SN1 reaction
SN1 is substitution, nucleophilic, unimolecular. The leaving group first departs on its own, forming a carbocation. The nucleophile then attacks. Rate = k[substrate] only. Tertiary substrates react fastest, in polar solvents, and the product is mostly racemic.
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Analogy
A two-act play: first the actor leaves the stage (slow), then someone new walks on (fast).
Example
Step 1 (slow): loses Br⁻, forms (CH₃)₃C⁺Step 2 (fast): water attacks cationRate depends only on [(CH₃)₃CBr]Product: (CH₃)₃C–OH
In practice
Tertiary alcohols turn into tertiary alkyl chlorides in concentrated HCl through an SN1 path.
Practice
Flashcards
- What is the rate law for SN1?Rate = k[substrate]. The nucleophile does not appear.
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 SN1 give a racemic mixture?
The carbocation intermediate is flat. The nucleophile can attack from either face with equal chance, giving both mirror-image products in about equal amounts. In practice, a little more inversion product forms, because the leaving group briefly shields one face, so the mixture is not perfectly 50:50.
How do I choose between SN1 and SN2?
Look at the substrate first. Methyl and primary favour SN2, and tertiary favours SN1. Then look at the nucleophile and solvent: strong nucleophiles with polar aprotic solvents favour SN2, and weak nucleophiles in polar protic solvents favour SN1. Secondary halides can go either way.