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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]

Malonic acid is propanedioic acid, a dicarboxylic acid in which two carboxyl groups are separated by one methylene group:Its neutral molecular formula is ${\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_4$ and its average molar mass is approximately $104.061\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$.^[ref]

Two of the four hydrogen atoms belong to carboxyl OH groups; the other two belong to the central carbon. Complete neutralization of both carboxyl groups therefore requires two equivalents of a suitable base, not four merely because the formula contains four hydrogens. An original stoichiometric model isFor $1.00\mathrm{mmol}$ acid, the two-equivalent amount of $0.100\mathrm{mol}{\mathrm{L}}^{-1}$ hydroxide solution is $20.0\mathrm{mL}$. The pH at an experimental endpoint also depends on acid-base equilibria and the measurement method; the equivalent count alone does not determine it. Malonate names the corresponding deprotonated system, whose protonation state should be specified.

Heating can promote decarboxylation, producing acetic acid and carbon dioxide. Malonic-acid derivatives underlie the carbon-chain construction described in the malonic ester synthesis.^[ref] For the unsubstituted acid, the stoichiometric balance isAn original ideal mass balance for $1.00\mathrm{mmol}$ converted by only this reaction gives about $60.052\mathrm{mg}$ acetic acid and $44.009\mathrm{mg}$ carbon dioxide from $104.061\mathrm{mg}$ starting material. Following evolved gas and residual material would test conversion, whereas a change in sample mass alone would not distinguish this reaction from evaporation or another decomposition pathway.

Terephthalic acid is benzene-1,4-dicarboxylic acid, an aromatic dicarboxylic acid with two carboxyl groups attached at opposite positions of a benzene ring. Its neutral molecular formula is ${\mathrm{C}}_8{\mathrm{H}}_6{\mathrm{O}}_4$. The positions of the groups are part of its identity, not information recoverable from the elemental formula alone.^[ref]

The two carboxyl groups make it a useful difunctional building block. Reaction with ethylene glycol, which has two alcohol groups, forms ester connections in polyethylene terephthalate, a polyester. This is step-growth polymerization: growing molecules react through their remaining functional groups.^[ref]

An original repeat-unit bookkeeping calculation isUsing rounded masses, $166.13+62.07-2(18.015)=192.17\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ per repeat-unit contribution. The subtraction represents two esterification connections per repeat in the long-chain description; it is not an elementary reaction equation for one isolated pair of monomers forming a complete polymer.

For a finite linear chain assembled from $n$ acid molecules and $n$ glycol molecules, with no ring closure or side reaction, there are $2n-1$ connections. The chain therefore loses $2n-1$ water molecules, rather than $2n$. End groups supply the difference: its mass is approximately $n(192.17)+18.015\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ under the same rounded-mass convention.

The molar mass and end-group balance consequently matter alongside conversion. Measuring consumption of acid groups and formation of ester connections tests the reaction model; identifying an aromatic diacid does not determine the final chain length or material properties.