AS & A-Level Chemistry 16 — Acyl compounds, nitrogen chemistry and polymers

The following trend concerns representative simple aqueous examples such as ethanoic acid, phenol and ethanol; other substituents can change the order. The relative acidity of organic hydroxy compounds is fundamentally determined by the stability of the conjugate base formed after a proton is lost. The general order of decreasing acidity is: carboxylic acids > phenols > water > alcohols. Carboxylic acids are the strongest of these acids because the resulting carboxylate ion is highly stabilised by the symmetrical delocalisation of the negative charge over two electronegative oxygen atoms. Phenols are weaker acids than carboxylic acids, but they are stronger than water and alcohols. The phenoxide ion is partially stabilised because one of the lone pairs on the oxygen atom delocalises into the extensive pi-electron system of the benzene ring, spreading the negative charge. Alcohols are extremely weak acids because the alkyl group exerts an electron-donating inductive effect, which concentrates the negative charge on the alkoxide oxygen atom, making the ion highly unstable and prone to accepting a proton.

The acidity of a carboxylic acid can be increased by substituting electron-withdrawing groups onto the alkyl chain. Highly electronegative atoms, such as chlorine, exert a negative inductive effect. This withdraws electron density away from the $\text{COO}^-$ group through the sigma bonds, dispersing the negative charge and further stabilising the carboxylate ion. Trichloroethanoic acid is, therefore, a stronger acid than chloroethanoic acid, which is stronger than ethanoic acid. Furthermore, the closer the electronegative substituent is to the carboxyl group, the stronger its stabilising inductive effect on the conjugate base.