Explain the SN1 and SN2 mechanisms with one example each. Which alkyl halides prefer each mechanism, and why?

1. 1 · Define in one line. In nucleophilic substitution, a nucleophile replaces the halogen of an alkyl halide, and X leaves as X⁻.
2. 2 · SN2 mechanism.
   • CH₃Cl + OH⁻: backside attack, one step
   • Transition state HO···CH₃···Cl
   • CH₃Cl + OH⁻ → CH₃OH + Cl⁻; rate = k[CH₃Cl][OH⁻]; inversion
3. 3 · Draw the SN2 picture. Draw OH⁻ on the left, the carbon with three H atoms in the centre and Cl on the right; then the transition state in brackets with dotted bonds; then HO-CH₃ with the H atoms flipped. Label: backside attack, transition state, inversion.
4. 4 · SN1 mechanism.
   • (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻ (slow)
   • (CH₃)₃C⁺ + OH⁻ → (CH₃)₃COH (fast)
   • Rate = k[(CH₃)₃CBr]
5. 5 · Which halides prefer which. SN2: CH₃X > 1° > 2° > 3°, because of steric hindrance. SN1: 3° > 2° > 1° > CH₃X, because of carbocation stability; allylic and benzylic halides are also fast.
6. 6 · Close in one line. So crowding decides SN2, and carbocation stability decides SN1.

Scoring tip: Write the rate law for each mechanism; it shows why one is numbered 1 and the other 2.