Isomerism and hybridization Flashcards
what is stereochemistry?
the study of molecules as 3D structures
what is a mirror image?
the reflection of an object in a mirror
what is chirality?
an object and its mirror image are nonsuperposable
what is achirality?
an object that lacks chirality; an object and its mirage image are superposable
What makes an object achiral?
If it has one or more element of symmetry
What are two (2) examples of elements of symmetry?
- Plane of symmetry
2. Center of symmetry
What is a plane of symmetry?
An imaginary plane passing through an object dividing it so that one half is the mirror image of the other half
What is a center of symmetry?
A point situated so that for all atoms in a molecule, an identical atom lies on opposite sides and equidistant from that point.
What is the simplest way to determine if an object is chiral?
If an object has no element of symmetry it is chiral.
What are stereoisomers?
Isomers that have the same molecular formula and connectivity but a different orientation of their atoms in space.
What is a conformational isomer?
Isomers that can be interconverted exclusively by rotations about a single bond.
What is a configurational isomer?
Isomers that have the same connectivity of atoms but the attached atoms are in different positions.
What is an enantiomer?
Stereoisomers that are nonsuperposable mirror images (i.e. chiral objects) of each other.
What is the difference between the terms “chiral” and “enantiomers?”
“Chirality” is a characteristic of objects while “enantiomers” refers to a relationship between a pair of objects.
What is a chiral center?
A tetrahedral atom (usually carbon) bonded to four different groups.
What is the most common cause of chirality in organic molecules?
A chiral center
What is a stereocenter and what is a specific type of stereocenter?
An atom about which exchange of two groups produces a stereoisomer. (For example, the carbons bonded to two groups to form cis,trans isomers.) Chiral centers are a type of stereocenter.
What two types of objects can comprise configurational isomers?
- Chiral
2. Achiral
What is it called when a pair of configurational isomers are chiral objects?
Enantiomerism
What’s an example of of a pair of configurational isomers that are achiral objects?
Some cis,trans isomers (ex. 1,4-dimethylcyclohexane)
What is cis,trans isomerism?
Isomerism that exists because of restricted rotation about a double bond.
What is a diastereomer?
Stereoisomers that are not mirror images of each other.
What will cause diastereomerism?
Two or more stereocenters in a molecule.
Why are conformational isomers considered stereoisomers?
Because the atoms are arranged differently in space.
What is the stereoisomer relationship between gauche and anti conformations?
Diastereomers because they are not mirror images.
What is the stereoisomer relationship between the two gauche conformations?
Enantiomers because they are mirror images and not superposable. These conformations have chirality.
Describe how chirality can be present in molecules without chiral centers
When (1) the barrier to interconversion between conformational isomers is large and (2) the enantiomers cannot interconvert through a planar (achiral) conformation, the molecule will be chiral and the enantiomers can be separated.
What is absolute configuration?
Which of two possible isomers an enantiomer is
How do you assign an absolute configuration? (3 steps)
- Assign priority using same priority system as E,Z
- Put the lowest priority group of the tetrahedral atom behind the atom
- Trace from highest priority to second highest priority and assign R if clockwise and S if counterclockwise.
What is the name of isomers which lack a chiral center but do not interconvert because of hindered rotation? (in otherwords, form diastereomers through isolatable conformations)
Atropisomers
Describe the relationship of all chiral centers between a pair of enantiomers
All chiral centers are the opposite between two enantiomers. (e.g. (2S,3R,4R) and (2R, 3S,4S))
What is the maximum number of stereoisomers for a molecule with n stereocenters?
2^n
What causes a molecule to have less stereoisomers than the maximum number predicted?
A configuration may result in a meso compound. Each meso compound decreases the total number of stereoisomers by 1 because its mirror image is not a different molecule.
What is a meso compound (meso structure)
A compound that (1) possesses 2 or more chiral centers, (2) is achiral and (3) has chiral stereoisomers.
Describe common characteristics of a meso compound (4 components)
Two chiral centers, each with the same four groups, and one is R and the other is S. There is an internal plane of symmetry.
Describe the physical and chemical properties of enantiomers in an achiral environment
Enantiomers have identical physical and chemical properties in an achiral environment.
