A process for preparing a mixture comprising at least one compound of the formula (Ia) and at least one compound of the formula (Ib) wherein one or more compounds of the formula where in each case: R 2 is (C 1 -C 12)-alkyl, (C 1 -C 12)-haloalkyl, (C 6 -C 10)- aryl, (C 6 -C 10)-haloaryl, (C 7 -C 10)-aralkyl, (C 7 -C 10)- haloaralkyl, (C 4 -C 10)-cycloalkyl or (C 4 -C 10)- halocycloalkyl, R 5 is (C 1 -C 12)-alkyl, (C 1 -C 12)-haloalkyl, (C 6 -C 10)-aryl, (C 6 -C 10)-haloaryl, (C 7 -C 10)-aralkyl, (C 7 -C 10)-haloaralkyl, (C 4 -C 10)-cycloalkyl or (C 4 -C 10)-halocycloalkyl, are converted in a continuously operated tubular reactor having a length of more than 100 times the characteristic length, while mixing with NH 3 in liquid or supercritical form, where the characteristic length refers to the diameter in the case of a round cross-sectional reactor area, the side length of the square in the case of a square cross-sectional reactor area, and the geometric average of length (a) and width (b) of the rectangular cross section in the case of a rectangular cross-sectional reactor area; and where the mixing of the compound(s) of the formula and NH 3 is effected under the following parameters: the mixing time for attainment of a coefficient of variation of 0.10 or less (CoV ≤ 10 -1) is less than 30 seconds. wherein one or more compounds of the formula where in each case: R 2 is (C 1 -C 12)-alkyl, (C 1 -C 12)-haloalkyl, (C 6 -C 10)- aryl, (C 6 -C 10)-haloaryl, (C 7 -C 10)-aralkyl, (C 7 -C 10)- haloaralkyl, (C 4 -C 10)-cycloalkyl or (C 4 -C 10)- halocycloalkyl, R 5 is (C 1 -C 12)-alkyl, (C 1 -C 12)-haloalkyl, (C 6 -C 10)-aryl, (C 6 -C 10)-haloaryl, (C 7 -C 10)-aralkyl, (C 7 -C 10)-haloaralkyl, (C 4 -C 10)-cycloalkyl or (C 4 -C 10)-halocycloalkyl, are converted in a continuously operated tubular reactor having a length of more than 100 times the characteristic length, while mixing with NH 3 in liquid or supercritical form, where the characteristic length refers to the diameter in the case of a round cross-sectional reactor area, the side length of the square in the case of a square cross-sectional reactor area, and the geometric average of length (a) and width (b) of the rectangular cross section in the case of a rectangular cross-sectional reactor area; and where the mixing of the compound(s) of the formula and NH 3 is effected under the following parameters: the mixing time for attainment of a coefficient of variation of 0.10 or less (CoV ≤ 10 -1) is less than 30 seconds. R 2 is (C 1 -C 12)-alkyl, (C 1 -C 12)-haloalkyl, (C 6 -C 10)- aryl, (C 6 -C 10)-haloaryl, (C 7 -C 10)-aralkyl, (C 7 -C 10)- haloaralkyl, (C 4 -C 10)-cycloalkyl or (C 4 -C 10)- halocycloalkyl, R 5 is (C 1 -C 12)-alkyl, (C 1 -C 12)-haloalkyl, (C 6 -C 10)-aryl, (C 6 -C 10)-haloaryl, (C 7 -C 10)-aralkyl, (C 7 -C 10)-haloaralkyl, (C 4 -C 10)-cycloalkyl or (C 4 -C 10)-halocycloalkyl, are converted in a continuously operated tubular reactor having a length of more than 100 times the characteristic length, while mixing with NH 3 in liquid or supercritical form, where the characteristic length refers to the diameter in the case of a round cross-sectional reactor area, the side length of the square in the case of a square cross-sectional reactor area, and the geometric average of length (a) and width (b) of the rectangular cross section in the case of a rectangular cross-sectional reactor area; and where the mixing of the compound(s) of the formula and NH 3 is effected under the following parameters: the mixing time for attainment of a coefficient of variation of 0.10 or less (CoV ≤ 10 -1) is less than 30 seconds.