An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group -SO2OH, are relatively stronger acids. The relative stability of the conjugate base of the acid determines its acidity. Other groups can also confer acidity, usually weakly: -OH, -SH, the enol group, and the phenol group. In biological systems, organic compounds containing only these groups are not generally referred to as organic acids.
A few common examples include:
In general, organic acids are weak acids and do not dissociate completely in water, whereas the strong mineral acids do. Lower-molecular-weight organic acids such as formic and lactic acids are miscible in water, but higher-molecular-weight organic acids such as benzoic acid are insoluble in molecular (neutral) form.
On the other hand, most organic acids are very soluble in organic solvents. p-Toluenesulfonic acid is a comparatively strong acid used in organic chemistry often because it is able to dissolve in the organic reaction solvent.
Exceptions to these solubility characteristics exist in the presence of other substituents that affect the polarity of the compound.
Simple organic acids like formic or acetic acids are used for oil and gas well stimulation treatments. These organic acids are much less reactive with metals than are strong mineral acids like hydrochloric acid (HCl) or mixtures of HCl and hydrofluoric acid (HF). For this reason, organic acids are used at high temperatures or when long contact times between acid and pipe are needed.
The conjugate bases of organic acids such as citrate and lactate are often used in biologically-compatible buffer solutions.
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