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

Question

Professor PC Thomas Classes · Answer

(b) : We know that higher the stability, lower is the heat of hydrogenation. But-2-ene is more stable than but-1-ene because the double bond present in but-2-ene is in the centre of the molecule $\left(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}ight)$. 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 :\ $$\begin{array}{cc}
	ext{CH}_3	ext{CH}_2	ext{CH}{=}	ext{CH}_2; & 	ext{CH}_3	ext{CH}{=}	ext{CHCH}_3; \
30 \ 	ext{kcal/mole} & 	extit{cis} \ 28.6 \ 	ext{kcal/mole} \
& 	extit{trans} \ 27.6 \ 	ext{kcal/mole} \
\
{	ext{CH}_2{=}	ext{CH}{-}	ext{CH}{=}	ext{CH}_2} \
{	ext{butadiene}} \
{957.1 \ 	ext{kcal/mole}}
\end{array}$$
 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 van der Waals strain than trans-isomer and thus cis-isomer is less stable.\