Question

Which has least heat of hydrogenation?

• A. cis-2-butene
• B. trans-2-butene
• C. 1,3-dimethyl-2-butene
• D. 2-methyl propene

Answer 1

Answer: (C)

1,3-dimethyl-2-butene

1,3-dimethyl-2-butene or 2-methylpent-2-ene has the least heat of hydration.
The heat of hydration is inversely proportional to the stability of alkene.
The more substituted alkene is more substituted.
1,3-dimethyl-2-butene is more substituted and more stable. Hence, it has the least heat of hydrogenation.
1,3-dimethyl-2-butene is trisubstituted alkene whereas others are disubstituted alkenes.