Describe the physical and chemical properties of diastereomers
Diastereomers have different physical and chemical properties even in an achiral environment.
What is a Fischer projection?
A two-dimensional projection of a molecule where the horizontal molecules are by convention in front and vertical molecules are by convention behind.
Why is a Fischer projection not equivalent to the same Fischer projection after being rotated by 90 degrees?
The convention of having horizontal groups in front and the vertical groups behind make rotations impermissible.
How do you determine if a carbon has 4 different attached groups in a ring?
Determine the connectivity by going all the way around the ring, if there is a point of difference in the order of the groups you encounter, they are not the same groups.
What is angle of specific rotation?
A physical property of an asymmetrical molecule that measures how much it rotates a plane of polarized light.
What causes optical activity?
Electromagnetic fields of a chiral molecule interact with the field of electromagnetic radiation asymmetrically causing rotation of polarized light
What is clockwise rotation of polarized light called?
Dextrorotatory
What is counterclockwise rotation of polarized light called?
Levorotatory
What can be said of the optical activity of products formed from optically inactive reactants?
Optically active products cannot be formed
What can be said of the optical activity of the products formed from chiral reactants?
Can be optically active or inactive
What is the utility of the fact that chiral reactants can form optically active or inactive products?
Whether the products are optically active or inactive gives insight into the mechanism
What can be said of the relative amounts of products formed by reactions introducing a second chiral or stereocenter?
The products need not be in equal amounts
What is the explanation for why unequal amounts of products may be formed in a reaction introducing a second stereocenter?
The two possible transition states are diastereomeric and have unequal activation energies
What is a conjugated diene?
A diene in which the double bonds are separated by one single bond (e.g. R-C=C-C=C-R)
What is a cumulated diene?
A diene in which the two double bonds share an sp-hybridzed carbon. (e.g. R-C=C=C-R)
What is a diene?
Compounds that contain two carbon-carbon double bonds.
What are is the geometry of carbons in a conjugated system?
Trigonal planar (sp2 hybridization with an unhybridized p orbital)
What is an isolated diene?
A diene in which the two double bonds are separated by more than one single bond.
What is another name for a cumulated diene system?
Allenic system
Why do the p orbitals not overlap in a cumulated system?
The sp hybridization of the shared carbon requires the p orbitals of the sp2 carbons to be 90 degrees apart for sigma overlap.
Compare the stability of conjugated systems to unconjugated systems
Compounds containing conjugated double bonds are more stable than isomeric compounds containing unconjugated double bonds.
Describe the geometry of a conjugated system
The four sp2 hybridized atoms of conjugated systems are restricted to a planar geometry.
What causes the geometry of a conjugated system?
Maximal overlap of [all four] pi orbitals occurs when they are parallel and thus planar.
What is the significance of the geometry of a conjugated system? (2 components)
(1) Because of the extra stability of all four pi orbitals being planar, there is a barrier to rotation about the single bond. (2) This creates isolatable s-cis and s-trans planar conformations.
What does allylic refer to?
Next to a carbon-carbon double bond.
What is an allylic carbocation?
A carbocation in which an allylic carbon bears the positive charge.
What is the two-step mechanism for the 1,2 and 1,4 electrophilic addition to a conjugated diene?
- Nucleophilic pi bond attacks electrophile and creates an allylic carbocation. The ability of remaining pi bond to shift creates a resonance hybrid with two possible carbocation locations.
- Make a new bond between a nucleophile and a carbocation. The original carbocation is 1,2 addition and the “resonance” carbocation is 1,4 addition.
What is the rate determining step of 1,2 and 1,4 electrophilic addition to a conjugated diene?
Formation of allylic carbocation stabilized by resonance
What is kinetic (rate) control?
When the distribution of products determined by the relative rates of formations of each product
What is the major factor making a product more kinetically favorable than an alternative?
A lower activation energy
What is thermodynamic (equilibrium) control?
When the distribution of products is determined by the relative stability of the products
What is the major factor making a product more thermodynamically favorable than an alternative?
A lower internal energy
What is the simplest way to switch the distribution of products between kinetic and thermodynamic control?
