Why do tertiary halides prefer SN1?

Tertiary halides form a stable tertiary carbocation (stabilised by the electron-donating alkyl groups), so the SN1 pathway is favoured. They are also too crowded for the back-side attack that SN2 needs.

What is the stereochemical outcome of SN2?

SN2 gives inversion of configuration (Walden inversion) because the nucleophile attacks from the side opposite the leaving group, flipping the molecule like an umbrella in the wind.

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Difference Between SN1 Reaction and SN2 Reaction

Nucleophilic substitution in haloalkanes follows two mechanisms - SN1 and SN2 - that differ in steps, rate and stereochemistry.

In short

SN1 is a two-step reaction that goes through a carbocation intermediate, is favoured by tertiary substrates and gives racemisation; SN2 is a single-step reaction with a transition state, is favoured by primary substrates and gives inversion of configuration.

SN1 Reaction vs SN2 Reaction - Comparison Table

Basis	SN1 Reaction	SN2 Reaction
Steps / molecularity	Two steps (unimolecular)	One step (bimolecular)
Rate depends on	Substrate only	Substrate and nucleophile
Intermediate	Carbocation	Transition state (no intermediate)
Favoured substrate	Tertiary (3°)	Primary (1°)
Stereochemistry	Racemisation	Inversion (Walden inversion)
Solvent	Polar protic	Polar aprotic

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Frequently Asked Questions - SN1 Reaction vs SN2 Reaction

Why do tertiary halides prefer SN1?: Tertiary halides form a stable tertiary carbocation (stabilised by the electron-donating alkyl groups), so the SN1 pathway is favoured. They are also too crowded for the back-side attack that SN2 needs.
What is the stereochemical outcome of SN2?: SN2 gives inversion of configuration (Walden inversion) because the nucleophile attacks from the side opposite the leaving group, flipping the molecule like an umbrella in the wind.

Difference Between SN1 Reaction and SN2 Reaction

SN1 Reaction vs SN2 Reaction - Comparison Table

Frequently Asked Questions - SN1 Reaction vs SN2 Reaction

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