Question

How many optical isomers are possible for lactic acid:

• A. 4
• B. 0
• C. 6
• D. 2

Answer 1

The Correct Option is D

The correct option is (D): 2.

Answer 2

Lactic acid, \( CH_3-CH(OH)-COOH \), has one carbon (the alpha carbon) attached to four different groups: a hydroxyl, a carboxyl, a methyl, and a hydrogen. That makes it a single chiral, or asymmetric, centre. The number of optical isomers a molecule can form follows the rule \( 2^n \), where \(n\) is the number of chiral centres. Let's check each option against this rule.

1. 4: Four optical isomers would need two chiral centres, since \( 2^2 = 4 \). Lactic acid has only one chiral carbon, so this count does not fit its structure.
2. 0: Zero optical isomers would mean the molecule has no chiral centre and is optically inactive. That is not true here, because the alpha carbon of lactic acid carries four different substituents and is genuinely asymmetric.
3. 6: Six is not a value the \( 2^n \) rule gives for a whole number of chiral centres near lactic acid's structure, since \( 2^2 = 4 \) and \( 2^3 = 8 \), so this count cannot match the actual number of stereocentres present.
4. 2: With exactly one chiral centre, \( n = 1 \), so the number of optical isomers is \( 2^1 = 2 \). These two isomers are the mirror-image enantiomers, D(+) lactic acid and L(-) lactic acid, which matches lactic acid's known stereochemistry.

Counting the chiral centres confirms that lactic acid, with its single asymmetric carbon, can exist as only two optical isomers.

Therefore, the correct answer is 2.