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| In [[acid–base]] [[chemistry]], '''homoassociation''' (an [[IUPAC]] term)<ref name=IUPAC>http://goldbook.iupac.org/H02842.html</ref> is an association between a [[base (chemistry)|base]] and its [[conjugate acid]] through a [[hydrogen bond]]. The alternate term ''homoconjugation'' also has wide usage,<ref>Izutsu, K., "Acid-Base Dissociation Constants in Dipolar Aprotic Solvents," Blackwell Scientific Publications: Oxford, U.K., 1990; </ref> but is ambiguous because it has another meaning in [[organic chemistry]] (see [[Conjugated system#Mechanism]]).
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| Most commonly, homoassociation leads to the enhancement of the [[acid dissociation constant|acidity]] of an acid by itself. The effect is accentuated at high [[concentration]]s, i.e. the ionization of an acid varies nonlinearly with concentration. This effect arises from the stabilization of the [[conjugate base]] by its formation of a hydrogen bond to the parent acid. A well known case is [[hydrofluoric acid]], which is a significantly stronger acid when concentrated than when dilute due to the following equilibria:
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| :2 HF <math>\rightleftharpoons</math> H<sub>2</sub>F<sup>+</sup> + F<sup>−</sup> ([[Molecular autoionization|autoionization of HF]]) | |
| :HF + F<sup>−</sup> <math>\rightleftharpoons</math> HF<sub>2</sub><sup>-</sup> (homoassociation)
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| Overall:
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| :3 HF <math>\rightleftharpoons</math> HF<sub>2</sub><sup>-</sup> + H<sub>2</sub>F<sup>+</sup>
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| The [[bifluoride]] anion (HF<sub>2</sub><sup>−</sup>) encourages the ionization of HF by stabilizing the F<sup>−</sup>. Thus, the usual ionization constant for hydrofluoric acid (10<sup>−3.15</sup>) understates the acidity of concentrated solutions of HF.
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| The effect of homoassociation is often high in nonaqueous [[solution]]s, wherein dissociation is often low. [[Carboxylic acid]]s and [[phenol]]s exhibit this effect.<ref>See for example Agnes Kütt, Ivo Leito, Ivari Kaljurand, Lilli Sooväli, Vladislav M. Vlasov, Lev M. Yagupolskii, and Ilmar A. Koppel "A Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Brønsted Acids in Acetonitrile" J. Org. Chem., 2006, volume 71, pp 2829–2838. {{DOI|10.1021/jo060031y}},</ref>
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| ==References==
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| <references/>
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| [[Category:Equilibrium chemistry]]
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| [[Category:Acid-base chemistry]]
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