THE MUSK ODOUR 425 classification of "musk". Perhaps Beets' wording of "a somewhat uniform odour type" is descriptive. The whole field of musk-chemistry is so vast that this discussion must be limited to a few well-defined types of structures and an attempt will be made to indicate how changes in structure influence odour and where possible, to describe the odours involved. THE BENZENE MUSKS From a structural point of view the two simplest molecules possessing a musk-like odour are 3.5-ditertiarybutylbenzaldehyde (I) and 3.5-ditertiary butylacetophenone (II). The former is too unstable to be of interest in perfumery. The latter has a fine, sweet musk odour reminiscent of musk ketone and ambrette with just a touch of the sandalwood note, and one is tempted to wonder why it is not produced commercially. The correspond- ing propiophenone (III) is somewhat similar in character but less intense and with more accent on the woodiness. Two other simple musk-like compounds are: 2-methyl 4.6-ditertiarybutylbenzaldehyde (IV) and 2.4- ditertiarybutyl 5-methoxybenzaldehyde (V). It is of course possible to place some of the nitromusks in the benzene section but they form a more logical section of their own. THE TETRALIN MUSKS Although many tetralin compounds giving some degree of musk odour have been produced, the main interest centres around the structure: CH, HaC CH, I CHa I-IaC CH, 0= I H,C HaC CHa H3 (VI) (VII) In the absence of the R radicle the compound is odourless, but if R is a methyl group then the compound possesses a fine musk odour. In the case where R is an ethyl group, the compound 1.1.4.4-tetramethyl-6-ethyl- 7-acetyl-l.2.3.4-tetrahydronaphthalene (VII) possesses an odour which is outstanding in the musk field. It has the fine dean quality of musk ketone with some of the intensity of ambrette. In addition it has a subtle shading of the woodiness found in the macrocyclic musks.

426 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Another tetralin musk which possesses a clean pleasant but rather weak odour is 1.1-dimethyl-7-tertiarybutyl-5-acetyl-l.2.3.4-tetrahydronaphthalene (wn) :- Beets • has put forward the following requirements for musk odour in the tetralin and indane classes :-- (1) More than 14 and less than 20 carbon atoms with an optimum at C16 to C18. (2) An indane or a tetralin skeleton. (3) An acyl and a secondary or tertiary alkyl group as separate substituents in the aromatic nucleus with an optimum for acetyl and tertiary butyl. (4) One of the carbon atoms of the non-aromatic ring bound to the aromatic nucleus is tertiary or quaternary with an optimum when it is quaternary. TRE I•)A•E MUSKS It has been suggested by Beets • that in an indane derivative the presence of an acyl group and of two tertiary or quaternary carbon atoms as sub- stituents in the benzene nucleus, are the minimum requirements for musk odour. Such tertiary or quaternary carbon atoms may both be part of the non-aromatic ring or one of them may be introduced as a separate alkyl group. The stronger musks are obtained when an acyl group and two quaternary carbon atoms are present. In the indane structure (IX) the following effects are found: C' / •x• 6 7 1 C•O I CH• (vm) (ix) The compound 1.1-dimethyl-6-tertiarybutylindane is odourless, but the introduction of a carbonyl group in the 4 position produces the musk odour. The aldehyde group (formyl) produces a fairly strong musk with an odour like musk ketone with the ambrette effect also in evidence. The methyl ketone (acetyl) is strongly musk-like, the ethyl ketone (propionyl) compound has a sweet intense musk odour and is very strong and lasting.