Лако-красочные материалы — производство

Phenol is a material of major commercial importance. One of its earliest uses was as a disinfectant (carbolic acid). Earlier in the twentieth century, it became important as a feedstock for resins such as Bakelite®, and in the latter part of the century it also became very important as a precursor for caprolactone and caprolactam and hence polyester and polyamide manufacture. The two major methods for phenol produc­tion nowadays are by the catalytic oxidation of benzoic acid and catalytic decomposition of cumene hydroperoxide (Scheme 4.55).

Diphenyl oxide, prepared from phenol, is important in rose and other floral fragrances. The addition of ethylene oxide to phenol gives phenoxyethanol and hence its esters, the most important of which is the isobutyrate (Scheme 4.56). Etherification gives materials such as anisole (methyl phenyl ether), estragole [3-(/>-methoxyphenyl)prop-l — ene, a constituent of tarragon] and anethole [l-(/?-methoxyphenyl)- prop-l-ene, which occurs in and is strongly characteristic of aniseed].

Scheme 4.56

Anethole and estragole occur in sulfate turpentine. Distillation from this source provides much of the anethole and estragole required, the shortfall in supply being made up by material synthesized from anisole. The addition of isobutylene to phenol gives a mixture of o- and p-t — butylphenols, which can be separated and hydrogenated to the corre­sponding cyclohexanols and then esterified to produce the acetates. The acetates are both very important fragrance ingredients and are used in large quantities. They are known as OTBCHA and PTBCHA, acro­nyms for o — and /?-/-butylcyclohexyl acetate, respectively. OTBCHA is also known as Ortholate® and has an apple odour with some fresh woody notes. PTBCHA has a warm, sweet, fruity, woody character. Both are mixtures of cis — and trans-isomers. The cis-isomer of PTBCHA is stronger than the trans- and has a desirable jasminic — floral character. In consequence, PTBCHA is sold in regular and high cis — grades. It is possible to separate the isomers completely, but the cost of doing so is prohibitive.

Addition of a one carbon unit to phenol in a Friedel-Crafts type of reaction gives rise to a family of perfume ingredients of great

importance, as shown in Scheme 4.57. Carboxylation gives salicylic acid. Acetylation of the phenolic group of salicylic acid gives aspirin and thus the acid is an important commodity chemical for the pharmaceutical industry. The esters of salicylic acid are important to the fragrance industry. Methyl salicylate is the major component of oil of wintergreen. Sportsmen and women readily recognize its odour, since it is characteristic of liniments prepared from that oil. The most important salicylates to perfumery are the amyl, hexyl and benzyl derivatives, which are used in very significant quantities. These have persistent, floral, herbaceous odours and make excellent blenders and fixatives for floral perfumes. The odour which we have all come to recognize as characteristic of sun-tan lotion is largely that of these higher salicylate esters.

Addition of formaldehyde to phenol normally produces a resin. However, under controlled, catalytic conditions, it is possible to obtain the hydroxymethyl derivatives in high yield. o-Hydroxymethyl —

phenol is known as saligenin and can be oxidized to salicylaldehyde. Similarly, the /7-isomer can be oxidized to /7-hydroxybenzaldehyde. Methylation of the latter gives anisaldehyde, which has a hawthorn odour, and Perkin condensation of the former leads to coumarin. Coumarin is present in newly mown hay and its sweet, hay-like note is used widely in perfumery. Thus, one reaction leads to the precursors for two important fragrance materials. The ratio of o — to /7-substitution can be controlled to some extent by the choice of catalyst and conditions, and so the reaction product mixture can be adjusted to suit the demand for the two end products. (Originally, salicylaldehyde was prepared from phenol using chloroform in the Reimer-Tiemann reaction.)

Oxidation of phenol leads to o-dihydroxybenzene (catechol, Scheme 4.58). This oxidation can be carried out in a number of ways, but the most important commercially is to use hydrogen peroxide as the oxidant with iron salts as catalysts. The monomethyl ether of catechol is known as guaiacol and can be prepared from catechol by partial methylation using dimethyl sulfate or an equivalent reagent. Guaiacol is a precursor for the terpenophenol family of sandalwood chemicals, as discussed earlier in the section on terpenes. Formylation of guaiacol gives vanillin (Scheme 4.58), the character-impact component of vanilla. The ethyl analogue can be prepared similarly and is known as ethylvanillin. Formylation of the methylene ether of catechol gives methylenedioxybenzaldehyde, which is commonly known as either piperonal or heliotropin. The former name comes from the correspond­ing acid, piperonylic acid, which is a degradation product of the pungent principles of pepper. The latter, and more common name, is derived from the fact that the odour of the aldehyde is strongly reminiscent of heliotropes, since it is the major fragrant component of those flowers. The vanillic group of compounds are used to give a heavy and long-lasting sweetness to fragrances, a character which is currently much in vogue with perfumes such as Tocade and Angel.

Vanillin and ethylvanillin are not particularly stable chemically. This is not surprising, since they possess both an aldehyde and a phenolic group. In functional products, where the pH is not neutral, they undergo a variety of reactions leading to discoloration. For example, inclusion of vanillin in a white soap will, after a matter of days, produce a colour close to that of chocolate.

The marketing phenomenon known as ‘trickle-down’ is when the odour of a fine fragrance is adapted so that a range of cosmetics, toiletries, soaps and so on can have the same fragrance to produce a line of products. If the fine fragrance contains vanillin, trickle-down

is difficult because of the discoloration issue. One task of the fragrance chemist is to overcome such problems and two solutions to this, developed by chemists at Quest, are shown in Scheme 4.58. Protection of the phenolic group of vanillin through the isobutyrate ester gives Isobutavan®. Reduction of the aldehyde group of ethylvanillin to a methyl group gives Ultravanil®. (Interestingly, the corresponding methyl ether, obtained by reduction of vanillin, has a strong smoky odour with no trace of vanillic sweetness.) Both of these compounds provide vanillic notes, but are much more stable in use than vanillin itself.

Aldol reaction between heliotropin and propionaldehyde, followed by hydrogenation, gives the hydrocinnamaldehyde derivative known as

Helional® or Aquanal®. This has a floral note, and is somewhat sweeter than the hydrocinnamaldehydes described in the section on benzene-based materials.

The catechol derived materials can also be approached by a different route, using readily available natural products, as shown in Scheme 4.59. Clove oil is available at moderate cost and in moderate quantity from several tropical countries, such as Indonesia and the Malagasy Republic. Similarly, sassafras oil is available from Brazil. The major component of clove oil is eugenol, 3-(3-methoxy-4-hydroxyphenyl)- prop-l-ene, which can be isomerized to isoeugenol using basic or metallic catalysts. Ozonolysis of isoeugenol gives vanillin. Similarly, heliotropin can be obtained from safrole, 3-(3,4-methylenedioxy — phenyl)prop-l-ene, the major constituent of sassafras oil. The various ethers and esters of these materials shown in Scheme 4.59 are used in perfumery. Eugenol itself is, by far, the most important of these. As well as being present in cloves, eugenol is an important contributor to the odour of carnations and is used in fragrances to that effect. Generally, the catechol route is cheaper than the eugenol-safrole route to vanillin and heliotropin, but local economics may tip the balance in the other direction particularly for heliotropin (which is the more expensive of the two).