NEET 2026 Chemistry Question 76 Solution

Addition of Br₂ across an alkene in non-polar CCl₄ proceeds through a cyclic bromonium ion intermediate, resulting in stereospecific ANTI addition. According to the stereochemical addition rule (TAM: Trans + Anti = Meso), bromination of trans-but-2-ene MUST produce MESO-2,3-dibromobutane.

• • In meso-2,3-dibromobutane in its anti conformation, there is an inversion center: if front carbon has Br UP and Me DOWN-RIGHT, the back carbon must have Br DOWN and Me UP-LEFT.
• • In the Newman projection given in Statement I, both Me groups are on the right and both H are on the left. Inverting through the center fails. This depicts the (2R, 3R) or (2S, 3S) chiral enantiomer (threo-pair), NOT the meso isomer!
• • Statement I is incorrect.

Reaction of an alkene with cold, alkaline KMnO₄ (Baeyer's reagent) proceeds via a cyclic permanganate ester intermediate, delivering both hydroxyl groups to the SAME face of the double bond (SYN addition). According to the stereochemical addition rule (CSM: Cis + Syn = Meso), syn-dihydroxylation of cis-but-2-ene produces MESO-butane-2,3-diol.

• • In the Newman projection in Statement II, the groups are in a near-eclipsed conformation where front OH aligns with back OH, front H aligns with back H, and front Me aligns with back Me.
• • This eclipsed conformation contains an internal plane of symmetry (σ), which is the definitive visual hallmark of the meso isomer.
• • Statement II is correct.

Statement I is incorrect, but Statement II is correct.

• • Matches Option (4).