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JEE Main levelSingle correct optionDifficulty 2 of 3st017

Which of the following compounds shows cis-trans (geometrical) isomerism?

A $CH X_{3} - CH = C = CH - CH X_{3}$
B $CH X_{3} - CH = C = C = CH - CH X_{3}$
C $(CH X_{3}) X_{2} C = C = CH X_{2}$
D $CH X_{2} = C = C = CH X_{2}$

Answer: B

In a cumulene with an odd number of

C = C

bonds (three in hexa-2,3,4-triene) the substituents at the two ends lie in one plane, so cis and trans forms exist. Penta-2,3-diene has two

C = C

bonds, so the end planes are perpendicular and it is axially chiral instead. The last two have identical groups on at least one terminal carbon.

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