Question

Rank the following alkenes in order of decreasing heats of hydrogenation: (largest first)

• A. $2>3>4>1$
• B. $2>4>3>1$
• C. $1>3>4>2$
• D. $1>4>3>2$

Answer 1

The correct option is D $1>4>3>2$

More stable the alkene, less is the heat of hydrogenation. Among the given alkene, 2 is most stable as it is stabalized through hyperconjugation by 8 alpha-hydrogens. In case of 3 and 4, the number of alpha hydrogen 6, whereas for 1 has the least alpha hydrogen equal to 2. Hence heat of hydrogenation will be maximum for 1.
Between 3 and 4, 4 will have higher heat as it has a net dipole moment whereas 3 has zero dipole moment.
Hence, the required order is - $1>4>3>2$