Question

Reactivity of hydrogen atoms attached to different carbon atoms in alkane has the order:

• A. tertiary > primary > secondary
• B. tertiary > secondary > primary
• C. secondary > tertiary > primary
• D. secondary = tertiary = primary

Answer 1

Answer: (B)

tertiary > secondary > primary

Differences in reactivity can only be attributed to differences in $C-H$ bond dissociation energies. The weaker bonds are broken more easily than the strong bonds. The difference in $C-H$ bond dissociation energy reported for $\text{primary}\left(1º\right)>\text{secondary}\left(2º\right)>\text{tertiary}\left(3º\right)$ sites agrees with the halogenation observations. The reactivity order of hydrogen attached to central $C$ for substitution reaction is $M{e}_{3}CH>M{e}_{2}C{H}_2>MeC{H}_3$. It is due to radical formed by the reactive hydrogen elimination, which is stabilized by hyperconjugation with adjacent hydrogen atoms.