b) in a second step, bringing the product resulting from the first step into contact with a monochlorinated organic compound of general formula R6 R7 R8 CCl or of the general formula R9 R10 R11 CCl, in which R6 and R7 are identical or different alkyl radicals comprising from 1 to 6 carbon atoms, R8 is a hydrogen atom or an alkyl radical comprising from 1 to 6 carbon atoms which is identical to or