Q1(f). Why DMSO accelerates many SN2 reactions

Answer in brief

DMSO dissolves many ionic reagents and strongly solvates cations while leaving anionic nucleophiles relatively reactive.

Key points

• Its polar S–O group coordinates a counter-cation; it lacks O–H or N–H groups and cannot form hydrogen-bond cages around anions.
• The less solvated anion attacks the electrophilic carbon more readily, lowering the effective barrier relative to a protic medium.
• The rate law is v = k[RX][Nu⁻]; backside attack gives inversion at a stereogenic carbon.
• The effect is most relevant to anionic nucleophiles; substrate crowding and competing elimination still control SN2 feasibility.