compounds as the members of a homologous series. For example, the lower fatty aldehydes have unpleasant odours, those with ten carbon atoms (and also double linkages, which in itself may affect odour) form some of the most delicate scents, while the higher members are odourless. The absence of odour in the higher members may be possibly associated with the low volatility exhibited by compounds of high molecular weight. Certain osmophores have practically equal effects; for example, benzaldehyde, nitrobenzene, benzonitrile, and phenyl azoimide have practically identical odours, and among the “artificial musks,” a nitro group may be replaced by the azoimido group without the odour being modified. As a general rule, homologues have similar odours, but many exceptions are known. For example the methyl and ethyl ethers of β-naphthol have the odour of neroli; on the other hand, of the esters of anthranilic acid, the methyl has the odour of orange blossoms, the ethyl has a slight odour, and the isobutyl is odourless. The introduction of a methyl group into the benzene ring generally involves little or no change in odour; but when it (and more especially higher alkyl radicals) is introduced into side chains the odour may be entirely changed. For example, benzene and its homologues have similar odours; phthalide is odourless, but the isopropyl and butyl phthalides, in which substitution occurs in the side chain, smell of celery. Especially characteristic are the derivatives of phenylacetylene. This hydrocarbon is distinctly unpleasant; on the other hand, para-ethyl and para-methyl phenylacetylene smell of anise. While the triply-linked carbon system is generally associated with strong and unpleasant odours, the doubly linked system gives pleasant ones. Thus the unpleasant phenylacetylene, C6H5·C⋮CH, is contrasted with styrolene, C5H5·CH.CH2, which occurs in storax, and phenylpropiolic aldehyde with cinnamic aldehyde, C6H5·CH:CH·CHO, which occurs in cassia and cinnamon. The reduction of a double to a single linkage may not destroy odour. Thus hydrocinnamic aldehyde, the reduction product of cinnamic aldehyde, smells of jasmine and lilac, and melilotin, which occurs in yellow melilot (Melilotus officinalis), has the same odour (woodruff) as its oxidation product coumarin. The orientation of the substituent groups in the benzene nucleus also affects odour. In general, the meta compounds are odourless, while the ortho and para may have odour. Thus p-methoxyacetophenone has a pleasant odour, the meta compound is odourless, o-aminoacetophenone, o-aminobenzaldehyde, and o-nitrophenol have strong odours, while the meta and para bodies are odourless. Of the three trinitrobenzenes only the symmetrical form gives origin to perfumes.
The concentration and even the solvent has considerable effect on the odour of a substance. Many of the artificial principles—vanillin, heliotropine, ionone, &c.—have very different odours in strong and in dilute solution; phenyl acetic acid and β-naphthylamine are odourless when solid, but have disagreeable odours when dissolved. Traces of impurities often have the effect of making odourless or pleasant-smelling compounds quite intolerable. Acetylene as generated from calcium carbide, and carbon disulphide prepared from its elements are quite intolerable, though when pure they are, at least, not unpleasant; artificial benzaldehyde must be very carefully purified before it can be used in the preparation of the more delicate scents. In all cases the natural scents are complex mixtures of many ingredients, and a variation in the amount of any one may completely alter the scent. Such mixtures would be difficult to reproduce economically; the perfumer is content with a product having practically an identical odour, with or without the natural substance which it is designed to compete with.
