 Dicarboxylic acid

A dicarboxylic acid is a carboxylic acid containing two carboxyl groups. The groups may be attached to an aliphatic framework, as in malonic acid, or an aromatic framework, as in terephthalic acid.^[ref] The definition counts functional groups rather than specifying chain length, ring substitution or acid strength.

A dicarboxylic acid can supply two acidic protons through its carboxyl groups. In the simplest diprotic acid model, write the successive equilibriaThe two acid dissociation constants $K_1$ and $K_2$ characterize different equilibria. Two chemically similar groups do not justify treating the two proton-removal steps as identical. Other ionizable groups, if present, require additional equilibria.

For an original ideal dilute-solution example, use concentration-form constants $K_1=10^{-3}\mathrm{mol}{\mathrm{L}}^{-1}$, $K_2=10^{-5}\mathrm{mol}{\mathrm{L}}^{-1}$ and hydrogen-ion concentration $H=10^{-4}\mathrm{mol}{\mathrm{L}}^{-1}$. Mass balance and the two equilibrium relations give species fractionsHere $({\alpha }_0,{\alpha }_1,{\alpha }_2)=(1,10,1)/12$. The average charge is $-1$ per acid-derived species even though only $5/6$ of the molecules are monoanions: equal neutral and dianion populations balance around that mean. The constants are stipulated for illustration, not assigned to a particular compound. Precise thermodynamic calculations use activities rather than equating them with concentrations.

A titration that fully removes both carboxyl protons consumes two moles of hydroxide per mole of initially neutral dicarboxylic acid, in the absence of other reactions. For example, 0.50 mmol requires 1.00 mmol hydroxide by stoichiometry. Two stoichiometric equivalents do not guarantee two separately resolved features on a titration curve; the acid dissociation constants and measurement conditions determine their separation.

Two carboxyl groups also provide two possible connections in esterification. Terephthalic acid and a diol can therefore build a polyester, illustrating how the same functional-group count matters both to acid-base measurements and to polymer connectivity.^[ref]