
is its configuration in relation to another chiral molecule
-is compared between the R and S enantiomers
Absolute configuration of a chiral molecule
describes the spatial arrangements of these atoms or groups within the molecule relative to each other
-is determined y using the R/S naming convention
Optical Activity
-Pairs of enantiomers (opposite R and S) have identical chemical and physical properties with one exception: OPTICAL ACTIVITY
- a compound is optical active if it has the ability to rotate plane-polarized light.
- If a plane-polarized light is passed through a solution of an optically active compound, the molecule rotates the orientation of the polarized light by an angel alpha. The enantiomer of this compound will rotate light but he same amount but in the opposite direction
Dextrorotatory
a compound that orates the plane of polarized light to the right or clockwise is indicated by a (+)
Levorotatory
a compound that rotate light toward the left or counterclockwise is labeled (-)
How can you not and can determine the direction of rotation of a optically active compound?
it cannot be determined from the structure of a molecule and is not related to its R and S designation.
-it must be determined experimentally!
How do determine the amount of rotation and what does it depend on
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
Racemic Mixture
or racemate, is a mixture of equal concentrations of both he (+) and (-) enantiomers
-the rotations cancel each other and no optical activity is observed
Diastereromers
Are stereoisomers that are not mirror images of each other
-more than one chiral center and thus multiple forms of stereoisomers exist
what is the general formula for determine how many stereosiomers exist for “n” chiral centers
2^N
Epimers
diastereomers that differ at only one carbon
-important in chemistry of carbohydrates
Meso Compounds
molecules with multiple chiral centers that also have an internal plane of symmetry
- one half of the molecule has an S enantiomer and the other had has its matching R enantiomer
- molecule is not optically active
Conformational Isomers
are compounds that differ only by rotation about or more single bonds
-these isomers represent the same compound but in a slightly different position
Most stable conformation for a newman projection
Staggered anti-when the two biggest groups are located 180 degrees from each other and there is not overlap of atoms along the line of sight
-called staggered anti because the two groups are antiperiplanar to each other
Gauch conformation
occurs when the two groups are 60 degrees apart
Totally eclipsed
highest energy state and is went the two groups overlap with each other
-zero degrees apart
eclipsed
is when the two groups are 120 degrees apart
how does ring strain arise in cycloalkanes?
angle strain-results when bond angles deviate from their ideal values
torsional strain- results when cyclic molecules must assume conformations that have eclipsed interactions
nonbonded strain-(Van Der Waals Repulsion)- resulsts when atoms or groups compete for the same space
How do cycloalkane alleviate angle strain, torsional strain, and nonbonded strain?
cycloalkanes attempt to adopt nonplanar conformation
What conformation does Cyclobutane go into to relieve stress?
puckered- which is a slight V shape
What conformation does cyclopentane go into to relieve stress?
envelope- looks like an airplane with no wings
What conformation does cyclohexane go into to relieve stress?
exists mainly in three conformations: charm, boat, and the twist/skewboat
ionic bonds
electron is transferred from one atom to another
covalent bonds
in which paris of electrons are sheared between two atoms
quantum number “n”
corresponds to the energy level in an atom
-essentially a measure of size
quantum number l=0 corresponds to what type of orbital?
s orbital
quantum number l=1 corresponds to what type of orbital?
p orbital
quantum number l=2 corresponds to what type of orbital?
d orbital
quantum number l=3 corresponds to what type of orbital?
f orbital
Molecular Orbital
when two atomic orbital are combined it forms molecular orbital
- are obtained mathematically by adding the wave functions of the atomic orbitals
- if the sign of the wave functions are the same, a lower-energy bonding orbital is produced
- if the signs are different, a high energy ANTIBODING ORBITAL is produced`
Saturated compound
means they the compound has the maximum number of hydrogen atoms attached to each carbon
What is the general formula for a alkane?
CnH2n+2
Primary Carbon
is bonded to only one other carbon atom
Secondary Carbon
is bonded to only 2 carbon atoms
Tertiary Carbon
is bonded to only 3 carbons
Quaternary Carbon
is bonded to only 4 carbons
Alkane Physical Properties
- vary in a predictable manner
- in general as the molecular weight of a straight chain alkane increases, the melting point, boiling point, and density also increase
- branched molecules have slightly lower boiling point and melting point than their straight-chain isomers
- Greater branching reduces the surface area of the molecule, decreasing the intermolecular attractive forces (london dispersion forces)- the molecules are held togeth less tightly, thus lowering the BP
At room temp what straight chain compounds are gases, liquids, and solids?
gases: CH4 through C4H10
Liquids: C5H12 through C16H34
Solids (waxes and harder solids): C17H36 and so on
Nucleophiles
are molecules that are attracted to positive charge
- nucleophile means nucleus lover
- are electron rich species that are often but not always negatively charged
- are attracted to atoms with partial or full positive charges
Nucleophile strengths
-if a group of nucleophiles are based on the same atom (oxygen for example) the stronger the base the stronger the nucleophile. Because bases act as electron donors and stronger nucleophiles are also better electron donors.
