How many stereoisomers does 5-chlorohexa-2,3-diene, CH3-CH=C=CH-CH(Cl)-CH3, have? Count enantiomers separately.

JEE Advanced levelSingle correct optionDifficulty 3 of 3st050

How many stereoisomers does 5-chlorohexa-2,3-diene, $CH X_{3} - CH = C = CH - CH (Cl) - CH X_{3}$ , have? Count enantiomers separately.

Two stereogenic units are present. The allene axis has

H

CH X_{3}

on C2 and

H

CH (Cl) CH X_{3}

on C4, so it is an axis of chirality (2 forms). C5 bears

H

Cl

CH X_{3}

and the allenyl group, so it is a stereocentre (2 forms). No symmetry relates them, so

2 \times 2 = 4

stereoisomers (two enantiomeric pairs). An answer of

8

would wrongly treat a

C = C

as an extra geometrical unit.