Question

The compound(s) showing optical activity is/are

• A.
• B.
• C.
• D.

Hint:

Using a simple frame or just bolding for the box
Key Points:
Optical activity requires chirality (non-superimposable mirror image).
Presence of a chiral center (carbon with 4 different groups) usually leads to chirality.
Exception: Meso compounds have chiral centers but are achiral due to internal symmetry (e.g., a plane of symmetry).
Glycine (A) is achiral. Lactic acid (B) and Glyceraldehyde (C) are chiral.
Tartaric acid (D) exists as chiral enantiomers and an achiral meso form. The diagram shows the meso form, but the compound name itself includes optically active forms.

Answer 1

The Correct Option is B

Optical activity is exhibited by chiral molecules, which are non-superimposable on their mirror images. The most common cause of chirality in these examples is the presence of a chiral center (a carbon atom bonded to four different groups).

• (A) Glycine: The central carbon atom is bonded to two hydrogen atoms, a carboxyl group (-COOH), and an amino group (-NH₂). Since it has two identical groups (H), it is achiral and optically inactive.
• (B) Lactic Acid: The central carbon atom (C2) is bonded to four different groups: hydrogen (-H), hydroxyl (-OH), methyl (-CH₃), and carboxyl (-COOH). Therefore, C2 is a chiral center, and lactic acid is chiral and optically active.
• (C) Glyceraldehyde: The central carbon atom (C2) is bonded to four different groups: hydrogen (-H), hydroxyl (-OH), aldehyde (-CHO), and hydroxymethyl (-CH₂OH). Therefore, C2 is a chiral center, and glyceraldehyde is chiral and optically active.
• (D) Tartaric Acid: This molecule has two potentially chiral centers (C2 and C3). Each is bonded to H, OH, COOH, and the other C(H)(OH)COOH group.
  • The specific Fischer projection shown has a plane of symmetry between C2 and C3 (the top half is the mirror image of the bottom half). This represents meso-tartaric acid, which is achiral due to internal compensation and thus optically inactive.
  • However, tartaric acid also exists as two chiral enantiomers ((+)-tartaric acid and (-)-tartaric acid) which lack this plane of symmetry and are optically active.
  • Since the question asks which compound(s) show optical activity, and tartaric acid can exist as optically active forms, this option is likely considered correct in the context of the possible isomers, despite the specific meso structure depicted.