RO->HO->RCO2>ROH>H20
Nucleophiles in protic solvent
Protic solvent(a solvent that is able to form hydrogen bonds)
- in a protic solvent large atoms or ions tend to be better nucleophiles
- larger ions more easily shed their solvent molecules and are more polarizable
CN->I->RO->HO->Br->Cl->F->H2O
Nucleophiles in aprotic solvent
aprotic solvent- a solvent that cannot form hydrogen bonds
- the nucleophiles are “naked”- they are not solvated
- nucleophile strength is directly related to basicity
F->Cl->Br->I-
Leaving groups
the best leaving groups are those that are weak bases, because these can accept a negative charge and dissociate to form a stable ion in solution
-HALOGEN
I->Br->Cl->F-
other leaving groups beside halogens
ex: OH group
SN1 reaction
-unimolecular nucleophilic substitution reaction
Why is a SN1 reaction unimolecular?
because the rate of the reaction is dependent upon only one molecule in the reaction or the rate expression is first order!
What is the rate determining step in SN1
is the dissociation of the substrate (starting molecule) to form a stable, positively charged ion called a CARBOCATION
-the formation and stabilization of the carbocation determines all other aspects of SN1
SN1 mechanism
involves two steps:
- the dissociation of a substrate molecule into a carbocation and a leaving group
- followed by the carbocation with a nucleophile to form a substituted product
How are carbocations stabilized in SN1 Reactions
stabilized by polar solvents that have lone electrons paris available to donate (water or ethyl alcohol)
-stabilized by charge delocalization throughout the molecule
-the more highly substituted carbocations are more stable-due to hydrocarbon substituent groups donate electron density toward the positive charge
Rate of SN1 reaction
- the slowest step is the dissociation of the molecule to form a carbocation intermediate- a step that is energetically unfavorable thus the RATE LIMITING STEP
- rate of the reaction depends entirely on the concentration of the substrate
- THE RATE DOES NOT depend on the concentration or the nature of the nucleophile
Structural factors that increase the rate of reaction of SN1
highly substituted alkanes allows for distribution of the positive charge over a greater number of carbon and hydrogen atoms- thus more stable carbocations
- Generally the order of reactivity is tertiary> secondary> primary> methyl
- primary and methyl SUBSTRATES DO NOT REACT
Solvent effects that increase the rate of reaction of SN1
Highly polar solvents are better at surround and isolating ions than are less polar solvents
- Polar protic solvents such as WATER AND ALCOHOLS work best because they can form hydrogen bonds with the leaving groups and solvating it and preventing it from retiring to the carbocation.
- also lone electron pairs on oxygen or nitrogen atoms in the solvent molecule can stabilize the carbocation intermediate
Nature of the leaving group increasing the rate of the reaction of SN1
Weak bases dissociate more easily from the alkyl chain and thus make better leaving groups
-increasing the rate of the carbocation formation
Nature of the nucleophile increasing the rate of the reaction of SN1
SN1 reactions do not require a strong nucleophile
-SN1 reactions run equally well with either strong (fully charged) or weak (electron-rich but uncharged) nucleophiles
Stereochemistry of SN1 reactions
SN1 reactions involve carbocation intermediates that are sp2 hybridized and have trigonal planar geometry.
-the attacking nucleophile can approach the carbocation from either above or below with equal probability therefore creating either the R or S enanatiomer with equal probability
-If the original compound is optically active because of the presence of a chiral center, then a racemic mixture will be produced.
Sn2 reactions
bimolecular nucleophilic substitution
- involves a nucleophile pushing its way into a compound while simultaneously displacing the leaving group
- its rate determining and only step involves two molecules: the substrate and the nucleophile
SN2 Mechanism
are concerted reactions-meaning the entire mechanism occurs in single coordinated process
- the nucleophile attacks the reactant from the backside of the leaving group, forming a trigonal bipyramidal TRANSITION STATE
- as the reaction progresses the bond to the nucleophile strengthens and the bond to the leaving group weakens.
