2. Isomerism

Question 1

What are constitutional isomers?

Answer 1

constitutional isomers are two molecules with the same molecular formula, but different connectivity (C5H12)

Question 2

What are conformational isomers?

Answer 2

conformational isomers are two molecules with the same formula and connectivity, but different 3D orientation due to rotation around a sigma bond.

Conformers are actually the same molecule, but may exist slightly differently in space.

Question 3

Staggered / eclipsed conformational isomers, which ism more stable?

Answer 3

Viewed as a Newman projection, staggered conformers are more stable as they have less steric hindrance and less electron repulsion.

Question 4

Explain anti, syn, and gauche conformations of conformational isomers.

Answer 4

When a sigma bond has substituents on either side, the location of those substituents matters.

Anti –> staggered conformation where the bulky groups are in opposite directions (180 apart) .
Syn - eclipsed conformation where the bulky groups are in symmetrical positions
Gauche - staggered conformation where the bulky groups are 60 degrees apart.

Anti = most stable
Syn = least stable

Question 5

In what situation may a gauche position be more stable than an anti position, for conformers?

Answer 5

If the substituent groups can form an intramolecular hydrogen bond, the gauche conformer will be more stable.

Question 6

Cyclohexane conformational isomers: What is the chair and boat?

Answer 6

cyclohexane can also exist as different conformers.

The chair conformer is lowest energy whereas the boat conformer is the less stable isomer.

Question 7

t or f, two equally energetic chair conformations exist for cyclohexane.

Answer 7

true, this is the concept of a ring flip, which can occur if there is sufficient thermal energy (at 298 K there is)

Question 8

What occurs during a ring flip?

Answer 8

All equatorial hydrogen’s become axial
All axial hydrogen’s become equatorial
This applies to any substituent

However, they do not change their position relative to the plane. If an axial H is out of plane, then it becomes an out of plane equatorial H.

Question 9

What is a 1,3-diaxial interaction? What chair conformation is most stable for cyclohexane?

Answer 9

Substituents in an axial position can undergo unfavorable 1,3-diaxial interactions. Therefore, to increase stability, we want subs to be in the equatorial position.

Question 10

t or f, to perform a ring flip, number the carbon atoms and then push up or down on the far right carbon.

Answer 10

True, this is helps with visualization.

Question 11

What are stereoisomers?

Answer 11

stereoisomers are molecules with the same formula and connectivity, but differ in their spatial arrangement around around a chiral center (a center with 4 different substituents around it)

Question 12

t or f, any molecule that cannot be superimposed onto itself is said to be chiral.

Answer 12

true

a molecule that CAN be superimposed onto itself is achiral.

Question 13

what is a stereocenter?

Answer 13

A stereocenter or stereogenic center is simply a chiral carbon (an sp3 hybridized carbon with 4 different subs)

Question 14

what is the maximum number of stereoisomers a molecule can have with n chiral carbons.

Answer 14

the maximum number of stereoisomer conformations a molecule can have is 2^n where n is the number of chiral carbons.

Question 15

A molecule that is symmetrical but has two chiral carbons (or n chiral carbons), has how many stereoisomers?

Answer 15

When a molecule is symmetrical, it will have 1 less isomer since, if the constituents are in the same plane, the molecule is identical.

E.g. 1,3-dibromohexane has 3 stereoisomers (draw them).

Question 16

What id deuterium (D) and Tritium (T)?

Answer 16

D is 2H (hydrogen with 2 neutrons).
T is 3H (hydrogen with 3 neutrons).

used to radio-label in chemistry

Question 17

Absolute configuration (R or S): explain the 4 steps in finding a molecules absolute configuration.

1. priority
2. bonds
3. configure
4. name

Answer 17

1. assign priority based on atomic number (Z)
   - tie, we go down the line of connectivity
   - isotope goes to higher atomic mass (A) T > D > H
2. multiple bonds are counted as 2 single bonds. E.g. C=O is considered two separate bonds to 2 oxygen atoms
3. place the lowest priority sub in the back, facing away
4. count the other subs from Highest to Lowest.

clockwise = R (i.e. to the right) 
counterclockwise = S

Question 18

t or f, when assigning absolute configuration, if the lowest priority group is pointing towards you, then you can just trace the priory path as is, and then flip the absolute configuration sign.

Answer 18

True!

you can also switch two constituents, draw the path, flip the sign.

Question 19

What are enantiomers?

