Hydrolysis of aryl and alkyl isothiocyanates in aqueous perchloric acid

Abstract

The slow hydrolysis of aromatic and aliphatic isothiocyanates in water is promoted by added perchloric acid. The hydrolysis leads first to the thiocarbamic acid, but this species decomposes rapidly to the (protonated) amine, and is not normally detected. Convenient rates of hydrolysis are obtained at 50 °C when [HClO₄]≳ 6.0 mol dm^–3. The effects of substituents, temperature, and acid concentration on the observed rate constant have been studied. Aliphatic isothiocyanates are somewaht more reactive than aromatic derivatives, but the effect of substituent changes is generally small, with electron release favouring reaction. Substituents close to the nitrogen atom hinder reaction. The value of ΔS^‡ is typically –120 to –220 J K^–1 mol^–1, and analysis of the acidity dependence by the excess acidity approach shows m^‡≃ 0.8. Addition of water to the isothiocyanate NC double bond via a mechanism invloving simultaneous proton transfer to nitrogen and nucleophilic attack by water at carbon with a cyclic transition state is proposed. The carbamic acids formed by the aliphatic isothiocyanates are sufficiently basic for them to be increasingly trapped as their protonated forms when [HClO₄] > 9.0 mol dm^–3.