Organic acid

Lore & Background

Organic acids are organic compounds that exhibit acidic properties, most commonly through a carboxyl group (–COOH), as seen in carboxylic acids, though sulfonic acids (–SO2OH) are relatively stronger. Other groups such as thiol (–SH), enol, and phenol can also confer weak acidity. The strength of an organic acid is determined by the relative stability of its conjugate base. In general, these acids are weak and do not fully dissociate in water, unlike strong mineral acids. Lower molecular mass examples like formic and lactic acids mix readily with water, while higher molecular mass ones such as benzoic acid remain insoluble in their neutral form. Most organic acids, however, dissolve well in organic solvents; p-toluenesulfonic acid is a comparatively strong acid often used in organic chemistry due to its solubility in organic reaction solvents. Exceptions to these solubility patterns occur when other substituents alter the compound’s polarity. Organic acids are widely distributed in nature as normal constituents of plants and animal tissues, and they are also produced through microbial fermentation of carbohydrates, particularly in the large intestine. They may occur as sodium, potassium, or calcium salts, or even stronger double salts. In biological systems, compounds containing these acidic groups are generally referred to as organic acids.

Reader's Guide

Organic acids are significant for their role in food preservation, where undissociated acids penetrate bacterial cell walls and disrupt physiology, particularly in pH-sensitive bacteria such as Escherichia coli and Salmonella species. They are used in oil and gas well stimulation as less reactive alternatives to mineral acids, and in rust removal via dissolution of iron oxides and chelation of metal ions. In animal feeds, organic acids improve performance and reduce intestinal pathogens without the public health concerns of antibiotic growth promoters. Research has tested acetic, formic, and propionic acids for wood preservation, but leaching and chemical degradation limited effectiveness. Their conjugate bases, like citrate and lactate, are used in biologically compatible buffer solutions.

Did You Know?

• Organic acids are generally weak acids and do not dissociate completely in water, unlike strong mineral acids.
• Lower molecular mass organic acids such as formic and lactic acids are miscible in water, while higher molecular mass ones like benzoic acid are insoluble in neutral form.
• Organic acids are used in food preservation because undissociated acids can penetrate bacterial cell walls and disrupt normal physiology of pH-sensitive bacteria.
• Citric and oxalic acids are used for rust removal because they dissolve iron oxides without damaging the base metal as stronger mineral acids do.