Salicylaldehyde (HO-C6H4-CHO) is the common name for 2-hydroxybenzaldehyde, an oily organic liquid that has the odor of buckwheat. It is a simple derivative of the hexagonal ring compound benzene (C6H6). One of benzene’s hydrogen atoms is replaced by an aldehyde group (-CHO). A hydroxyl group (-OH) replaces another hydrogen atom attached to an adjacent carbon. Thanks in part to these reactive side groups and their “one-two” location on the benzene ring, salicylaldehyde is a powerful tool in chelation chemistry and in ring-generating condensation chemistry.
The most important use of salicylaldehyde involves chelates — molecules that act, as the name suggests, like crab claws. One or more aldehyde or derivative molecules attaches to a target metal atom through coordination bonding, which occurs when both electrons are provided by just one atom. For salicylaldehyde copper chelate, the four oxygen atoms of the two aldehyde molecules symmetrically surround, then attach to, a lone copper atom. Metal chelates, partly because of their stability in unfavorable environments, are used in fertilizers.
In some instances, the chelate itself is not what is commercially desired. Rather, it is the ability to form chelates at the point of use that is valuable. When this is the situation, salicylaldehyde is almost always chemically modified before use. A particular derivative is formed and is designed to be applicable in a very specific situation. One example is salicylaldehyde isonicotinoyl hydrazone, which has been proven capable of reducing dangerous levels in a not uncommon heart malady called catecholamine cardiotoxicity.
Another chelation use of the aldehyde requires reacting it first with hydroxylamine to form salicylaldoxime. This oxime of the aldehyde is practical to the analytical chemist because it generates brilliant and distinct colors in the presence of transition metal ions. Transition metals are those metals whose d-subshell of orbital electrons is incomplete and include such metals as nickel, iron, chromium, copper and cobalt. Insoluble transition metal complexes are formed and are then collected, dried and quantified through appropriate laboratory procedures. A similar titration procedure employs the phenylhydrazone to quantify organometallic reagents.
Other important, non-chelate related organic syntheses using salicylaldehyde include condensation reactions. Condensation reactions combine two or more molecules to form larger, more complex species. One example is the reaction of the aldehyde with ethyl malonate in the presence of other minor reactants to form the aldol condensation product 3-carbethoxycoumarin, a two-ring structure. Not all reactions employing salicylaldehyde create more complex species; some, such as the oxidation to catechol, are simple conversion reactions.