2-Hydroxybenzaldehyde [90-02-8] , salicylaldehyde, C7H6O2, Mr 122.12, is a yellowish, oily liquid with an odor resembling that of bitter almonds; it is volatile in steam. It occurs naturally in cassia oil and in the essential oils of several plants of the genus Spirea, mp 1.6 °C, bp 196 °C at 101.3 kPa, bp 92 °C at 2.93 kPa, density r 1.167 g/cm3 at 20 °C, refractive index nD20 1.5718, dynamic viscosity h 2.50×10–3 Pa · s at 25 °C, surface tension s 42×10–3 N/m at 25 °C, dipole moment m (measured in liquid benzene) 9.54×10–30 C m (2.86 D) heat of combustion 27.29 kJ/g at 20 °C and constant pressure.
2-Hydroxybenzaldehyde is soluble in many organic solvents, in 20 % aqueous sodium carbonate, and in 10 % aqueous sodium hydroxide. At 86 °C, 1.7 g can be dissolved in 100 g of water. With iron(III) chloride, 2-hydroxybenzaldehyde gives an intense violet color. It forms yellow alkali salts through inner complexing.
The two most important processes for the manufacture of 2-hydroxybenzaldehyde begin with phenol. The best known process is based on the Reimer-Tiemann reaction, in which phenol is reacted with chloroform in the presence of an aqueous alkali. Moderate yields of 2-hydroxy- and 4-hydroxybenzaldehyde are obtained in a ratio of about 85 : 15. The use of aqueous methanol as a reaction medium [64] improves the yield, especially that of 4-hydroxybenzaldehyde. According to a modern variation [65] of the Reimer-Tiemann reaction, the process is carried out in the absence of water. An alkali-metal phenolate reacts with chloroform and a suspension of an alkali-metal hydroxide in an inert organic solvent.
One industrial process starts with phenol and forms 2-hydroxybenzyl alcohol (saligenin) by reacting triphenyl metaborate with formaldehyde [66]. The alcohol is catalytically oxidized in air to give 2-hydroxybenzaldehyde [67]. It is more economical to react phenol with formaldehyde in the presence of alkaline catalysts and in the absence of boric acid [68]. The yield is approximately 85 %. The ratio of 2-hydroxybenzyl alcohol to 4-hydroxybenzyl alcohol can be altered by varying the catalyst. The oxidation of the hydroxybenzyl alcohols is carried out in an aqueous solution with a platinum – lead – carbon catalyst and gives the corresponding hydroxybenzaldehydes in yields of more than 98 %.
Several other processes are based on side-chain chlorination of 2-cresol and saponification of the resulting dichloromethyl group to form the aldehyde group. The phenolic hydroxyl group is protected prior to chlorination by esterification with an inorganic or organic acid chloride, such as phosphorus oxychloride [69] , phosgene [70] , tetrachlorosilane [71] , or acetyl chloride [72]. Hydrolysis splits the ester. 2-Hydroxybenzaldehyde can also be produced by electrolytic reduction of salicylic acid on a rotating amalgam cathode [73] and by catalytic reduction of a salicylic acid halide [74].
2-Hydroxybenzaldehyde is one of the most important derivatives of benzaldehyde. Estimated annual production worldwide is 4 – 6 kt. Because of its pleasant aromatic odor, 2- hydroxybenzaldehyde is used in perfumes and, on a large scale, as a starting material in the production of cumarin.
Its condensation products with amines have complex-forming properties and are used as additives, e.g., in petrochemistry. 2-Hydroxybenzaldehyde is also an intermediate for dyes, pharmaceuticals, plastics, photographic chemicals, agricultural chemicals, and electroplating chemicals.
[64] Dow Chem. Co., US 3365500, 1964 (D. F. Pontz).
[65] Sumitomo Chem. Co., EP-A 68725, 1981 (K. Hamada, G. Suzukamo).
[66] Rhône-Poulenc S. A., FR 1328945, 1962 (P. Marchand, J. B. Grenet).
[67] Rhône-Poulenc S. A., FR 2305420, 1975 (J. Le Ludec).
[68] H. Fiege, K. Wedemeyer, K. A. Bauer, A. Krempel, R. G. Mölleken: "Fragrance and Flavor Substances," in R. Croteau (ed.): Fragrance Flavor Subst. Proc. Int. Haarman & Reimer Symp. 2nd 1979, 63 – 73; Chem. Abstr. 95 (1981) 150058 a.
[69] Tenneco Chem., Inc., US 3641158, 1969 (A. J. Deinet, D. X. Klein).
[70] BASF, DE-OS 1925195, 1969 (H. Hoffmann, J. Datow, G. Wenner).
[71] Tokuyama Soda Co., JP-Kokai 83124729, 1982; Chem. Abstr. 99 (1983) 158016 p.
[72] Yoshitomi Pharmaceutical Ind., JP 7303831, 1968 (K. Saruwatari, T. Gono, K. Tsubone); Chem. Abstr. 79 (1973) 18387 x.
[73] K. S. Udupa, G. S. Subramanian, H. V. K. Udupa, Ind. Chem. 39 (1963) 238 – 241;Chem. Abstr. 59 (1963) 10986 b.
[74] Seimi Kagakagu Co., JP 6813204, 1965 (S. Abe, K. Sato, T. Asami, T. Amakasu, T. Itakura); Chem. Abstr. 70 (1969) 28646 j.
