Assertion (A): Hexa-2,3,4-triene, $CH X_{3} - CH = C = C = CH - CH X_{3}$ , exists as cis and trans isomers, whereas penta-2,3-diene exists as a pair of enantiomers. Reason (R): The substituent planes at the two ends of a cumulene are coplanar when the number of cumulated $C = C$ bonds is odd and perpendicular when it is even.

ABoth A and R are true and R is the correct explanation of A
BBoth A and R are true but R is not the correct explanation of A
CA is true but R is false
DA is false but R is true

Answer: A

The two terminal

sp X_{2}

carbons are joined by a rod of cumulated bonds. With an odd number of

C = C

bonds (three here) the end planes are coplanar, giving a planar molecule with cis/trans isomerism. With an even number (two in penta-2,3-diene) the end planes are mutually perpendicular, giving an axially chiral, non-planar molecule. Both statements are true and R explains A.

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Answer: A

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