Efficient and environment friendly process for chloromethylation of substituted benzenes

A process for preparing a compound of formula I, comprising contacting a compound of formula II with a chloromethylating agent generated in-situ by reaction of a formaldehyde precursor and hydrogen chloride, in a solvent and in presence of a catalytic amount of a carboxylic acid of formula III,

• wherein R1, R2 and R3 are independent of each other,
• R1 represents H, R or —OR, wherein R is a substituted or unsubstituted C1-C4 alkyl group or substituted or unsubstituted C3-C6cycloalkyl group;
• R2 represents hydroxy group —OH or alkoxy group —OR, wherein R is a substituted or unsubstituted C1-C4 alkyl group or substituted or unsubstituted C3-C6cycloalkyl group;
• or R1 and R2 jointly form an alkylenedioxy group represented by —O—(CH2)n—O— wherein n is 1, 2 or 3;
• R3 is a substituent at any position of the aromatic ring other than position 1, 3 and 4 and represents H, R, —OR, wherein R is a substituted or unsubstituted C1-C4 alkyl group, substituted or unsubstituted C3-C6cycloalkyl group or SH;
• and
• R4 represents an alkyl group containing one to six carbon atoms;
• wherein the catalytic amount of the carboxylic acid of formula III is in a range of from 0.5% to 10% by weight of the compound of formula II.