Diastereomers, Fischer Projections and D/L Configuration

Fischer projections put 3D molecules on paper. With them you can tell enantiomers from diastereomers at a glance, name erythro and threo pairs, and assign D/L configuration.

Builds on: Part 10 · Lactic Acid, Tartaric Acid and Counting Stereoisomers.

Video coming soon

Diastereomers

2-Bromo-3-chlorobutane

Two different chiral centres give 2² = 4 stereoisomers: two enantiomer pairs (I/II and III/IV). Any member of one pair is a diastereomer of either member of the other. Diastereomers differ in m.p., b.p. and solubility, so they can be separated.

Three Fischer projections of a diol. — Three diol projections.

Enantiomer or diastereomer?

Read both centres

(I) and (III) are reflected at both centres: enantiomers. (II) differs from each at only one centre: a diastereomer of both.

D/L configuration

The reference: glyceraldehyde

D-(+)-Glyceraldehyde has OH on the right; L-(−) on the left. Compounds made from D-glyceraldehyde without touching that centre are D. In sugars, the last-but-one carbon decides. D/L is configuration; d/l is the sign of rotation.

Fischer projections of glyceraldehyde. — D- and L-glyceraldehyde.

Fischer projections A, B and C. — (B) shown turned by 180°.

Exam question

Turn and check

Turning (B) by 180° puts COOCH₃ on top with both OH on the left: the mirror image of (A). So (A) and (B) are enantiomers, while (A) and (C) differ at one centre and are diastereomers. Some keys give (b); the projections as drawn give (d).

JEE-style question

Your turn

Fischer projections (A), (B), (C): which is correct?

(a)(A) and (B) are identical

(b)(A) and (B) are diastereomers

(c)(A) and (C) are enantiomers

(d)(A) and (B) are enantiomers

Show the answer and the traps

Turn (B) by 180°: COOCH₃ comes to the top, and both OH go to the left. That's (A) reflected, so (A) and (B) are enantiomers: option (d).

(A) and (C) differ at only one centre, so they're diastereomers, not enantiomers. Some keys give (b); turn and check, and it's (d).

Watch out

Common mistakes

Turning a Fischer projection by 90°.That gives the enantiomer; only 180° keeps the molecule.

Linking D with (+).D/L is configuration and d/l is the sign of rotation; they are unrelated.

Practice

Try these

1. What does turning a Fischer projection by 90° in the plane give?

Its enantiomer.

2. Fischer projection: CHO on top, H left, OH right, CH₂OH at the bottom. D or L?

D-Glyceraldehyde (OH on the right).

3. How many stereoisomers does CH₃CH=CH–CH(OH)CH₃ have?

4: one chiral centre (×2) and a cis-trans double bond (×2).