Diastereomers: stereoisomers that are not mirror images.

Unlike enantiomers, they differ in all properties (m.p., b.p., d, etc.).

enantiomers	diastereomers
A-B	A-C, A-D
C-D	B-C, B-D

Maximum possible # of stereoisomers = 2ⁿ

where n = # of stereogenic centers (*)

1* = 2¹ = 2 } 1 pair of enantiomers

2* = 2² = 4 stereoisomers

4* = 2⁴ = 16 stereoisomers

Sucrose (table sugar, disaccharide)

# of maximum stereoisomers possible = 2⁹ = 512

Tartaric Acid

plane of symmetry

enantiomers identical, achiral, meso compound

point of symmetry

meso compound : optically inactive, achiral diastereomer of compound > 1 stereogenic center

Fischer Projections

For > 1 stereogenic center, align the stereogenic centers vertically with all substituents extending to the left or right

consider each stereogenic center separately

Fischer projections do not indicate the preferred conformation.

D,L Stereoisomers

optical activity : (+, d) (-, l)

absolute configuration : R, S

all saccharides (sugars) that have the same configuration as (+)-glyceraldehyde at the stereogenic center most distant from the carbonyl functional group are designated D. (carbonyl = aldehyde, ketone, carboxylic acid, etc.)

The opposite configuration is designated L.

(+)-(R)-glyceraldehyde is D (-)-(S)-glyceraldehyde is L

D-(-)-erythrose D-(-)-threose

D-(+)-glucose

All naturally occurring sugars are D.

Amino Acids

a-Amino Acids

Alanine

The -NH₂ group (amino group) corresponds to the hydroxy group (-OH) on glyceraldehyde for D vs. L determination.

All naturally occurring amino acids are L.

Resolution of Enantiomers

enantiomers : same achiral properties, cannot be separated by common techniques (distillation, recrystallization, normal chromatography)

diastereomers : different achiral properties, can be separated by common techniques

In order to separate enantiomers, they must be temporarily converted to a diastereomer.

Steps in separating enantiomers

1. Start with mixture of enantiomers, R(1) and S(1) (racemate if 50:50)

2. React with pure enantiomer of another suitable compound, for e.g. let it be S enantiomer of (2). Then a pair of diastereomers would form:

R(1)S(2) and S(1)S(2).

R(1) and S(1) + pure S(2) ® R(1)S(2) + S(1)S(2).

3. Separate the diastereomers by some physical method

4. Then convert the diastereomers back into their original compounds in separate containers. If done carefully, should have samples of pure enantiomers

The solubility of the diastereomers in methanol is different. The S (R,R) salt will crystallize first.

Enantiomers can also be separated by chromatography if a chiral stationary phase is used.

The intermolecular attractive forces (e.g. dipole-dipole attraction) between

enantiomers and chiral substance on stationary phase will be different. The enantiomers will move at different rates on the TLC plate, GC column, or in column in liquid chromatography.

Reactions Involving Chiral Compounds

A. chiral molecule reacts with achiral molecule

- enantiomers will react at same rate

- same products form except for absolute configuration at stereogenic center

e.g. Either hand (chiral) can hold a book (achiral).

B. two chiral molecules react

chiral molecule A + chiral molecule B

A (R) + B (R)

A (S) + B (R)

reactions may occur at significantly different rates

even different products (structures) may form

e.g. A right handed glove (chiral) will only fit on a right hand (chiral). You could attempt to put a right handed glove on a left hand, but it won’t work as well.