Chloromethyl methyl ether (CMME) is a compound with formula CH3OCH2Cl. It is a chloroalkyl ether. It is used as an alkylating agent and industrial solvent to manufacture dodecylbenzyl chloride, water repellents, ion-exchange resins, polymers, and as a chloromethylation reagent. In organic synthesis, it is used for introducing the methoxymethyl (MOM) protecting group, and is thus often called MOM-Cl or MOM chloride. A convenient method to prepare MOM chloride in situ is by using dimethoxymethane and an acyl chloride in the presence of catalytic Lewis acid. A very similar method, using a high-boiling acyl chloride, can be used to prepare pure material. This method yields > 93% pure material with dimethoxymethane as the only contaminant. In contrast, the venerable procedure from formaldehyde, methanol and hydrogen chloride yields material contaminated with a significant amount of the dangerous bis-chloromethyl ether and requires fractional distillation. CMME is a known human carcinogen. Chronic exposure can increase the incidence of respiratory cancers, including small cell carcinoma. It is one of 13 chemicals regulated by the Occupational Safety and Health Administration despite not having an established permissible exposure limit.