Haloalkanes and Haloarenes (Class 12): SN1 versus SN2 Explained Simply

The comparison you must be able to write from memory

Why the order of reactivity reverses

SN1 runs through a carbocation, and carbocation stability rises 3° > 2° > 1°, so tertiary halides react fastest by SN1.

SN2 needs the nucleophile to attack the carbon from the side opposite the leaving group. Bulky groups block that approach, so the order reverses: primary halides react fastest by SN2. The same structural feature that stabilises the intermediate in one mechanism obstructs the transition state in the other.

That back-side attack is also why SN2 gives inversion of configuration — the Walden inversion, often compared to an umbrella turning inside out in the wind.

Why haloarenes are so unreactive

• The C—Cl bond has partial double-bond character from resonance with the ring, which shortens and strengthens it.
• The carbon is sp² hybridised, so it is more electronegative and holds the shared pair more tightly than the sp³ carbon of a haloalkane.
• The electron-rich ring repels an approaching nucleophile.
• Forming a phenyl cation is very unfavourable, so the SN1 route is effectively closed too.

FAQs

Which halide is fastest by SN2?

Primary. Less steric crowding lets the nucleophile attack from the back side.

Why does SN1 give a racemic mixture?

The carbocation intermediate is planar, so the nucleophile can attack from either face with roughly equal probability.

Why are haloarenes less reactive than haloalkanes?

Resonance gives the C—X bond partial double-bond character, the sp² carbon binds more tightly, and the electron-rich ring repels nucleophiles.

What is the Walden inversion?

The reversal of configuration at the carbon centre during an SN2 reaction, caused by back-side attack.