SOME ASPECTS OF PERFUMERY CHEMICALS perfumer has graciously approved of the odour, it is found that the price of the product is too high. Now impurities which influence the odour of an aromatic can be roughly classified into three categories: Firstly, traces of starting materials. Secondly, by-products and isomers produced as a result of side reactions during manufacture, and Thirdly, traces of decomposition products formed in the final material on storage. We are concerned here, of course, with proportions of impurities which hardly influence the physical constants of the products in which they occur, but appreciably change their odour. As an example of the first category, traces of starting materials, we may mention anisaldehyde or aubepine ex p-cresol. It is well known that the slight phenolic note due to traces of p-cresol is difficult to remove. Another example in this category is that of some esters, in the manufacture of which certain amines are used as catalysts, and it is quite often difficult to remove the last traces of such amines. The only means of producing materials with satisfactory odours in such cases is to use different starting materials. For example, anethole may be used instead of p-cresol for the production of anisaldehyde, and in the case of esters different catalysts have to be employed. As an example of the second category of impurities, the by-products and isomers produced as a result of side reactions, we may mention terpineol manufactured irom terpin hydrate. Four isomers are obtained in this process: alpha-terpineol, beta-terpineol, gamma-terpineol and terpinenol. The com- mercial grade is a mixture of alpha and beta terpineol, and a camphory note of certain qualities is due to the presence of traces of the other two isomers. Then there is the case of linalyl acetate. As you know, under certain conditions linalool is converted to terpineol by the action ot acetic anhydride. If such conditions are given during the manufacturing process of linalyl acetate, the traces of terpineol so formed are acetylated to terpinyl acetate which then causes that objectionable resinous note which is sometimes found in linalyl acetate. Once present, it is very difficult, if not almost impossible, to remove these traces of terpinyl acetate. The last category of impurities, you will remember, includes decomposi- tion products formed on storage. Here may be mentioned aldehydes which are oxydised to acids with pronounced odours. Also certain esters may be mentioned which hydrolyse on standing, such as butyrates which develop a note of butyric acid on storage. Now the chemist will always endeavour to produce chemicals in as pure a state as he possibly can. He is a scientist, his criterion is chemical analysis, and he has figures to guide him specific gravity, refractive index, percentage

40 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of aidehyde, alcohol, ester, or whatever the case may be, these are the properties by which he determines the quality of a product. The perfumer, on the other hand, is not a scientist, he is an artist. His analytical instrument is his nose, and the trained nose is much more sensitive than, say, a refractometer. It will detect traces of impurities which cannot be detected by the ordinary routine methods of chemical and physical analy- sis. It is true that such methods as ultra-violet and infra-red spectroscopy are recently being used more widely for the evaluation of aromatics, but their use is still far from being universally adopted, and we therefore have to depend largely on the perfumer's nose. As I said, the perfumer is an artist, and as an artist he is not logical. For whereas he expects one type of chemicals to be so pure that no impurities can be detected by the most sensitive of analytical instruments--the per- fumer's nose--he expects, on the other hand, other types of chemicals to contain impurities. In fact, he will only accept such chemicals when they are impure and will reject them in their pure state. Let me give you some examples of such chemicals, again using the classification mentioned earlier on. First of all, chemicals containing traces of starting materials. The odour of the Schiff's Base formed by the action of hydroxycitronellal on methyl anthranilate is, in my opinion, due to traces of both these starting materials. If one were to remove them this product would become useless to the perfumer. It is just this very proportion in which hydroxycitronellal and methyl anthranilate are present in this Schiff's Base, together with its fixative properties caused by its low volatility, which makes it such a valuable aromatic. Certain acetals may also be mentioned in this connection. There certainly is no doubt about it that there are many acetals which possess odours distinct from those of the parent aidehyde. But whenever I have the opportunity of smelling certain other acetals I cannot help feeling that the odour of traces of the parent aidehyde contributes to no small extent to the odour character of the acetal. Now we come to the second category, the chemicals which contain by-products and isomers formed during their preparation. It has been frequently stated that if one were to prepare certain products in an absolutely pure state one would obtain materials without any odour at all. The acetate of trichloromethyl phenyl carbinol is a case in point. Several years ago Dr. Mitchell published a paper in Perfumery • Essential Oil Record' in which he stated that on repeated crystallisation of the commercial product an odourless ester was obtained. Obviously the rose odour which the perfumer expects this ester to possess is due to some by-product formed as a result of a side reaction. It would be senseless to remove it and produce an odourless perfumery chemical.