How many stereoisomers does hepta-2,5-dien-4-ol, $CH X_{3} - CH = CH - CH (OH) - CH = CH - CH X_{3}$ , have? Count enantiomers, diastereomers and any geometrical isomers about C=C; a meso form counts as one stereoisomer. Remember that C4 is a stereocentre only when its two alkenyl groups are different.

Answer: D

The molecule is symmetric about C4. For (E,E) and (Z,Z) the two alkenyl groups at C4 are identical, so C4 is not a stereocentre, giving one isomer each. For (E,Z) the two groups are different, so C4 is a stereocentre and gives R and S, which are enantiomers. The total is

1 + 1 + 2 = 4

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How many stereoisomers does hepta-2,5-dien-4-ol, CHX3​−CH=CH−CH(OH)−CH=CH−CHX3​, have? Count enantiomers, diastereomers and any geometrical isomers about C=C; a meso form counts as one stereoisomer. Remember that C4 is a stereocentre only when its two alkenyl groups are different.

Answer: D

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