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Dicarboxylic Acids: Examples, Properties and Uses

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Dicarboxylic acids are organic compounds carrying two carboxyl (-COOH) groups in one molecule, with the general structure HOOC-R-COOH. The simplest members are oxalic acid (the two groups joined directly), malonic, succinic, glutaric, and adipic acids, plus the aromatic phthalic acid.

They matter far beyond the laboratory. Adipic acid polymerises with hexamethylenediamine into nylon-6,6, terephthalic acid into PET bottles and polyester, and phthalic anhydride into plasticisers and resins. Several also occur in metabolism - succinic and malic acids are Krebs cycle intermediates - so they link organic chemistry to biochemistry and industry.

Key Concepts

  • Nomenclature: named as alkanedioic acids (ethanedioic = oxalic, butanedioic = succinic, hexanedioic = adipic); aromatic ones keep common names like phthalic acid.
  • Stepwise acidity: the first proton (Ka1) is lost more easily than the second (Ka2), because removing a proton from an already negative ion is harder.
  • Cyclic anhydrides: on heating, succinic and glutaric acids lose water to form five- and six-membered cyclic anhydrides; adipic acid instead gives a cyclic ketone.
  • Dibasic neutralisation: one mole of dicarboxylic acid needs two moles of NaOH for complete neutralisation.
  • Polycondensation: dicarboxylic acids react with diols or diamines, splitting out water at each link, to build polyesters and polyamides.

Formulas

  • Neutralisation: HOOC-R-COOH + 2NaOH --> NaOOC-R-COONa + 2H2O.
  • Anhydride formation: HOOC-CH2-CH2-COOH --> (CH2CO)2O + H2O (succinic anhydride).
  • Nylon-6,6: n HOOC-(CH2)4-COOH + n H2N-(CH2)6-NH2 --> [-OC-(CH2)4-CO-NH-(CH2)6-NH-]n + 2n H2O.
  • These assume pure reagents; in industry a slight excess of one monomer controls the chain length.

Worked Example

A 0.63 g sample of oxalic acid dihydrate, (COOH)2.2H2O (molar mass 126 g/mol), is titrated with 0.1 M NaOH. Moles of acid = 0.63 / 126 = 0.005 mol. Being dibasic, it needs twice as many moles of NaOH: 0.010 mol. Volume of 0.1 M NaOH = 0.010 / 0.1 = 0.10 L, i.e. 100 mL. If the burette reads near 100 mL the sample is confirmed pure; a much lower reading would reveal an impurity or a weaker acid.

Common Misconceptions

  • A dicarboxylic acid is exactly twice as strong as a monocarboxylic acid. Strength is set by Ka1 alone, and the second proton comes off weakly - the solution is nowhere near twice as acidic as acetic acid at the same concentration.
  • Every dicarboxylic acid forms a cyclic anhydride on heating. Only five- and six-membered rings form easily, so succinic and glutaric acids do; oxalic and malonic acids decompose to carbon oxides instead.
  • Dicarboxylic acids are laboratory curiosities. Adipic acid is made on a scale of millions of tonnes a year for nylon, and citric acid, a tricarboxylic cousin, flavours soft drinks worldwide.

Real-World Applications

  • Adipic acid plus hexamethylenediamine makes nylon-6,6 for textiles, carpets, and engineering plastics.
  • Terephthalic acid plus ethylene glycol makes PET for bottles and polyester fibre.
  • Phthalic anhydride makes plasticisers that keep PVC flexible and alkyd resins for paints.
  • Oxalic acid removes rust and cleans marble; succinic acid is a food additive and drug intermediate.

Dicarboxylic acids pack two acid groups into one molecule, giving stepwise acidity, anhydride chemistry, and polymer-forming power. From nylon and PET to Krebs cycle intermediates, they show how a small structural change - one extra -COOH - creates an entire industrial chemistry.

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