Лако-красочные материалы — производство

Until the middle of the nineteenth century, perfumes were largely for personal application and, furthermore, that use was restricted mostly to the wealthiest strata of society because of the cost of producing the natural materials required as ingredients. The development of organic chemistry in the nineteenth century began to make synthetic chemicals available and their use in fragrances began to grow. The incorporation of synthetics into perfumery received a huge fillip in 1921 when Coco Chanel launched her famous perfume, No 5. Chanel №5 owes its unique character to the inclusion of synthetic aliphatic aldehydes, 2- methylundecanal in particular. The success of this fragrance inspired other fragrance houses to experiment with synthetic materials and the modern age of perfumery was born. The synthetic materials were cheaper to produce than natural materials, thus making perfume accessible to all. Furthermore, more robust molecules could be pro­duced which would survive in acidic, basic and even oxidizing media. Thus, it became possible to put perfume into household products in which natural oils could not be used because of degradation of their components and resultant changes in odour and colour.

The use of essential oils is also restricted by their chemical stability. Many of the components of natural oils do not survive in products such as bleaches, laundry powders and even soaps. For example, the major component in jasmine oil is benzyl acetate, which is hydrolysed in all of these products owing to their high pH (13-14, 10-11 and 9-10, respectively), and it is also susceptible to the oxidants present in the first two. The indole present in jasmine causes soap to discolour. The discovery and application of synthetic fragrance materials towards the end of the nineteenth century and throughout the twentieth was therefore a momentous event in the history of the industry. Nowadays, fragrances can be used in all the consumer goods produced for personal and household care and they can be afforded by everyone. More detail of the use and performance of fragrances in products is given in later chapters; the reason for mentioning this at this point is to highlight the importance of economic considerations.

Initially, the synthetic perfumery materials were introduced through serendipitous use of the products discovered through advances in chemical technology. For example, the nitromusks were discovered by Baur while he was working on explosives related to TNT. As tech­niques for isolation, characterization and synthesis of organic chemi­cals improved, the search for new fragrance materials became more structured. In this, the fragrance industry follows the same path as the pharmaceutical industry. The first step is to identify the materials which nature uses. Thus, the chemical components of natural oils were separated by distillation and/or chromatography and their structures determined by chemical analysis and/or spectroscopy. (Details of the application of these techniques are given in Chapter 12.) Having identified the molecular structure of an odorant, the next task is to synthesize a sample that is identical to it. Synthesis serves as the final proof of the correct determination of the structure, but it also makes it possible to produce the material without relying on the natural source. Synthetic compounds whose structures are the same as those of the natural material are referred to as ‘nature identical’. This classification of materials is important in legislative terms; it is easier to obtain clearance for a nature-identical material than for one which has no natural counterpart. However, the natural materials may contain structural features which make them difficult to synthesize or suscep­tible to degradation in the products to which perfumes are added. The next step is therefore to synthesize materials that are close in structure but not identical to the natural one. The effect of changes of structure on the odour, and other properties, of the materials can then be studied and further analogues synthesized to produce an optimum balance of odour, performance and cost. A more detailed account of this process is dealt with in Chapter 15. For the moment, the example of the chemistry of jasmine compounds serves to illustrate the overall path from natural to synthetic material.

The components of an essential oil may be classified into three

groups. Some components add little or even nothing to the odour of the oil, but may serve another purpose. For instance, they could be fixatives. The components in the second group add odour and are important in forming the total impression of the oil but, smelt in isolation, are not associated immediately with the oil. The third group of compounds are the character impact compounds. These are the materials which give the characteristic notes to the oil and which, when smelt in isolation, are instantly associated with the oil. Figure 3.5 shows a GLC trace of jasmine oil and materials of each type can be seen in it. Isophytol and benzyl benzoate have very little intrinsic odour and serve mostly as fixatives. Benzyl acetate is the major component of jasmine oil and plays a significant part in the total odour. However, it possesses a fruity note which could be, and indeed is, found in many other oils.

The character-impact compounds of jasmine are jasmone and methyl jasmonate. These two are instantly recognizable as jasmine in character and are essential to the odour of the oil. Their structures are shown in Scheme 3.10, which shows their syntheses through a common inter­mediate. Jasmone was first synthesized by Crombie and Harper (1956), but the synthesis in Scheme 3.10 is that of Buchi and Egger (1971). Buchi’s synthesis illustrates the main problem in the synthesis of nature-identical jasmone and methyl jasmonate; that is, the inclusion of the cw-double bond in the side chain. The most convenient method of introducing this feature is through Lindlar hydrogenation of an acetylenic compound. In terms of total synthesis of natural products, this is a relatively trivial step and is easy to carry out on a laboratory scale. However, several synthetic steps are be required to prepare the five-carbon unit for the side chain and two more are needed to introduce and hydrogenate it. On a manufacturing scale, this leads to high process costs, especially since two of the stages involve the handling of hazardous reagents, viz. acetylene and hydrogen. If the side chain of jasmone is replaced by a saturated one, the synthesis is made much easier, and so dihydrojasmone is much less expensive than jasmone. Stetter and Kuhlmann’s (1975) two-step synthesis of dihy­drojasmone from readily available starting materials is shown in Scheme 3.11. If the endocyclic double bond and the methyl substituent

Figure 3.5 GLC trace of jasmine oil. Peak A = benzyl acetate (26.1% of volatiles by relative peak area); Peak В = jasmone (3.3% of volatiles by relative peak area); Peak C = methyl jasmonate (0.6% of volatiles by relative peak area); Peak D = benzyl benzoate (11.5% of volatiles by relative peak area); Peak E— iso-phytol (5.6% of volatiles by relative peak area)

base

I

I

Scheme 3.10

on the ring are also ignored, the synthesis becomes even more amenable to operation on a commercial scale (Scheme 3.12). Scheme 3.12 shows the preparation of pentylcyclopentanone, but use of different aldehydes in the initial aldol condensation gives rise to a series of homologous compounds, each with a unique blend of jasmine and fruity notes. Scheme 3.12 also shows the route used to prepare methyl dihydro — jasmonate commercially. This chemistry is described in more detail in Chapter 4. Cyclopentanone is available in bulk at low cost by the pyrolysis of the calcium or barium salts of adipic acid, the precursor of Nylon 66®. This is an example of how the fragrance industry capita­lizes on the availability of inexpensive feedstocks from much larger scale industries, in this case the textile industry.

Natural jasmine oils cost £3000-5000/kg, the nature-identical materials are about one tenth of that price and the price of the simpler analogues is a further order of magnitude, or even more, lower. In addition, because they lack the double bonds, the synthetic materials are more stable in products, such as laundry powder, which contain bleaching agents. All of these materials are used in fragrances, but there

is a correlation between price and tonnage. Obviously, the less expensive a material is, the more it is used. Jasmine absolute can only be used economically, in more than trace amounts, in the most expensive fine fragrances, whereas, 2-heptylcyclopentanone is cheap enough to allow its use in reasonably high proportions in most fragrances, including low-cost ones for use in laundry powders and household cleaners. The example of jasmine is typical of the interplay of inspiration from nature, technical possibility and economic pressure which has given rise to the variety of fragrance materials in use today and is described in Chapter 4.

G.