CHO co

Methylation: T> 120°c Scheme 4.5 Nucleophilic substitution on DMC by BAC2 and BAL2 mechanisms.

carbonyl carbon of DMC, giving the transesterification product. Under these conditions, DMC can replace phosgene;

ii. At higher temperatures (usually T > 160°C), DMC acts primarily as a methylating agent: a BAL2 (bimolecular, base-catalyzed, alkyl cleavage, nucleophilic substitution) mechanism predominates where the nucleophile attacks the methyl group of DMC.

Of the two, only the methylation reaction is irreversible, because the CH3OCO2H that is formed decomposes to methanol and CO2.

Since both methylation and methoxycarbonylation generate CH3O", both reactions can be conducted in the presence of catalytic amounts of base. This avoids the formation of unwanted inorganic salts as by-products, and the related disposal problems. In principle, the methanol produced can be recycled for the production of DMC.11 In contrast, methylation with methyl halides or DMS, and carbonyla-tion with phosgene all generate stoichiometric amounts of inorganic salts.

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