20. The Variety of Organic         Compounds   Previous PageNext Page
        Carboxylic Acids

Formic acid is one of the main irritants in ant and bee stings, and acetic acid is familiar in vinegar. Propionic acid, which is porduced by certain strains of bacteria, is responsible for the characteristic odor and taste of Swiss cheese, and butyric acid causes the smell of rancid butter. Caproic acid has a ripe goat smell. Valeric acid has the odor that we associate with Limburger cheese and manure, although it first was extracted from the root of the valerian or heliotrope plant. It once was considered medicinally valuable as a sedative, but it is not clear whether its value was physiological or psychological, in terms of the threat of a second dose.

Because one hydrogen atom in a carboxylic acid is attached to the more electronegative oxygen atom rather than to a carbon, these acids are polar and capable of forming extensive hydrogen-bonded networks. They are acids because this same proton dissociates in aqueous solution (right). The explanation of the ready dissociation is similar to that for carbonic acid and phenol. In undissociated acid the proton is bound to one oxygen, and a double bond extends from the carbon to the other oxygen. In the carboxyl ion the electrons of that double bond and the formerly bonding electron pair of the departed proton all are delocalized throughout the entire O-C-O group. This delocalization stabilizes the ion and shifts the equilibrium in favour of dissociation.

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