Concerning the Products of the Reaction of Methyl Bromide and Ethyl Bromide with Potassium Hydroxide in Aqueous Methanolic Solutions and the Progress of this SN2-Reaction

Article abstract of DOI:10.1002/prac.19983400111

Investigations of the reaction of methyl bromide and ethyl bromide with potassium hydroxide in methanolic and aqueous methanolic solutions show that the main products of these reactions are dimethyl ether and ethylmethyl ether. The reaction rates measured in methanolic or aqueous methanolic solutions are the same whether potassium hydroxide or potassium methoxide are used. These results are caused by an equilibrium between hydroxide and methoxide ions with which we could establish the equilibrium constant near 0.6. This means that a solution of sodium hydroxide c=0.1 moll^-1 in methanol contains roughly 99.8% of methoxide ions. The reaction rates in methanolic as well as in aqueous methanolic solutions are strict second order. The reaction rate measured at several temperatures permitted the calculation of E_A≠, ΔH≠, ΔS^≠ and ΔG^≠. Furthermore the kinetic investigations show that the nucleophilicity of methoxide ions is lower compared to hydroxide ions. The calculation of the Swain-Scott-parameter n results in a nucleophilicity scale in order to methoxide, hydroxide, ethoxide ions. The kinetic investigations of the reaction of ethyl bromide with methoxide and hydroxide ions in methanolic solutions demonstrate that at high temperatures the rate constant of methoxide ions is higher than that of hydroxide ions. The opposite case can be observed at lower temperatures. At the temperature of 20°C the rate constants of both reactions are equal. This is to do with the isokinetic effect which one is rarely able to observe at room temperatures.