Question 1

What is the SN1 mechanism?

Accepted Answer

SN1 (Substitution Nucleophilic Unimolecular) is a two-step mechanism. In the slow, rate-determining step the C–X bond breaks by heterolysis to form a carbocation; in the fast second step the nucleophile attacks that carbocation. Because only the substrate participates in the rate-determining step, the rate law is Rate = k[substrate].

Question 2

Why does SN1 give a racemic mixture?

Accepted Answer

The carbocation intermediate is sp² hybridised and trigonal planar, with an empty p orbital perpendicular to the plane. Both faces of this flat carbon are equally accessible to the nucleophile in an achiral environment, so attack occurs with near-equal probability from either side, producing a 50:50 mixture of the R and S enantiomers — a racemic mixture with zero optical activity.

Question 3

Why is a tertiary halide most reactive towards SN1?

Accepted Answer

SN1 reactivity depends entirely on how easily the carbocation forms. Tertiary carbocations are stabilised by hyperconjugation from nine β-C–H bonds and the +I effect of three alkyl groups, so they form fastest. The order is 3° > 2° > 1° > CH₃ — the exact opposite of SN2. Allylic and benzylic halides are also highly reactive because their cations are resonance-stabilised.

Question 4

What is the rate-determining step of SN1?

Accepted Answer

The first step — ionisation of the C–X bond to give the carbocation — is the rate-determining step. It has the higher activation energy of the two transition states, which is why the energy profile shows two humps with the carbocation sitting in the valley between them.

Question 5

How do SN1 and SN2 differ?

Accepted Answer

SN1 is unimolecular (Rate = k[RX]), proceeds through a planar carbocation intermediate with two transition states, gives racemisation, favours 3° substrates and polar protic solvents, and allows rearrangements. SN2 is bimolecular (Rate = k[RX][Nu⁻]), is concerted with one transition state, gives 100% inversion, favours methyl/primary substrates and polar aprotic solvents, and never rearranges.

SN1 Mechanism: The Carbon Goes Flat — Carbocation & Racemisation in 3D

Practice

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