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

Step 1: Understanding the Concept:
Lactic acid has the structure \( CH_3-CH(OH)-COOH \). We need to find how many optical isomers this molecule can form.

Step 2: Key Formula or Approach:
Optical isomers come from chiral centres, carbon atoms bonded to four different groups. The number of possible optical isomers follows \( 2^n \), where \(n\) is the number of chiral centres in the molecule.

Step 3: Detailed Explanation:
Look at the alpha carbon of lactic acid. It is bonded to a hydroxyl group \( (-OH) \), a carboxyl group \( (-COOH) \), a methyl group \( (-CH_3) \), and a hydrogen atom \( (-H) \). All four groups on this carbon are different, so this carbon is a chiral centre.
No other carbon in the molecule carries four different substituents, so lactic acid has exactly one chiral centre, \( n = 1 \).
Applying the formula:
\[ \text{Number of optical isomers} = 2^n = 2^1 = 2 \]
These two isomers are non-superimposable mirror images of each other, known as D(+) lactic acid and L(-) lactic acid.

Step 4: Final Answer:
\[ \boxed{2} \]