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JEE Main levelOne or more correct optionsDifficulty 2 of 3st026

Which of the following allenes (cumulenes with two $C = C$ bonds) are chiral?

AButa-1,2-diene, $CH X_{2} = C = CH - CH X_{3}$
B2,4-Dimethylpenta-2,3-diene, $(CH X_{3}) X_{2} C = C = C (CH X_{3}) X_{2}$
CPenta-2,3-diene, $CH X_{3} - CH = C = CH - CH X_{3}$
D1,3-Dichloroallene, $ClCH = C = CHCl$

Answer: C, D

An allene is axially chiral only if each terminal carbon carries two different groups. In penta-2,3-diene each end has

H

and

CH X_{3}

, and in 1,3-dichloroallene each end has

H

and

Cl

: both chiral. Buta-1,2-diene has a

CH X_{2}

end (two identical

H

) and the tetramethyl compound has identical groups at both ends, so both are achiral. Correct: A and C.

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