Question

What is the order of the ease of replacement of different types of H atoms by halogen atoms in alkanes?

Answer 1

1. The order of ease of replacement of hydrogen atoms by halogen atoms in alkanes is:
   Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl,
2. Halogenation of alkanes (e.g., chlorination or bromination) takes place via a free radical mechanism.
3. During the reaction, a carbon-hydrogen bond breaks, and a carbon free radical is formed.
4. The stability of the free radical determines how easily a hydrogen atom is replaced.
5. Tertiary free radicals are the most stable because the carbon is bonded to three other carbon atoms and stabilized by hyperconjugation and inductive effects.
6. Secondary free radicals are less stable than tertiary, but more stable than primary.
7. Primary free radicals are less stable, and methyl radicals (from methane) are the least stable.

Hence, tertiary hydrogens are replaced most easily, while methyl hydrogens are replaced with great difficulty.