2-(2-Acetylamino-5-chlorophenyl)-2,2-difluoroethanoic acid and 2-(2-acetylamino-5-methylphenyl)-2,2-difluoroethanoic acid, and 2-(2-acetylaminophenyl)-2,2-difluoro-N-phenylacetamide and 2-(2-acetylaminophenyl)-N-(4-chlorophenyl)-2,2-difluoroacetamide: examples of variation in molecular packing and hydrogen-bonding motif induced by substituent change