SN2 reaction
SN2 is substitution, nucleophilic, bimolecular. The nucleophile attacks from the back as the leaving group departs, all in one step. Rate = k[substrate][nucleophile]. Stereochemistry is inverted. Methyl and primary carbons react fastest.
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Analogy
An umbrella blowing inside out in the wind: the shape flips as the new group arrives and the old one leaves.
Example
Rate = k[RBr][OH⁻]OH⁻ attacks from the side opposite BrProduct: (S)-butan-2-olInversion of configuration
In practice
Chemists use SN2 to build new bonds, for example making an ether or a nitrile from a primary alkyl halide.
Practice
Flashcards
- What is the stereochemical result of SN2?Inversion of configuration.
Interview questions
- Compare SN1 and SN2 in terms of rate law, substrate, stereochemistry and solvent.
- SN2: second order, one step, inversion, primary substrates
- SN1: first order, two steps, racemisation, tertiary substrates
- SN2 favours strong nucleophiles, polar aprotic
- SN1 favours polar protic solvents
Questions people ask
Why does SN2 work poorly on tertiary halides?
The nucleophile must reach the back of the carbon. In a tertiary halide, three bulky groups block this path, so the reaction is extremely slow. This steric hindrance makes the order of SN2 reactivity methyl > primary > secondary, with tertiary almost unreactive.
What makes a good leaving group?
A good leaving group is stable once it has left with the bonding electrons. Weak bases such as iodide, bromide, chloride and tosylate are good. Strong bases such as hydroxide are poor. This is why alcohols are often converted into tosylates or halides first.