11.6 g of the N-methyl-N-(11-hydroxyundecyl)amine obtained according to Section 1, 8 g of potassium carbonate powder, 7.18 g of 4-fluorobenzaldehyde and 2 drops of the conventional and known phase transfer catalyst Aliquat® 336 (fatty quaternary ammonium chloride) in 50 ml of dimethylpropyleneurea were stirred for 72 hours at 95° C. Aliquat 336 is a commercial product and constitutes methyl-trioctylammonium chloride. It is a mixture of C8 - and C10 -chains, C8 being predominant. It is used as a phase transfer catalyst and liquid ion exchanger. The reaction mixture was then diluted with 700 ml of water and extracted with three times 50 ml of a solvent mixture of ether and ethyl acetate in a volume ratio of 1:1. The combined extracts were washed with three times 50 ml of water and then dried with sodium sulfate and, after the sodium sulfate had been separated off, were evaporated down. The resulting residue was freed from unconverted benzaldehyde by drying at 60° C. under reduced pressure from an oil pump, this procedure being monitored by thin layer chromatography. The dry product was purified by boiling with active carbon in acetone 12.45 g of 4-[N-methyl-N-(11-hydroxyundecyl)]-amino]-benzaldehyde were obtained, whose IR spectrum (KBr pellet) had bands at 3,400, 2,900, 2,850, 1,660, 1,580, 1,520, 1,460, 1,440, 1,380, 1,310, 1,240, 1,160, 1,050 and 820 cm-1.
