Question

The correct statement for the given reaction is

• A. Y is optically inactive due to external compensation
• B. Y is optically inactive due to internal compensation
• C. X is predominantly cis-alkene
• D. Y does not have chiral centres

Answer 1

Answer: (B)

Y is optically inactive due to internal compensation

Answer 2

The starting material is (2S,3R)-2,3-dibromopentane. The reaction with NaI in acetone is a dehalogenation reaction of vicinal dibromides to form an alkene. This reaction proceeds via an E2 mechanism involving anti-periplanar elimination of the two bromine atoms.

The product X is pent-2-ene, CH$_3$-CH=CH-CH$_3$. The substituents on the double bond are H and CH$_3$ on each carbon. E-pent-2-ene has the two CH$_3$ groups on opposite sides of the double bond.

CF$_3$CO$_3$H is trifluoroperoxyacetic acid, which is a peroxy acid. Peroxy acids react with alkenes to form epoxides (oxiranes). This reaction is a syn addition of oxygen to the double bond.

Since X is E-pent-2-ene, the reaction with a peroxy acid will form an epoxide. The epoxide formed from E-alkene by syn addition is a meso compound if the alkene is symmetrical and has identical groups on both sides of the double bond.

The epoxide is 2,3-epoxypentane. O /
CH$_3$-CH-CH-CH$_3$

This molecule has a plane of symmetry passing through the oxygen and the midpoint of the C-C bond.

A meso compound is optically inactive due to internal compensation. It has chiral centers but is achiral due to the presence of a plane or center of symmetry. So, Y is optically inactive due to internal compensation.