Лако-красочные материалы — производство

The alkylation of toluene to give hydrocinnamic derivatives is dis­cussed in the previous section, alongside the same reaction of benzene. The tree of products in this section all derive from the oxidation of toluene, as shown in Scheme 4.53.

Air oxidation of toluene gives predominantly benzoic acid. This is used in perfumery for the preparation of benzoate esters, benzo — phenone and various other compounds, but this use is dwarfed by the other industrial uses of benzoic acid. It is used in many different ways, for instance, as a precursor for nylon monomers. Crude benzoic acid contains a small amount of benzaldehyde, which is easily extracted from it. In view of the huge volume of benzoic acid produced, the volume of benzaldehyde recovered makes a substantial contribution to that used by the perfumery industry.

Chlorination of toluene under radical conditions (either through the

use of an initiator or by photolysis) gives a mixture of mono-, di- and trichlorotoluene. In practice in industry, the reaction is run with an excess of toluene present which means that benzyl chloride is the major product. A little benzal chloride is produced and can be separated and hydrolysed to give benzaldehyde. The major use of benzyl chloride is in the production of benzyl alcohol and its esters; the alcohol is produced by hydrolysis of the chloride. The esters can be prepared by esterifica­tion of the alcohol, but it is better economically to prepare them directly from the chloride by reaction with a salt of the corresponding acid. By far the most important of this group of products is benzyl acetate, the major component of jasmine oils. Grignard addition of benzyl chloride to acetone leads to a family of ingredients based on dimethylbenzylcarbinol. The most important member of the family is the acetate, known by the acronym DMBCA.

Although benzaldehyde has an odour which is very characteristic of almonds, it is the chemicals derived from it, rather than its own odour, that make it an important material to the perfumery industry. The major benzaldehyde derivatives are shown in Scheme 4.54. The most important are shown on the left of the scheme.

Claisen ester condensation gives cinnamic acid and its esters, the most important of which is methyl cinnamate, followed by benzyl cinnamate. Aldol condensation of benzaldehyde with other aldehydes

Scheme 4.54

gives the series of а-substituted cinnamaldehydes. The lowest member of the series, cinnamaldehyde, is used to some extent in fragrances, but its main use is as the starting material for the corresponding alcohol, cinnamyl alcohol. This is an important component of spicy perfumes in which a cinnamon note is required. Its esters, the acetate in particular, are also used for their odours. Much more important are the higher members of the series, amylcinnamic aldehyde (АСА) and hexylcin­namic aldehyde (HCA). They possess odours reminiscent of the fatty background note of jasmine, although neither are found in jasmine oils. Most synthetic jasmine perfumes use one or both of these compounds as the foundation on which the fragrance is built. They are inexpensive materials and so can be used in large proportions in perfume formulae. They are also very fibre-substantive materials and are therefore of great importance in laundry products, such as detergents and fabric condi­tioners.

Addition of chloroform to benzaldehyde followed by esterification with acetic anhydride gives the trichloro derivative known as rose crystals or, more commonly by the misnomer, rose acetone. Such misnomers are not uncommon with older fragrance materials. Some are accidental, but others were probably intended to deceive competi­tors in the days before analytical chemistry progressed to the stage where such deception is easy to uncover.

Benzaldehyde undergoes the Prins reaction with homoallylic alco­hols to give a variety of perfume ingredients, mostly with green, herbaceous odours. These products have very intense odours and so are used at relatively low levels in fragrances. The product obtained from the Prins reaction of benzaldehyde and 3-methylbut-3-ene-l-ol (isoprenol) can be hydrogenated to give 3-methyl-5-phenylpentan-l-ol, the pyran ring being broken open by hydrogenolysis of the benzylic ether bond during the hydrogenation. This material is known under various tradenames, such as Mefrosol® and Phenoxanol®. It has a very pleasant, fresh, white-floral odour.