- The leaving group is displaced as the bond to the nucleophile becomes complete
SN2 RATE
- the single step of SN2 reaction involves two reacting species thus the substrate and the nucleophiles concentrations play a role in determining the rate of SN2 reactions
- follows SECOND ORDER KINETICS
rate expression for SN2 reaction
rate= k{substrate}{nucleophile}
Structural factors that can affect the rate of SN2 reactions
- the nucleophile must have unhindered access to the central carbon of the substrate
- Thus SN2 reactions occur most readily with substrate with little branching in order to minimize steric hindrance
The order of reactivity:
methyl>primary> secondary > tertirary
**tertiary substrates do not react by the SN2 mechanism
Solvent Effects that can affect the rate of SN2 reactions
SN2 reactions occur most readily in polar APROTIC solvents–the solvents are unable to form hydrogen bonds
- typical polar aprotic solvents include acetone and DMSO
- since these solvents cannot form hydrogen bonds, they do not create a salvation shell around the nucleophile and therefore do not interfere with its attack on the substrate
Nature of the leaving group affecting the rate of SN2 reactions
-weak bases dissociate more easily from the alkyl chain and thus make better leaving groups, increasing the ease of displacement by the nucleophile
Nature of the nucleophile affecting the rate of Sn2 reactions
- the nucleophile must attack a neutral molecule and displace the leaving group it must then be a strong nucleophile
- this means that the nucleophile is negatively charge ionic spices and can be either a strong or weak base
Stereochemistry of SN2 reactions
- Since the nucleophile attacks from one side of the central carbon and the leaving group departs from the opposite side, the reaction FLIPS the bonds attachéd to the carbon.
- If the reactant is chiral, optical activity will be retained, but will be inverted between R and S as long as the nucleophile and leaving group have the same priority relative to the other groups
SN1 Vs Sn2 nucleophile
SN1: any nucleophile
SN2: strong (charged) nucleophile
SN1 VS SN2 solvents
SN1: polar protic
SN2: Polar aprotic
SN1 vs SN2 leaving groups
SN1: Cl-, Br-, I-, weak bases
OH if protonated to form OH2^+
SN2: Cl-, Br-, I-, weak bases
SN1 VS SN2 substrate reactivity
SN1: 3>2>1>methyl
SN2: methyl>1>2>3
Free radical substitution reactions
alkanes can react by a free radical substation mechanism in which one or more hydrogen atoms are replaced by Cl, Br, or I atoms
- Involves 3 steps
1) Initiation: diatomic halogens are either cleaved by UV light (shown as UV or hv) or peroxide (H-O-O-H or R-O-O-H), resulting in the formation of free radicals (heat can also be used, but is not specific for radical formation) - Free radicals are uncharged species with unpaired electrons. They are extremely reactive and readily attack alkanes
2) Propagation: is when radical produces another radical the can continue the reaction. A free radical reacts with an alkane, removing a hydrogen atom to form HX, and creating an alkyl radical. The alkyl radical can then react with X2 to form an alkyl valid and generate another X. thus propagation the radical
3) Termination: two free radicals combine with one another to form a stable molecule
Combustion reaction of Alkane
reaction of an alkane with molecular oxygen (O2) to form carbon dioxide, water, and heat
-combustion is often incomplete, producing significant quantities of carbon monoxide instead of carbon dioxide
Pyrolysis
occurs when a molecule is broken down by heat in the absence of oxygen.
- also called CRACKING is most commonly used to reduce the average molecule weight of heavy oils and to increase the production of more desirable volatile compounds
- in alkanes, the C-C bonds are cleaved, producing smaller chain alkyl radicals
Disproportionation
a process in which a radical transfers a hydrogen atom to another radical, producing an alkane and an alkene
degree of unsaturation
CnHm
N=1/2(2n+2-m)
general formula for alkene
CnH2n
Physical properties of alkenes
- similar to those of alkanes
- melting and boiling points increase with increasing molecular weight and are similar in value to those of the corresponding alkane
- Terminal alkenes usually boil at a lower temp than internal alkenes
- Trans-alkenes generallly have higher melting points than cis-alkenes because their higher symmetry allows better packing in their solid state
- Trans-alkenes generally have lower boiling points than cis-alkenes because they are less polar`
Elimination reactions of alcohols and alkyl halides
the molecule loses either HX (where X is a halide) or a molecule of water from two adjacent carbons to form a double bond.
E1
unimolecular elimination
- is a two step process preceding through a carbocation intermediate
- rate of reaction is only dependent on the concentration of the substrate
- the elimination of a leaving group and a proton results in the production of a double bond
What conditions favor E1
- same factors favored by SN1
- protic solvents
- highly branched carbon chains
- good leaving groups
- weak nucleophiles in low concentrations
What conditions favor E1 over SN1
high temps favor E1, but directing a reaction toward either SN1 or E1 is difficult thus the product will be a mixture of both E1 and SN1 products
E2
bimolecular elimination
- its rate is dependent on the concentration of tow species, the substrate and the base
- there are two possible hydrogens that can be removed from carbon on either side of the leaving group, resulting in two different products. The more substituted double bond is formed preferentially`
E2 vs. SN2
1) steric hindrance does not greatly affect E2 reactions. Therefore highly substituted carbon chains, which form the most stable alkenes, undergo E2 easily and SN2 rarely
2) A strong bulky base such as t-butoxide favors E2 over SN2. While SN2 is favored over E2 by strong nucleophies that are weak bases such as CN- or I-
Types of reactions alkenes undergo
- reduction of hydrogen
- addition by electrophiles and free radicals
- hydroboration
- a variety of oxidations
- polymeriation