We now give an account of the artificial scents, principally arranged according to their chemical relations. The fatty esters are interesting as providing many of the fruit essences; in fact, by appropriate blending, any fruit odour can be reproduced. Their use, however, is inhibited by the fact that they irritate the respiratory organs, producing coughing and headaches. Isobutyl carbinol acetic ester (amyl acetate), (CH2)2 CH CH2·CH2·OC·CH3, forms when in dilute alcoholic solution the artificial pear oil; a similar odour is possessed by isoamyl-n-butyrate, C3H7·CO2·C5H11. n-Octyl acetate, C8H11·O2C·CH3, has the odour of oranges. Isoamyl propionate, C5H11·O2C·C2H5, and ethyl-n-butyrate, C3H7·O2C·C2H5, have the odour of pineapple, the latter constituting the artificial pineapple oil of commerce. Isoamyl isovalerate, C5H11·O2C·C4H9, is the artificial apple oil. Of the fatty ketones, methyl nonyl ketone, CH3·CO·C9H19, which is the scent of oil of rue, and methyl ethyl acetone, CH3·CO·CH(CH3) (C2H5), which has the odour of peppermint, receive commercial application. Of exceptional importance in the chemistry of perfumes are the unsaturated open chain compounds containing at least eight carbon atoms. These are chemically considered, along with the related cyclic compounds, in the article Terpenes; here we notice their odours and occurrence in perfumes. Of the alcohols, l-linalol occurs in oil of lavender, bergamot, limet and origanum; d-linalol in coriander; citronellol and geraniol in rose, geranium and pelargonium oils. Of the aldehydes, citral or geranial has the odour of lemons; citronellal is the chief constituent of citronella oil. By condensing citral with acetone and treating the product with dilute sulphuric acid, the valuable violet substitute ionone results. This substance is a hydroaromatic ketone, and closely resembles the natural principle irone. By successive treatment with acetic anhydride (to form isopulegol), oxidation to isopulegone, and treatment with baryta citronellal yields the cyclic compound pulegone, the chief constituent of oil of pennyroyal. The olefinic terpenes are generally convertible into methyl heptenone, (CH3)2C:CH(CH2)2·CO·CH3, which has been synthesized from sodium acetonylacetone and amylene dibromide; this ketone occurs in several essential oils, and has the odour of rue. For the occurrence of cyclic terpenes in the essential oils reference should be made to the table below, which contains the names, sources and chief ingredients of the more important essential oils[1] The terpenes are printed in italics, the aliphatic and benzenoid compounds in ordinary type.
| Name of Oil. | Source. | Constituents. |
| Anise | Pimpinella anisum | Anethole, estragole. |
| Bay | Pimenta acris | Eugenol, methyl eugenol, chavicol, estragole, myrcene, phellandrene. |
| Bergamot | Citrus bergamia | Linalol, linalyl acetate, d-limonene, bergaptene. |
| Cajaput. | Melaleuca, sp. | Cineol. |
| Cassia | Cinnamonum cassia | Cinnamic aldehyde, cinnamyl acetate. |
| Caraway | Carum carvi | Carvone, d-limonene. |
| Camphor | Cinnamonum camphor | d-Pinene, phellandrene, terpineol, eugenol, safrole. |
| Chamomile | Anthemis nobilis | Isobutyl and isoamyl esters of angelic and tiglic acids. |
| Cinnamon | Cinnamonum Zeylanicum | Cinnamic aldehyde. |
| Clove | Eugenia caryophyllata | Eugenol. |
| Coriander | Coriandum salivum | Linalol. |
| Cumin | Cuminum cymium | Cumic aldehyde, cymene. |
| Eucalyptus | Eucalyptus globulus | Cineol, d-pinene, and fatty aldehydes. |
| Fennel | Foeniculum vulgare | Anethole, fenchone, d-pinene |
| Geranium | Andropogon schoenanthus | Geraniol, citronellol. |
| Jasmine | Jasminum grandiflorum | Methyl anthranilate, indol, benzyl alcohol, benzyl acetate, linalol, linalyl acetate. |
| Lavender | Lavendula vera | Linalol, l-linalyl acetate. |
| Lemon | Citrus limonum | Limonene, phellandrene, citral, citronellal, geranyl acetate, linalol. |
| Lemon-grass | Andropogon citratus | Citral. |
| Neroli | Citrus bigardia | l-Linalol, geraniol, limonene, methyl anthranilate. |
| Orange | Citrus aurantium | d-Limonene. |
| Peppermint | Mentha piperita | Menthol, menthyl acetate and valerate. |
| Pine-needle | Pinus sylvestris | d-Pinene, d-sylvestrene. |
| Rose | Rosa damascena | Geraniol, l-citronellol. |
| Rose Geranium | Pelargomum odoratissemum | Geraniol, citronellol. |
| Rosemary | Rosamarinus officinalis | Pinene, camphene, camphor, cineol, borneol. |
| Sage | Salvia officinalis | Pinene, cineol, thujone, borneol. |
| Sassafras | Sassafras officinalis | Safrole. |
| Spearmint | Mentha viridis | l-Linalol, l-carvone. |
| Star anise | Illicium anisatum | Anethole. |
| Tansy | Tanacetum vulgare | Thujone. |
| Thyme | Thymus vulgaris | Thymol. |
| Wormwood | Artemisia absinthum | Thujone and thujyl esters. |
| Ylang-ylang | Cananga odorata | l-Linalol, geraniol. |
- ↑ See J. B. Cohen, Organic Chemistry, p. 532; or J. Parry, Chemistry of Perfumes (1908).
