Introduction
This chapter formalises the mechanistic detail behind reactions used loosely in earlier chapters (alkene preparation, functional group interconversion), and is one of the most heavily tested areas of organic mechanism because it requires integrating several factors — substrate, nucleophile/base, solvent, temperature — simultaneously.
1. SN1 and SN2 substitution
SN2 (“substitution, nucleophilic, bimolecular”) is a single-step, concerted mechanism: the nucleophile attacks the carbon from the side opposite the leaving group exactly as the leaving group departs, producing inversion of configuration at that carbon (like an umbrella flipping inside out). Because the nucleophile must approach directly, SN2 is fastest at unhindered (methyl, primary) carbons and is disfavoured by bulky substituents.
SN1 (“substitution, nucleophilic, unimolecular”) proceeds in two steps: the leaving group departs first, forming a carbocation intermediate, which the nucleophile then attacks. Because the carbocation is planar, the nucleophile can attack from either face, giving a racemic (or partially racemised) mixture rather than a single configuration. SN1 is favoured at carbons that form stable carbocations (tertiary, or those stabilised by adjacent resonance) since the rate-determining step depends only on how easily the leaving group departs to form that cation.
2. E1 and E2 elimination
E2 (“elimination, bimolecular”) is a single concerted step in which a base removes a proton from a carbon adjacent to the leaving group at the same time the leaving group departs, forming a double bond directly — it requires a specific anti-periplanar geometry between the departing proton and leaving group.
E1 (“elimination, unimolecular”) shares its first step with SN1 (leaving group departs to form a carbocation), after which a base removes a proton from an adjacent carbon to form the double bond — since it shares the same carbocation intermediate, E1 and SN1 are always in direct competition with each other under the same reaction conditions.
3. Leaving groups, nucleophiles, and bases
A good leaving group is one that is stable once it has departed (typically the conjugate base of a strong acid, such as halide ions or tosylate) — a poor leaving group makes both SN1/E1 (slow ionisation) and SN2/E2 (slow direct displacement) difficult. A strong, small, unhindered nucleophile favours SN2; a species acting primarily as a base (attacking a proton rather than the carbon) favours elimination (E1/E2) over substitution.
4. Substrate, solvent, and temperature effects
Substrate structure is often the dominant factor: methyl/primary substrates favour SN2 (accessible carbon, no stable cation possible); tertiary substrates favour SN1/E1 (stable cation, too hindered for SN2 backside attack); secondary substrates can go either way depending on the other conditions. Polar protic solvents (water, alcohols) stabilise ions through solvation and favour SN1/E1; polar aprotic solvents (acetone, DMSO) do not solvate anions well, leaving nucleophiles more “naked” and reactive, favouring SN2. Higher temperature generally favours elimination over substitution, because elimination has a more favourable (more positive) entropy change, forming two product molecules effectively (alkene plus, separately, the protonated base) from one substrate.
5. Competition between substitution and elimination
In practice, most reaction conditions produce some mixture of substitution and elimination products, and predicting the major product requires weighing all the above factors together: substrate class first (methyl/primary strongly favours SN2; tertiary strongly favours E1/E2 with a strong base, or SN1/E1 with a weak one), then the specific nucleophile/base strength and bulk, then solvent, then temperature — a systematic four-factor check rather than a single memorised rule.
Key Takeaways
- SN2 is concerted with inversion of configuration and favours unhindered substrates; SN1 proceeds via a planar carbocation, giving racemisation, and favours substrates forming stable cations.
- E2 is concerted and requires anti-periplanar geometry; E1 shares its carbocation intermediate with, and directly competes with, SN1.
- Good leaving groups are needed for all four pathways; strong/small/unhindered nucleophiles favour SN2, while base character and bulk favour elimination.
- Substrate class, solvent polarity/protic character, and temperature together determine which of the four pathways dominates, and must be assessed systematically rather than in isolation.