# DBT JRF 2022 — Question 13

Exam: DBT JRF 2022 · Section: A · Topic: Biochemistry › Stereochemistry & Optical Activity · Difficulty: Medium

## Question

Which one of the following statements is true with respect to optical activity?

## Options

- **A.** A relationship exists between the (R) and (S) configurations of enantiomers and the direction [(+) or (−)] in which they rotate plane-polarized light.
- **B.** Equimolar mixture of D and L isomers of the same compound will be optically active.
- **C.** Meso compounds that are optically inactive may contain stereogenic centers or chiral centers.
- **D.** Optical activity is independent of temperature.

## Answer

Option C

## Explanation

Optical activity and stereochemistry: True statement: 'Meso compounds that are optically inactive may contain stereogenic (chiral) centers.' • A meso compound has two or more chiral centers BUT possesses an internal plane of symmetry (mirror plane within the molecule). • This internal symmetry causes the optical rotations of the two halves to cancel each other out — the compound is achiral overall and is optically INACTIVE. • Example: meso-tartaric acid has two chiral carbons but is optically inactive. Why other options are wrong: • Option 1: (R) and (S) designations do NOT predict the direction of optical rotation (+/−); this must be determined experimentally. • Option 2: A racemic mixture (equimolar D + L) is optically INACTIVE (rotations cancel). • Option 4: Optical activity IS affected by temperature (it changes the specific rotation). Answer: Option 3.

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