Nucleophilic substitution: SN2 and SN1
A nucleophile replaces the halide ion. SN2 is one step with backside attack (CH₃X fastest); SN1 goes through a carbocation (3° fastest).
Common nucleophiles and products (R-X + Nu)
| Reagent | Nucleophile | Product | Class of product |
|---|---|---|---|
| NaOH or KOH (aq) | HO⁻ | R-OH | Alcohol |
| H₂O | H₂O | R-OH | Alcohol |
| NaOR' | R'O⁻ | R-O-R' | Ether |
| NaI | I⁻ | R-I | Alkyl iodide |
| NH₃ | NH₃ | R-NH₂ | Primary amine |
| R'NH₂ | R'NH₂ | R-NH-R' | Secondary amine |
| KCN | C≡N⁻ (attacks through C) | R-CN | Nitrile (alkyl cyanide) |
| AgCN | Ag-CN (only the N end is free, so it attacks through N) | R-NC | Isonitrile (isocyanide) |
| KNO₂ | O=N-O⁻ (attacks through O) | R-O-N=O | Alkyl nitrite |
| AgNO₂ | Ag-O-N=O (attacks through N) | R-NO₂ | Nitroalkane |
| R'COOAg | R'COO⁻ | R'COOR | Ester |
| LiAlH₄ | H⁻ | R-H | Hydrocarbon |
| R'⁻M⁺ | R'⁻ | R-R' | Alkane |
Ambident nucleophiles
Some nucleophiles can attack through two different atoms; they are called ambident nucleophiles. Cyanide can use C or N. KCN is mainly ionic, so free CN⁻ attacks through carbon (a C-C bond is more stable than C-N) and gives an alkyl cyanide, R-CN. AgCN is mainly covalent, so only the nitrogen is free to donate its electron pair, giving an isocyanide, R-NC. Nitrite behaves the same way: KNO₂ gives an alkyl nitrite, R-O-N=O, while AgNO₂ gives a nitroalkane, R-NO₂.
SN2 mechanism
SN2 means substitution, nucleophilic, bimolecular: two species take part in the only step. Example: CH₃Cl with OH⁻ (proposed by Hughes and Ingold, 1937).
- 1OH⁻ approaches the carbon from the side opposite the chlorine (backside attack).
- 2Transition state: HO···CH₃···Cl. The C-O bond forms while the C-Cl bond breaks, all in one step. The three H atoms lie flat in one plane.A transition state is the unstable arrangement at the top of the energy hill; it cannot be isolated
- 3★CH₃Cl + OH⁻ → CH₃OH + Cl⁻Cl⁻ leaves. The three H atoms flip over like an umbrella blown inside out: inversion of configuration
- 4Rate = k[CH₃Cl][OH⁻], second order. Bulky groups near the carbon block the attack (steric hindrance), so CH₃X > 1° > 2° > 3°.
★ key step
SN1 mechanism
SN1 means substitution, nucleophilic, unimolecular: only the alkyl halide takes part in the slow step. It is favoured in **polar protic solvents** (water, alcohol, ethanoic acid), which stabilise the ions. Example: (CH₃)₃CBr with OH⁻.
- 1★(CH₃)₃C-Br → (CH₃)₃C⁺ + Br⁻Slow, rate-determining step: the C-Br bond breaks to give a carbocation
- 2(CH₃)₃C⁺ + OH⁻ → (CH₃)₃C-OHFast step: the nucleophile joins the carbocation
- 3Rate = k[(CH₃)₃CBr], first order. Carbocation stability is 3° > 2° > 1° > CH₃⁺ (alkyl groups push electrons towards the positive carbon), so 3° halides react fastest.
- 4Allylic and benzylic halides also react fast by SN1, because their carbocations are stabilised by resonance.
★ key step
SN2 vs SN1
SN2 is decided by crowding at the carbon; SN1 is decided by how stable the carbocation is.
Nucleophilic substitution is how chemists attach -OH, -CN or -NH₂ groups to a carbon chain. Many medicines and dyes made by India's chemical industry begin with a step like this.
3 marks: “Write the SN2 mechanism for the reaction of chloromethane with hydroxide ion. Why is it called SN2?” OH⁻ attacks from the back; transition state HO···CH₃···Cl; Cl⁻ leaves and the configuration inverts. Rate = k[CH₃Cl][OH⁻]: two species in the rate-determining step, so bimolecular (SN2).
2 marks: “Bromoethane gives propanenitrile with KCN but ethyl isocyanide with AgCN. Explain.” Cyanide is ambident. KCN is ionic; free CN⁻ attacks through C (C-C bond more stable), giving CH₃CH₂CN. AgCN is covalent; only N's lone pair is free, so CH₃CH₂NC forms.
2 marks: “Arrange in increasing order of SN1 reactivity, with a reason: CH₃CH₂Br, (CH₃)₃CBr, (CH₃)₂CHBr.” CH₃CH₂Br < (CH₃)₂CHBr < (CH₃)₃CBr, because carbocation stability is 1° < 2° < 3°.
SN2: two in the slow step, one step overall, back door, flip. SN1: one in the slow step, two steps overall, carbocation, mix.