Answer 19

These are stereoisomers (i.e. cannot be superimposed onto each other) that are mirror images of each other. think of holding your hand to a mirror

Question 20

t or f, enantiomers will always have the opposite absolute configurations.

Answer 20

True. At every stereocenter, enantiomers will have opposite configurations

Question 21

How do two enantiomers differ in terms of their physical properties?

Answer 21

They essentially have the same physical properties (BP, MP, etc) except for their optical activity. Enantiomers will bend plane-polarized light to the same magnitude in opposite directions.

Question 22

What are dextrorotatory (d / +) and levorotatory (l / -) ?

Answer 22

This refers to how enantiomers rotate plane-polarized light.
clockwise = dextrorotatory (d or +)
counter-clockwise = levorotatory (l or -) (to the left)

Question 23

What is specific rotation?

Answer 23

the magnitude of rotation of plane-polarized light for any compound is its specific rotation.

Question 24

What is a racemic mixture?

Answer 24

A racemic mixture contains 50/50 d and l enantiomers. this mixture will have a specific rotation of 0 since the enantiomers cancel each other out. A racemic mixture is NOT optically active.

Question 25

t or f, the R/S configuration is related to the +/- optically activity of enantiomers.

Answer 25

FALSE, no correlation.

Question 26

For every stereocenter of a compound, we can determine whether its stereochemistry is R or S. Explain diastereomers.

Answer 26

diastereomers are stereoisomers (non-superimposeable) that are NOT mirror images (which would be enantiomers).

In diastereomers, the R / S configurations will differ at at least one chiral center, but not all.

Question 27

If a molecule has three chiral centers all of which are R configuration, the same molecule with all three centers in the S configuration would be related to the first molecule how?

Answer 27

If you were to flip the first molecule over a mirror plane, you would get the second molecule. Thus, if all stereocenters are the same configuration the opposite is its enantiomer.

Question 28

A molecule that is R,S is related to a molecule that is S,R in what way?

Answer 28

enantiomers! Since if we flip one molecule, we get the other (mirror images).

Question 29

t or f, unlike enantiomers, diastereomers can have very different physical properties (MP, BP, etc).

Answer 29

True and this also applies to their optical activity, which has no relationship between each other (whereas enantiomers is equal and opposite).

Question 30

What is enantiomer resolution? How is it done?

Answer 30

Since enantiomers have identical physical properties, separating R and S configurations is difficult.

enantiomer resolution involves adding a resolving agent (known as a chiral probe) which binds the two steromers and changes some physical property, such as solubility.

Question 31

What are epimers?

Answer 31

Epimers are a subclass of diastereomers where only one chiral center is different between two molecules.

RRSR –> RRRR

these are epimers

Question 32

How do you name D and L sugars?

Answer 32

1. Place the CHO group at the top
2. look at the orientation of the OH on the highest numbered carbon (at the bottom)
   - pointing to the RIGHT = D
   - pointing to the left = L

Question 33

What is the epimeric carbon?

Answer 33

Recall, epimers only differ at one stereocenter. This stereocenter is called the epimeric carbon

Question 34

What are anomers?

Answer 34

When epimers form upon a ring closure, they form anomers.

e.g. sugars: the C5 OH group attacks the C1 carbonyl group to create a ring system

Question 35

At the anomeric carbon (stereocenter of anomers), how do you determine if it is an alpha or beta compound?

Answer 35

on the anomeric carbon…
OH pointing down = alpha
OH pointing up = beta

Question 36

What is a meso-compound?

Answer 36

A meso-compound is a molecule that contains more than one chiral center, but has an internal line of symmetry (ie. one half of the molecule replicates the other half of the molecule).

Question 37

t or f, since meso-compounds contain chiral centers, they have optical activity.

Answer 37

False, since meso-compounds have an internal line of symmetry, their optical activities cancel to zero. Thus, the molecule as a whole is achiral.

Question 38

A meso-compound with two chiral centers has x amount of isomers.

Answer 38

Typically 2^n = x # of isomers.

However, since a meso-compound has its own symmetry, its isomer is actually identical to itself.

so instead of 4 isomers, its 3.

Question 39

What are geometric isomers? How do you name them?

Answer 39

Geometric isomers are diasteromers that differ in orientation around a ring or double bond.

two highest priority groups on the same side = Cis (Z)
two highest priority groups on opposite sides = Trans (e)

Question 40

When do we use cis / trans and when do we use Z / E?

Answer 40

Cis / Trans = ring geometric isomers
Z / E = double bond geometric isomers
not strict.