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Optical activity; tartaric acid stereoisomers

Short answer

Optical activity is the ability to rotate plane-polarised light. Tartaric acid has (2R,3R) and (2S,3S) enantiomers (active) and a meso form (inactive) with a plane of symmetry. The compound HOOC–CHBr–CHBr–COOH with both Br on the same side is the meso form, optically inactive due to internal compensation.

What is optical activity? Give the stereoisomers of tartaric acid. Would you expect the following compound to be optically active? Explain. [Figure: HOOC–CHBr–CHBr–COOH, with H and Br on each of the two central carbons]

Optical activity

Optical activity is the property of a substance of rotating the plane of plane-polarised light. A substance that rotates it clockwise is dextrorotatory (+), and one that rotates it anticlockwise is laevorotatory (−). It is caused by molecular chirality, e.g. a carbon bonded to four different groups.

Stereoisomers of tartaric acid

   COOH         COOH         COOH
H—C—OH       HO—C—H       H—C—OH
HO—C—H       H—C—OH       H—C—OH
   COOH         COOH         COOH
 (2R,3R)      (2S,3S)      meso (2R,3S)
 L-(+)        D-(−)        inactive
  • (2R,3R) and (2S,3S) are enantiomers (optically active, equal and opposite rotation).
  • The meso form has a plane of symmetry (internal mirror) between C2 and C3, so it is optically inactive by internal compensation. It is a diastereomer of both active forms.
  • An equimolar mixture of the (+) and (−) forms is the racemic (±) acid.

Is HOOC–CHBr–CHBr–COOH optically active?

Assumption (figure not shown): the two Br atoms are on the same side of the Fischer projection, with H and Br on each central carbon. The compound is 2,3-dibromosuccinic acid, which has two identical chiral carbons.

   COOH
H—C—Br
H—C—Br
   COOH

With both Br atoms on the same side, the molecule has an internal plane of symmetry, so it is the meso form and is optically inactive (internal compensation), like meso-tartaric acid. If the two Br atoms were on opposite sides, the molecule would be chiral and optically active (d or l).

✓ Answer checked by Claude Opus 5.5 (high effort)

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