Question 1

Why does DMSO favour SN2 reactions with anionic nucleophiles?

Accepted Answer

DMSO is a polar aprotic solvent. It does not strongly hydrogen-bond to an anionic nucleophile, so the nucleophile remains less strongly solvated than in water or alcohol. This usually makes backside attack easier and increases the SN2 rate.

Question 2

What is the rate law for an SN2 reaction?

Accepted Answer

The rate law is Rate = k[R-X][Nu]. The reaction is bimolecular, so its rate depends on the concentrations of both the alkyl halide and nucleophile when the substrate, temperature and other conditions are held comparable.

Question 3

Why is a methyl substrate not suitable for an SN1 demonstration?

Accepted Answer

An SN1 reaction begins with ionization of the alkyl halide to form a carbocation. A methyl carbocation is not formed under ordinary substitution conditions because it is too unstable, so SN1 demonstrations use suitable secondary or tertiary alkyl halides instead.

Question 4

How do polar protic solvents favour SN1 reactions?

Accepted Answer

Polar protic solvents stabilise the carbocation and the leaving-group anion formed during the slow ionization step. A suitable secondary or tertiary substrate can therefore undergo SN1 more readily, while the rate remains Rate = k[R-X] under otherwise identical conditions.

Question 5

Why is tert-butoxide a stronger E2 promoter than ethoxide on 2-chloropropane?

Accepted Answer

Tert-butoxide is a strong, bulky base. 2-chloropropane has beta-hydrogens, so E2 elimination is possible, and its bulk makes it a more effective E2 promoter than ethoxide. The same bulk obstructs the close backside approach required for SN2.

Polar Protic vs Polar Aprotic: The Solvent Shell That Speeds Up SN2

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