Pyrolysis of thio- and isothio-cyanates. Part V. The pyrolysis of but-3-enyl and 1-methylprop-2-enyl isothiocyanates

Abstract

The homogeneous thermal elimination of isothiocyanic acid from but-3-enyl and 1-methylprop-2-enyl isothiocyanates has been investigated in the ranges 30–300 mmHg and 240–390 °C. The primary process is unimolecular, although secondary reactions of the products are more extensive than in the case of the isothiocyanates previously studied. The temperature dependences of the rate constants are given by equations (i) and (ii) for the but-3-enyl and 1-methylprop-2-enyl cases respectively. The increased reactivity brought about by unsaturation log₁₀k₁/s^–1= 12·40 ± 0·20 –(45,040 ± 320)/2·303 RT cal mol^–1(i), log₁₀k₁/s^–1= 11·84 ± 0·30 –(37,550 ± 750)/2·303 RT cal mol^–1(ii) in the carbon skeleton is due mainly to the allylic weakening of the carbon–nitrogen bond whereas conjugation with the forming double bond is unimportant.