Question

Why do 3° alkyl halides undergo substitution by S_N1 mechanism, whereas 1° alkyl halides by S_N2 mechanism?

Answer 1

1. In S_N1 mechanism, the first step is the formation of a carbocation intermediate by loss of the leaving group.
2. Tertiary alkyl halides form highly stable carbocations due to the inductive effect and hyperconjugation from three alkyl groups surrounding the positively charged carbon.
3. This carbocation stability lowers the energy barrier for its formation, making S_N1 favorable for 3° alkyl halides.
4. Tertiary alkyl halides have significant steric hindrance that blocks nucleophiles from attacking directly in a backside manner.
5. Because S_N2 requires backside attack, steric hindrance inhibits the S_N2 pathway for tertiary alkyl halides.
6. Therefore, 3° alkyl halides undergo substitution predominantly by S_N1 mechanism where the rate-determining step is carbocation formation.
7. Primary alkyl halides form highly unstable carbocations that are not favored energetically; hence, S_N1 mechanism is unlikely.
8. The electrophilic carbon in 1° alkyl halides is less hindered by steric bulk, allowing nucleophiles to easily access and attack from the backside.
9. S_N2 mechanism is a single-step process where the nucleophile attacks as the leaving group departs simultaneously.
10. Due to low steric hindrance, this backside nucleophilic attack is facilitated for 1° alkyl halides.
11. Consequently, 1° alkyl halides undergo substitution mainly by S_N2 mechanism, which depends on both the nucleophile and the substrate concentration.