in which case one r has the above defined meaning and the or each further radical r is alkyl the radicals r optionally being different application_number US 65792076 A description in general the process according to the invention is carried out in a temperature range between 40 u00b0 c and 65 u00b0 c preferably in a temperature range between 5 u00b0 and 20 u00b0 c the cyanogen halides which can be used are above all cyanogen chloride and cyanogen bromide which are readily accessible industrially and especially cyanogen chloride examples of solvents which may be mentioned are aliphatic cycloaliphatic araliphatic and aromatic hydrocarbons as well as mixtures thereof for example ligroin cyclohexane benzene toluene and the xylenes aliphatic cycloaliphatic araliphatic and aromatic halogenated hydrocarbons for example methylene chloride ethylene chloride chlorobenzene and dichlorobenzene aliphatic and aromatic nitro hydrocarbons for example nitromethane and nitrobenzene aliphatic and cyclic ethers for example diethyl ether isopropyl ether diisobutyl ether tetrahydrofurane and dioxane lower aliphatic alcohols such as methanol ethanol propanol isopropanol and butanol aliphatic ketones for example acetone diethyl ketone and methyl ethyl ketone esters of aliphatic carboxylic acids with lower aliphatic alcohols for example formic acid esters and acetic acid esters such as methyl acetate and ethyl acetate butyl acetate and amyl acetate carboxylic acid amides for example dimethylformamide and dimethylacetamide and also water and solutions and mixtures of water with other solvents especially those mentioned above as well as mixtures and solutions of the solvents especially the abovementioned solvents with and in one another preferred solvents are water lower aliphatic alcohols with up to 6 carbon atoms aromatic hydrocarbons aliphatic chlorinated hydrocarbons and mixtures thereof especially toluene methylene chloride isopropanol.