Process for selective preparation of hydroxybenzaldehydes

A process for preparing hydroxybenzaldehydes of the general formula (1) ##STR5## in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are identical or different and are hydrogen, fluorine, chlorine or bromine atoms, also alkyl(C.sub.1 -C.sub.6) groups which may be substituted by fluorine atoms, alkoxy(C.sub.1 -C.sub.4) groups, unsubstituted or substituted aryl(C.sub.6 -C.sub.6) groups, which may be substituted at the aromatic nucleus by fluorine, chlorine or bromine atoms or by alkyl(C.sub.1 -C.sub.4) or alkoxy(C.sub.1 -C.sub.4) groups, also unsubstituted or substituted aryl(C.sub.6 -C.sub.14)-alkyl(C.sub.1 -C.sub.6) groups, which may be substituted at the aromatic nucleus by fluorine, chlorine or bromine atoms or by alkyl(C.sub.1 -C.sub.4) or alkoxy(C.sub.1 -C.sub.4) groups, also a cyclopentyl, cyclopentadienyl, cyclohexyl, cyclohexenyl or cyclohexadienyl group which may be substituted by alkyl(C.sub.1 -C.sub.4) groups, also the phenoxy group which may be substituted by fluorine, chlorine or bromine atoms or by alkyl(C.sub.1 -C.sub.4) or alkoxy(C.sub.1 -C.sub.4) groups, and where the substituents R.sup.1, R.sup.2, R.sup.3 and R.sup.4 may with the hydroxybenzene ring carbon atoms on which they are located form one or two saturated or unsaturated isocyclic or heterocyclic rings which may be substituted by alkyl(C.sub.1 -C.sub.4) groups, and in which the hydroxyl and aldehyde groups are in the ortho or para position with respect to one another, which comprises admixing 1 mol of a phenol of the general formula (2) ##STR6## in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 have the meanings above, if desired in an organic solvent which is inert towards the reactants, in a pressure vessel with from about 5 to about 100 mol of hydrogen fluoride and from about 0.5+x to about 1.5+x mol of boron trifluoride, where x is the number of oxygen atoms contained in the starting compound of the stated formula (2), setting the temperature of this mixture to from about -10.degree. to about 100.degree. C. and subsequently passing carbon monoxide into this initially charged mixture until a pressure of from about 10 to about 150 bar is reached and allowing the mixture to react at the desired pressure reached.