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Advances in the Chemistry of Nitriles and Amides

Contact Author Mark Erman
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Nitriles and amides represent two important families of organic compounds containing respectively, a CN or a CONR1R2 moiety. Their chemistries are very much intertwined because either family can serve as a feedstock for the other. For instance, nitriles can be obtained by a direct dehydration of primary amides RCONH2, and vice versa, primary amides — by an addition of water to nitriles. Conversions of nitriles to N-substituted secondary or tertiary amides are also quite practical, although they often require more than one chemical step.

Regarding the applications of nitriles and amides, these two classes of compounds tend to occupy different functional niches. Nitriles are primarily used as fragrances, and amides as flavor and cosmetics components — coolants, sweeteners, and so forth, with some exceptions. Nitriles have long been known as more chemically stable odor alternatives for the corresponding aldehydes, but certainly have not been limited to this kind of application. The current aroma chemical market features about three dozen nitriles varying greatly in their chemical structure (examples are given in Figure 1). There are saturated and unsaturated aliphatic nitriles with both linear and branched carbon skeletons, cycloaliphatic and aromatic nitriles, and also those containing combinations of aromatic and aliphatic substituents. Parmanyl (Dragoco) represents an interesting “hybrid” of leaf alcohol and a nitrile molecule obtained by addition of cis-3-hexenol to acrylonitrile. Reflecting the growing demand for the optically active aroma chemicals, Takasago manufactures lcitronellyl nitrile. Firmenich patented a new family of nitriles containing a plinol-based fragment.

There is no universal synthetic approach to all classes of nitriles used in perfumery. One of the textbook methods for obtaining nitriles consists of the alkylation of cyanide salts with alkyl halogenides. Practical applicability of this method is limited not only by the notorious toxicity of the cyanides, but often by the commercial unavailability of starting alkyl halogenides. Another general method — ammonolysis of carboxylic acids, esters, or long-chain alcohols — is often a method of choice when the starting materials (acid, ester, alcohol) are readily available, inexpensive and relatively stable because the reaction conditions are rather harsh. For example, dodecanenitrile can be obtained in a high yield by the ammonolysis of lauric acid at 260°C over a titania/silica catalyst. Condensation methods lead to α,β-unsaturated nitriles. Thus, geranyl nitrile is obtained by the condensation of methyl heptenone with acetonitrile, a method, which might present a viable alternative to the oximation-dehydration route discussed below. Condensation of phenylacetonitrile with cyclohexanone produces Peonile (Givaudan).

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