See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: The meso compound of 2,3-butanediol has two stereocenters with opposite configurations (R and S), and an internal plane of symmetry. In a Fischer projection, meso-2,3-butanediol is represented with OH groups on opposite sides (one left, one right) — specifically the (2R,3S) or equivalently (2S,3R) configuration, which are the same compound due to the internal mirror plane. Step 1: Identify the configuration of meso-2,3-butanediol. Meso-2,3-butanediol has the two OH groups on opposite sides in a Fischer projection (one OH on the left at C2 and one OH on the right at C3, or vice versa). This gives one center R and the other S, making it achiral overall. Step 2: Analyze option (a). In (a), the Fischer projection shows: - C2: H on left, OH on right, CH3 on top - C3: H on left, CH3 (actually examining: H left, CH3 right), OH on bottom Wait — reading more carefully: C2 has H-left, OH-right; C3 has H-left, CH3-right, with OH at bottom. The horizontal substituents at C3 are H (left) and CH3 (right), and OH is at the bottom (which in Fischer means it is going downward along the chain — but OH should be a horizontal substituent for a stereocenter in Fischer). Re-examining: the structure in (a) shows C2 with H-left, OH-right and C3 with H-left, CH3-right, meaning both OH groups are on the RIGHT side of the Fischer projection. This represents the (2R,3R) or (2S,3S) enantiomeric pair — i.e., the (+) or (-) 2,3-butanediol, NOT the meso compound. In meso-2,3-butanediol, the OH groups must be on opposite sides in the Fischer projection. Step 3: Analyze options (b), (c), and (d). - Option (b): C2 has OH-up, H-left, CH3-right; C3 has H-left, CH3-right, OH-down. The horizontal groups at C2: H-left, CH3-right with OH at top (chain end = CH3, so OH is a substituent going up meaning it's on one side) — actually in a proper Fischer for 2,3-butanediol, OH groups should be horizontal. In (b), OH at top of C2 means it's the chain continuation? No — OH is on top and CH3 is on the side, suggesting this is drawn with OH on opposite sides relative to C3, making it meso. - Option (c): HO on left at C2, H on right at C2; H on left at C3, CH3 on right at C3 with OH below — the OH groups are on opposite sides, consistent with meso. - Option (d): The wedge-dash structure shows two stereocenters; when converted to Fischer, the OH groups end up on opposite sides, consistent with meso. Step 4: Conclusion. Option (a) has both OH groups on the same side of the Fischer projection (both on the right), which corresponds to the chiral (non-meso) form of 2,3-butanediol. Therefore, option (a) is NOT meso-2,3-butanediol. Therefore, the correct answer is A.