Homoconjugation
In chemistry, homoconjugation has two meanings. Most commonly, it refers to the enhancement of the acidity of an acid by itself. The effect is mainly observed at high concentrations. IUPAC recommends the term "homoassociation",[1] but this term has not gained wide usage."[2] Homoconjugation also refers to "orbital overlap of two π-systems separated by a non-conjugating group, such as CH2.[1]
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 significantly stronger acid when concentrated than when dilute due to the following equilibria:
- 2 HF → H+ + HF2-
The bifluoride anion (HF2-) encourages the ionization of HF by stabilizing the F-. Thus, the usual ionization constant for hydrofluoric acid (10-3.15) understates the acidity of concentrated solutions of HF. Carboxylic acids and phenols also exhibit this effect.
The effect of homoconjugation is often high in nonaqueous solutions, wherein dissociation is often low.
References
- ↑ 1.0 1.1 http://goldbook.iupac.org/H02842.html
- ↑ 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.