Changing the temperature (high temperature favors thermodynamic control)
Why do higher temperatures increase thermodynamic control? (2 components)
At higher temperatures a greater proportion of molecules have the energy to (1) cross the energy barrier of reacting backwards and (2) cross the larger energy barrier of the thermodynamic product
Which product of the electrophilic addition to a diene is the kinetic product?
The product formed by the most stable (highly substituted) carbocation.
Which product of the electrophilic addition to a diene is the thermodynamic product?
The product that is the most stable (highly substituted) alkene.
What is the diels-alder reaction?
Concerted pericyclic reaction between a conjugated diene and a dienophile.
What is a pericyclic reaction?
A reaction that takes place in a single step without intermediates and involves a cyclic distribution of orbitals.
What is a dienophile?
A compound containing a double bond that can react with a conjugated diene to give a diels-alder adduct.
What is a diels-alder adduct?
A cyclohexene resulting from the cycloaddition reaction of a diene and a dienophile.
What is a cycloaddition reaction?
A reaction in which two reactants add together in a single step to form a cyclic product.
What are the two (2) frontier molecular orbitals?
- highest occupied molecular orbitals (HOMOs)
2. lowest unoccupied molecular orbitals (LUMOs)
What does “highest” and “lowest” refer to in the names of frontier molecular orbitals?
the energy of the orbitals
What does “occupied” or “unoccupied” refer to in the names of frontier molecular orbitals?
if the orbital is populated with two electrons or is empty
How many carbons does the diene contribute to the cyclohexene ring?
Four
How many carbons does the dienophile contribute to the cyclohexene ring?
Two
Describe the change in the types of bonds between reactants and the diels-alder adduct?
Breaking of two pi bonds and forming of two sigma bonds
What is a major driving force for the diels-alder reaction?
The breaking of two weaker bonds and forming two stronger bonds means the product is more stable.
Describe the movement of the six pi electrons in a diels-alder reaction. (3 components)
- Pi electrons in dienophile form a sigma bond with diene.
- Pi electrons in diene form a sigma bond with dienophile.
- Pi electrons in diene move to the sigma bond that was formerly between the conjugated system to stabilize the ring.
What are three (3) reasons that make the diels-alder reaction enormously valuable in synthetic organic chemistry?
- One of the simplest reactions that can form a six-membered ring.
- One of the few reactions that can form two new carbon-carbon bonds at the same time
- Completely stereo-specific and quite regioselective
Which planar conjugated diene conformation is more stable and why?
S-trans because it avoids nonbonding steric interactions.
Which planar conjugated diene conformation is the reactive conformation in the diels-alder reaction and why?
S-cis because carbon atoms 1 and 4 are close enough to react with the carbon-carbon double or triple bond of the dienophile.
What can increase the reaction rate of diels-alder reaction?
If an electron-poor dienophile reacts with an electron-rich diene.
How can a dienophile be made to be electron-poor?
An electron-withdrawing substituent (ex. carbonyl because the oxygen will polarize the C-O bond and make it partially positive)
How can a diene be made to be electron-rich?
An electron-releasing substituent (ex. methyl because the C-H sigma bonds can donate electron density)
Describe the formation of bicyclic alkenes using diels-alder reactions
When a cyclic diene is used, the sp3 hybridized carbon will move out of the plane and form a bridgehead carbon while carbons 1 and 4 will still form sigma bonds with the dienophile.
Describe the stereospecificity of the diels-alder reaction
The configuration of both the dieneophile and the diene is retained.
The synthesis of chiral products from achiral starting materials in an achiral environment invariably leads to what kind of products?
A racemic mixture
What is resolution?
The separation of enantiomers
What are two (2) strategies for resolution?
- Convert into diastereomers, separate by physical properties, regenerate into enantiomers.
- Use a chiral environment to distinguish enantiomers.
What is a racemic mixture?
A 50/50 mixture of enantiomers.
What is an alkyne?
An unsaturated hydrocarbon containing one or more carbon-carbon triple bonds.
Describe the carbon-carbon triple bond in terms of orbitals.
Two overlapping sp hybridized orbitals form the sigma bond; two overlapping 2py orbitals form one pi bond; two overlapping 2pz orbitals form the second pi bond.
What is the IUPAC nomenclature for a carbon triple bond?
The infix -yn- (e.g. butyne, pentyne, hexyne etc.)
What is the common name for ethyne?
