Discussion
Among the following, the chiral compound is
![P: 4,4-diisopropylcyclohexanone N-tert-butyl imine; Q: penta-2,3-diene; R: hexa-2,3,4-triene; S: a spiro[3.3]heptane with two acetyl groups on one carbon and H and Me on the far carbon](/uploads/question-images/gate-cy-2024/q15.png)
Answer: B
The chiral compound is Q.
Q is penta-2,3-diene, an allene with two different groups (H and CH3) on each terminal carbon. In an allene the two terminal planes are perpendicular, so the molecule has no mirror plane and is axially chiral.
R has three cumulated double bonds; with an odd number the terminal substituents lie in one plane, giving cis-trans isomers but not chirality. P would be axially chiral only if the two groups on C4 differed, but both are isopropyl. S is a spiro[3.3]heptane, which is axially chiral only when each terminal carbon carries two different groups; here one carbon has two identical acetyl groups. So P, R and S are achiral.
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