There are some chemicals that most people agree smell bad. Butyric acid (found in rotting butter), isooctane (the main component of gasoline), and ammonia all fit this description. But some subtances have such an awful odor that they end up in a class of their own. Perhaps regrettably, some of these are also important synthetically. Such is this week's Molecule of the Week: putrescine.

The structure of putrescine is displayed in Figure 1. Note the resemblance to the molecule ethylene diamine. The two molecules are close relatives, though the latter does not smell nearly as rotten as the former.

Putrescine has the smell of rotting flesh (and is indeed largely responsible for the smell of corpses). It is produced in living cells by the three-step degradation of the naturally-occurring amino acid arginine (see the amino acid table for information on all the naturally-occurring amino acids). The degradative pathway, and the associated enzymes, are displayed in Figure 2.

Putrescine is useful chemically in many of the same roles as its aforementioned relative, ethylene diamine. A brief segue into the organic chemistry of alkylamines is thus warranted.

Most nitrogen-containing molecules are referred to as amines. Those that contain single-bonds to carbon are alkylamines. There is a lone pair of electrons on the nitrogen atom. This lone pair is available for bonding, though such a bond would (formally) give the nitrogen a positive charge. If a hydrogen atom is bound to the nitrogen it can be shed to give a neutral molecule in the event of new bond formation. This makes an alkylamine a good nucleophile, a molecule that tends to form bonds with positive or partially-positive atoms, such as the carbon in alkyl halides. This permits reactions such as that shown in Figure 3.

Alkylamines react with electrophiles other than alkyl halides. For example, treatment of properly designed ("activated") esters or carbonates with alkyl amines readily gives alkylamides. An example reaction is displayed in Figure 4 (without the electron-pushing used in Figure 3).

Note that in both example reactions the side-products (triethylammonium bromide in Figure 3, p-nitrophenol in Figure 4) are much more water-soluble than the desired products. This allows the products to be purified with relative ease.

Clearly alkylamines are of great synthetic utility. Molecules that have two amino groups (such as putrescine and ethylene diamine) are especially useful as molecular linkers since they can form new bonds at both ends. Certain diamines, especially the simple ethylene diamine, find uses in plastics manufacturing and other large-scale industrial processes.

Fortunately for chemists, in many applications requiring diamines, ethylene diamine can be used. Ethylene diamine is cheap (especially in large quantities) and not much more putrid than ammonia. Where longer chains are needed, however, the undesirable putrescine is available.