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Which one of the following has the smallest heat of hydrogenation per mole?

$\begin{array}{1 1}(a)\;\text{1-butane}&(b)\;\text{Trans-2-butane}\\(c)\;\text{Cis-2-butane}&(d)\;\text{1,3-butadiene}\end{array}$

1 Answer

We know that higher stability,lower is the heat of hydrogenation.Butene-2 is more stable than butene-1 because the double bond present in butene-2 is in the centre of the molecule$(CH_3CH=CHCH_3)$.Moreover trans-2 butene is more stable than cis-2-butene and thus its (i.e) trans-2-butene,heat of hydrogenation per mole will be less.
Values of heats of hydrogenation are listed here:
Trans-2-butene is more stable than cis-2-butene because in trans-2-butene the bulky groups are far apart whereas in cis-2-butene the bulky groups are crowded together.Due to this cis-isomer has more vander walls strain than trans-isomer and thus cis-isomer is less stable.
Hence (b) is the correct answer.
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