JEE Advanced 2021 · Paper 1 · Chemistry Q2

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ChemistrySingle correct+3 / -1Question 21 of 57

Among the following, the conformation that corresponds to the most stable conformation of meso-butane-2,3-diol is

Newman projection (A), shown

Newman projection of butane-2,3-diol. Front carbon: OH up, Me lower left, H lower right. Back carbon: H upper left, Me upper right, OH down. This drawing is the meso isomer in the conformation with OH/OH and Me/Me both anti.

Newman projection (B), shown

Newman projection of butane-2,3-diol. Front carbon: OH up, H lower left, Me lower right. Back carbon: H upper left, OH upper right, Me down. This drawing is the meso isomer with OH/OH gauche and Me/Me gauche (H anti to H).

Newman projection (C), shown

Newman projection of butane-2,3-diol. Front carbon: OH up, H lower left, Me lower right. Back carbon: Me upper left, OH upper right, H down. This drawing is the (2R,3R) isomer, not the meso compound.

Newman projection (D), shown

Newman projection of butane-2,3-diol. Front carbon: OH up, Me lower left, H lower right. Back carbon: H upper left, OH upper right, Me down. This drawing is the (2S,3S) isomer, not the meso compound.

Answer and worked solution

Official answer: B

Which drawings are meso? A meso conformation has a centre of inversion or a mirror plane.

(A) shows the meso compound with OH/OH and Me/Me both anti (a centre of inversion).
Rotating the back carbon of (A) by 120° gives (B), which is the same meso compound.
(C) and (D) are the chiral (2R,3R) and (2S,3S) isomers, so they are not meso-butane-2,3-diol at all.

Most stable. In (B) the two OH groups are gauche, so they can form an intramolecular hydrogen bond, while the two H atoms are anti. This outweighs the gauche Me/OH interactions. The all-anti form (A) cannot form the hydrogen bond.

(B)

Topics:Conformations Optical isomerism and chirality Hydrogen bonding

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