Лако-красочные материалы — производство

Acetylation of naphthalene gives methyl naphthyl ketone, and sulfonation followed by alkaline fusion gives naphthol (Scheme 4.60). The methyl and ethyl ethers of naphthol are prepared from naphthol by reaction with the corresponding alkyl sulfate under basic conditions. These ethers are usually known by the shorter names of yara and nerolin, respectively. Yara, nerolin and methyl naphthyl ketone possess floral odours and are moderately important perfume ingredients.

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Scheme 4.60

Oxidation of naphthalene gives phthalic acid, which can also be obtained by air oxidation of o-xylene. Phthalic acid is an important feedstock for polymers and plasticizers and so, once again, the fragrance industry is ‘piggy-backing’ on a larger industry to obtain
inexpensive ingredients. Higher esters of phthalic acid are used as plasticizers and solvents; the ester of most interest to the fragrance industry is the diethyl ester, which is used as an odourless solvent for fragrances.

Methyl anthranilate is obtained by the Hoffmann rearrangement of phthalimide, which is easily prepared from phthalic anhydride and ammonia. Methyl anthranilate occurs naturally in many flowers and has a very characteristic and intense, sweet smell. When added to an aldehyde, it forms the corresponding Schiff’s base. If the aldehyde carries an a-hydrogen atom, the Schiff’s base is in equilibrium with the corresponding enamine. The relative proportions of Schiff’s base and enamine present depend on the structure of the aldehyde, since the inductive and resonance effects seek to minimize the total free energy of the molecule. Perfumers refer to the products as Schiff’s bases irrespective of the exact composition. Indeed, Schiff’s bases formed from methyl anthranilate are so important to the industry that the term ‘Schiff’s base’, when used in perfumery, almost invariably implies a Schiff’s base of methyl anthranilate. The importance of these Schiffs bases to perfumery results from the fact that they increase both the chemical stability and tenacity of the aldehyde component. The Schiff’s bases are less reactive than the free aldehyde and, since the molecular weight is much higher, they are less volatile. Thus, loss of the aldehyde by both chemical reaction and evaporation is slowed down. Moreover, hydrolysis of the Schiff’s base releases both the aldehyde and the methyl anthranilate, both of which have intense odours. Thus, use of a Schiff’s base generates a long-lived fragrance composition of the two ingredients. Originally, the aldehydes used in these Schiff’s bases also had sweet, floral odours and so an harmonious accord was produced. Giorgio is an example of a fragrance which relies heavily on this type of accord.

A new fashion was created when Dior introduced Poison in which the aldehyde component of the Schiff’s base has a strident green character, creating a sharp contrast with the heavy sweetness of the methyl anthranilate. The pale yellow colour of methyl anthranilate is darkened when a Schiff’s base is formed from it, because of the hypsochromic shift in the ultraviolet absorbtion maximum that results from the extension of conjugation. This colour has an effect on the colour of any fragrance in which it is incorporated and has to be taken into account if colour might be deleterious in the final product. Many Schiff’s bases are available commercially, but some perfumers prefer to formulate their fragrances using aldehydes and free methyl anthranilate and allow the Schiff’s base to form spontaneously in the perfume.