Carbonium ion reactions

Abstract

A summary is given of the evidence for the existence of the triphenylmethyl carbonium ion in acid solutions and in inert solvents containing Friedel-Crafts catalysts. The reactions of this ion are listed, especially those with phenols. The reaction of triphenylcarbinol with phenols in acetic acid-sulphuric acid mixtures to form hydroxytetra-phenylmethanes (the Baeyer-Villager reaction) has been studied. The reaction Ph3COH + o-cresol → 4-OH, 3-CH3tetraphenylmethane + H2O has been used mainly. Methods of analysis for the reactants have been worked out. It was found that the reaction was second order and k2 increased with acid strength. The ionisation of the carbinol has been measured in the media used and it was found that in acids containing 0-20% H2S04 and a constant 5% H2O, k2 varied with the percentage ionisation of the carbinol, indicating that the triphenylmethyl carbonium ion was the active entity of the reaction. In acids containing > 20% H2SO4 this relationship does not hold, chiefly because the o-cresol reacts with the media, and partly because of the appearance of the reverse reaction H+ + Ph3CC6H3(OH)CH3 → Ph3C+ + C6H4(OH)CH3. By the use of deuterated o-cresol it was established that the reaction between o-cresol and triphenylcarbinol takes place in two steps, the first being rate-determining. [equation]. The energy of activation and the frequency factor of the reaction have been determined. The effect of the triphenylmethyl carbonium ion on the stannic chloride catalysed polymerisation of styrene was studied before the main work was begun. It was hoped that Ph3C would provide active centres for the initiation of polymerisation, as polymerisation is known to proceed through a carbonium ion mechanism under these conditions. Unfortunately the presence of Phc3C retarded the reaction. A short report of this work appears at the end of this thesis.