Why is dimethyl sulfate highly toxic as a methylating agent while dimethyl carbonate is much safer?

Why is dimethyl sulfate highly toxic as a methylating agent while dimethyl carbonate is much safer?
The toxicity gap between dimethyl sulfate and dimethyl carbonate comes down to leaving group ability in SN2 methylation reactions. Dimethyl sulfate generates a methyl sulfate anion, the conjugate base of a strong acid, making it an extremely active methylating agent that can readily alkylate biomacromolecules in the body. Dimethyl carbonate, by contrast, forms a methyl carbonate anion from a weak acid, so its methylation requires harsh conditions like high temperature and pressure. This lower reactivity is why DMC is considered a green solvent and a safer substitute for toxic methylating agents in fine chemical synthesis. Still, DMC is not completely harmless; it carries some toxicity and mutagenic potential, so handling precautions remain advisable.