Acetylene
How is it indicated if a hydrocarbon chain contains more than one triple bond?
The infixes -adiyn-, -atriyn- and so forth.
How are the common names for alkynes derived?
Prefixing the names of substituents on the carbon-carbon triple bond to the word “acetylene.” (ex. CH3C=CH is methylacetylene)
What are the smallest cycloalkynes that have been isolated? (2 components)
Cyclooctyne but it polymerizes rapidly at room temperature. Cyclononyne is stable at room temperature.
What is a terminal alkyne?
An alkyne in which the triple bond is between carbons 1 and 2.
What is significant about the chemistry of 1-Alkynes (terminal alkynes)?
The hydrogen bonded to the terminal carbon is sufficiently acidic to be removed by a strong base.
What bases are strong enough to remove a hydrogen from a terminal alkyne?
Sodium hydride (NaH), Sodium amide (NaNH2), Lithium diisopropylamide (LDA)
What is the product when a strong base reacts with a terminal alkyne?
Acetylide anion
Why is the hydroxide ion not a strong enough base to convert a terminal alkyne to an alkyne anion?
Water is a stronger acid than acetylene.
What is an alkylation reaction?
A reaction in which a new carbon-carbon bond to an alkyl group is formed.
Describe the alkylation of acetylide anions with methyl and primary haloalkanes
The acetylide anion is a nucleophile because of its unpaired electrons. It donates this pair of electrons to the carbon bonded to a halogen; the C-X electrons collapse on X and it leaves as an anion. This results in the acetylide replacing the halogen and forming an alkyne.
Describe the IUPAC numbering priority for double and triple bonds
They have the same numbering priority. Numbers are assigned by having the lowest numbers and in some case by alphabetization.
Why must the alkyl halides used in the alkylation reaction of acetylide and a haloalkane be methyl or primary?
Secondary and tertiary alkyl halides have too much hindrance from backside attack (Sn2 reaction)
What alkyne reduction reaction forms a cis alkene?
Pt, H2, Lindlar’s catalyst
What alkyne reduction reaction froms a trans alkene?
Dissolving-metal reduction (Li or Na with liquid ammonia)
What is a vinyl carbocation?
A carbocation where the positive charge is on a carbon participating in a carbon-carbon double bond.
Rank carbocation stability, including vinyl carbocations.
tertiary > secondary > primary > secondary vinyl > primary
What reaction allows the preparation of alkynes from alkenes?
Dehydrohalogenation
What is special about the reagent used for dehydrohalogenation of an alkene to form an alkyne?
A stronger base than OH- must be used.
Why does the use of the base KOH only allow the first dehydrohalogenation step to occur when trying to synthesize an alkyne?
OH- is not strong enough to remove a vinyl hydrogen.
Describe the relationship between the vinyl group and the vinyl carbocation.
Vinyl group is when the point of attachment to the main chain is on a carbon participating in a double bond (i.e. R-CH=CH2) and a vinyl carbocation is when the positive charge is on a carbon participating in a double bond.
Describe the relationship between the allyl group and the allylic carbocation.
Allyl group is when the point of attachment is on a carbon single bonded to a carbon participating in a double bond (i.e. R-CH2-CH=CH2) and an allylic carbocation is when the positive charge is on a carbon single bonded to a carbon participating in a double bond.
Describe the regioselectivity of HX electrophilic addition to an alkyne
HX adds to alkynes by Markovnikov’s rule
In electrophilic addition to an alkyne, what determines if of reactant will add one or two times to an alkyne?
The ratio of the reactants.
What reagents can be used for Markovnikov hydration of an alkyne?
Hg2+, H+ and H20
What reagents can be used for nonMarkovnikov hydration of an alkyne?
BH3 (or sia2BH), H202 and NaOH
What is sia2BH and what is its purpose?
Disiamylborane. Two of the hydrogens are replaced by a buly group so it reacts with the alkyne only one time.
What are tautomers?
Constitutional isomers in equilibrium with each other that differ in the location of a hydrogen atom and a double bond relative to a heteroatom (common O, N or S)
What is an enol?
A compound containing a hydroxyl group bonded to a double bonded carbon atom.
What is keto-enol tautomerism?
A type of isomerism involving keto (from ketone) and enol tautomers.
