Process for preparing compounds of the formula (Ia) in which R 1a to R 1e and R 2a to R 2d are each independently selected from the group consisting of hydrogen, halogen, (C 1 -C 6)-alkyl, (C 1 -C 6)-haloalkyl, (C 1 -C 6)-alkoxy, (C 3 -C 7)-cycloalkyl, (C 1 -C 6)-alkylthio, (C 3 -C 7)-cycloalkylthio, S q -(C 1 -C 6)-alkyl with q = 0, 1 or 2, (C 1 -C 6)-alkylcarbonyl, -CO-aryl, cyano and nitro or in which two adjacent R 1a to R 1e radicals in each case form a -O-CH 2 CH 2 - radical, and R 3a is selected from the group consisting of hydrogen and the following radicals: (C 1 -C 6)-alkyl, (C 1 -C 6)-haloalkyl, (C 3 -C 7)-cycloalkyl, (C 1 -C 6)-alkoxy, (C 1 -C 6)-alkylthio, (C 3 -C 7)-cycloalkylthio, - p-heterocyclyl, where these are each unsubstituted or substituted by one or more substituents selected from the group consisting of halogen, (C 1 -C 6)-alkyl, (C 1 -C 6)-haloalkyl, (C 1 -C 6)-alkoxy, cyano and nitro, and R 3b is selected from the group consisting of hydrogen and the following radicals: (C 1 -C 6)-alkyl, (C 2 -C 6)-alkenyl, (C 2 -C 6)-alkynyl, (C 1 -C 6)-alkoxy, (C 2 -C 6)-alkenyloxy, - p-heterocyclyl, where these are each unsubstituted or substituted by one or more substituents selected from the group consisting of halogen, (C 1 -C 4)-alkoxy and (C 1 -C 4)-alkylthio, or R 3a and R 3b together with the connecting nitrogen atom form a 3- to 8-membered, saturated or unsaturated ring, by reacting a compound of the formula with an amine of the formula R 3a R 3b NH in which the R 3a and R 3b radicals are each as defined above, in aqueous solution, wherein the pH is regulated by - initially charging a portion of the amine of the formula R 3a R 3b NH in water at the start of the reaction, and - adding the remaining amount of the amine of the formula R 3a R 3b NH to the reaction mixture over the course of the reaction in one or more steps. R 1a to R 1e and R 2a to R 2d are each independently selected from the group consisting of hydrogen, halogen, (C 1 -C 6)-alkyl, (C 1 -C 6)-haloalkyl, (C 1 -C 6)-alkoxy, (C 3 -C 7)-cycloalkyl, (C 1 -C 6)-alkylthio, (C 3 -C 7)-cycloalkylthio, S q -(C 1 -C 6)-alkyl with q = 0, 1 or 2, (C 1 -C 6)-alkylcarbonyl, -CO-aryl, cyano and nitro or in which two adjacent R 1a to R 1e radicals in each case form a -O-CH 2 CH 2 - radical, and R 3a is selected from the group consisting of hydrogen and the following radicals: (C 1 -C 6)-alkyl, (C 1 -C 6)-haloalkyl, (C 3 -C 7)-cycloalkyl, (C 1 -C 6)-alkoxy, (C 1 -C 6)-alkylthio, (C 3 -C 7)-cycloalkylthio, - p-heterocyclyl, where these are each unsubstituted or substituted by one or more substituents selected from the group consisting of halogen, (C 1 -C 6)-alkyl, (C 1 -C 6)-haloalkyl, (C 1 -C 6)-alkoxy, cyano and nitro, and R 3b is selected from the group consisting of hydrogen and the following radicals: (C 1 -C 6)-alkyl, (C 2 -C 6)-alkenyl, (C 2 -C 6)-alkynyl, (C 1 -C 6)-alkoxy, (C 2 -C 6)-alkenyloxy, - p-heterocyclyl, where these are each unsubstituted or substituted by one or more substituents selected from the group consisting of halogen, (C 1 -C 4)-alkoxy and (C 1 -C 4)-alkylthio, or R 3a and R 3b together with the connecting nitrogen atom form a 3- to 8-membered, saturated or unsaturated ring, - initially charging a portion of the amine of the formula R 3a R 3b NH in water at the start of the reaction, and - adding the remaining amount of the amine of the formula R 3a R 3b NH to the reaction mixture over the course of the reaction in one or more steps.