"Schiff bases" By Luca Turin

"Schiff bases" By Luca Turin

There are plenty of big mysteries left in olfaction, but one of my favorites is a small one, so small in fact that nobody seems to regard it as interesting: Schiff bases. They are chemical compounds named after Hugo Schiff (1835-1915), a German-born chemist who emigrated to Italy and became Professor in Turin and Florence (try doing that today). Schiff bases are the reaction product of aldehydes and amines and contain an unusual arrangement of atoms, -C=N-. Depending on your point of view, they are either a godsend or a nightmare. The reader who has some knowledge of organic chemistry is probably already wondering why aldehydes would ever encounter amines inside a bottle of perfume.

Aldehydes are everywhere since Chanel No. 5, but don’t amines smell fishy? Yes, all except one do, and there’s the first little mystery: methyl anthranilate. Draw the structure on a napkin, ask a rookie chemist to guess its smell, and she will probably say, “Harsh, fishy, aniline like.” Open a bottle and surprise her: American grape. Methyl anthranilate, an essential component of white floral compositions, is what made Narcisse Noir and l’Heure Bleue so wonderful. Anthranilate comes with a warning: mix it with aldehydes and after a day or two your perfume will go dark, your soap bar completely black, because the quantum mechanics of the -C=N- group are such that it absorbs light.

That’s the nightmare. The godsend is that you can make Schiff bases between anthranilate and just about any aldehyde, and they have amazing olfactory properties. For a start, they go on forever and are tremendously powerful. The reason for this is supposed to be that they slowly fall apart into their components and release them into the air, since they are considered too big (more than 16 carbons) to have their own smell. There, however, lies another mystery: most Schiff bases smell different and much stronger than the sum of their parts. If you doubt this, get a bottle of Giorgio. Its canary-yellow color is due to a small dose of a Schiff base between anthranilate (grape) and the aldehyde helional (a fresh, pale metallic smell).

Weld them together and you get the stuff of Giorgio, a prodigious, take-no-prisoners hybrid between fruit and iron that belongs in medieval legend. The perfumer Jean-Pierre Subrénat, who had a hand in Giorgio, told me that when the fragrance was composed in 1980 nobody thought that this small Beverly Hills firm would sell millions. Accordingly, they specified this then-new Schiff base of which only one barrel was in existence. When sales of Giorgio took off and the suppliers had to make more, they found to their horror that they could not recreate that particular smell for months while orders went unfilled. To this day nobody seems sure why this happened, but fragrance chemists seem to have steered clear of -C=N- bonds since.

Luca Turin works at the MIT; he lives in Boston.