What is unique about the products of a hydration of an alkyne?
The initial enol tautomerizes to more stable keto form by (1) shifting the hydrogen of the hydroxyl group to the adjacent double-bonded carbon and (2) forming a double bond between carbon and oxygen.
What are three tips when planning an organic synthesis?
- Count the carbons in the products versus the starting material so you know what fragments must be added or subtracted.
- Analyze the functional groups to see what reactions are possible.
- If complicated, work backwards.
Compare the reactivity of benzene with alkenes and alkynes
Does not react in the same electrophilic addition reactions as alkenes and alkynes
What is an aromatic compound?
Any compound that meets Huckel criteria for aromaticity
What is an arene?
Class of hydrocarbons that are aromatic
What is an aryl group?
A group derived by removal of an H from an arene. Given the simple Ar-
What is the resonance energy of benzene and what does it indicate?
The difference in internal energy between 1,3,5-cyclohexatriene and benzene. Benzene has a lower internal energy (by about 151 kJ/mol) than the hypothetical alkene because of its resonance.
What are the four parts of Huckel’s criteria for aromaticity?
- Be cyclic.
- Have one 2p orbital on each atom of the ring.
- Be planar or nearly planar.
- Have a closed loop of (4 n + 2) pi electrons.
What is an antiaromatic compound?
A compound that meets all of Huckel’s criteria except it has 4n pi electrons. It is especially unstable.
What is the Frost circle?
A graphic method for determine the relative energies of pi MOs for planar, fully conjugated, monocyclic compounds.
How do you make a Frost circle? (4 components)
(1) Draw a circle then inscribe in it a polygon of the same number of sides as the ring in question. (2) One of the vertices must be at the bottom of the circle. (3) The relative energies are given by the points where the vertices touch the circle. (4) MOs below the horizontal diameter are bonding orbitals and MOs above the horizontal diameter are nonbonding orbitals.
Why does the Frost circle work?
It reproduces geometrically the mathematical solutions to the wave equation
What is an annulene?
A cyclic hydrocarbon with a continuous alteration of single and double bonds
What is the name of benzene in annulene notation?
[6]annulene
Why is [10]annulene not aromatic?
The ten-membered ring is too small to accomodate inward pointing hydrogens. This steric interaction makes the ring twist and become nonplanar.
Why can neutral monocyclic unsatured hydrocarbons with an odd number of carbons not be aromatic compounds?
By necessity there must be at least one CH2 group in the ring and there would not be a closed loop of 2p orbitals.
How can you form an aromatic ion from cyclopentadiene?
Treatment with sodium amide will remove a hydrogen and form a cyclopentadiene anion. The carbon becomes sp2 hybridized with two electrons in its unhybridized 2p orbital, forming a closed loop of 2p orbitals.
What is the significance of cyclopentadiene’s anion being aromatic?
Because cyclopentadiene’s anion is aromatic, it is the most acidic hydrocarbon known.
What is the name of a group derived by removing a hydrogen from benzene?
Phenyl
How do you name benzene derivatives that are formed by substituting a single hydrogen?
Add a prefix to the word benzene or it may have a special name.
What is the benzene derivative where a H is replaced by CH3?
Toluene
What is the benzene derivate where a H is replaced by OH?
Phenol
What is the benzene derivative where a H is replaced by COOH?
Benzoic acid
What is the benzene derivative where a H is replaced by So3H?
Benzenesulfonic acid
What is the benzene derivative where a H is replaced by NH2?
Aniline
What is the benzene derivative where a H is replaced by COCH3
Acetophenone
How do you locate substituents in a disubstituted benzene? (3 ways)
- 1,2 is ortho
- 1,3 is meta
- 1,4 is para
How do you name a disubstituted benzene if one of the groups give the ring a special name?
Consider the special name the root for the name and start the numbering at that group.
How do you locate groups in a trisubstituted benzene?
use numbers to locate the group
If a substituted group gives the ring a special name (so it must be on carbon 1), do you include 1 in the name?
No
What is the name for the group NO2?
Nitro
Why do aromatic compounds not react with electrophiles as alkenes do?
Resonance stabilization energy
What is the major type of reaction for aromatic compounds?
Electrophilic Aromatic Substitution
