Microsynthesis of C¹⁴-Labelled Ethyl Iodide

Abstract

CARBON dioxide labelled with C¹⁴ can be converted in good overall yield to ethyl iodide by way of acetylene and ethylene on the scale of about 10 micromoles. Acetylene can be produced from carbon dioxide¹ in 90–95 per cent yield, and it may be reduced almost quantitatively to ethylene by the method of Traube^2,3, in which it is shaken for an hour with a large excess of freshly prepared chromous chloride solution at room temperature. The chromous chloride solution is prepared by the rapid reduction of a solution of 1 part green chromic chloride hexahydrate in 2 parts 25 per cent hydrochloric acid with zinc wool, until the solution turns almost a copper sulphate blue. About 0·4 ml. of this solution is pipetted into a glass reaction vessel (see diagram), frozen in liquid nitrogen and evacuated. The acetylene is condensed into the tube which is then sealed off at A and allowed to warm to room temperature. After agitation for an hour, carried out by intermittent rotation about its axis of the reaction vessel held horizontally, the vessel is sealed on to the high vacuum apparatus at B and the break-seal opened. All the dissolved gas is boiled from the chromous chloride solution, and ethylene condensed at liquid nitrogen temperature into a trap connected with a McLeod gauge, water and any residual acetylene being held back in a U-tube at liquid oxygen temperature. The yield in terms of standard gas volume is 96-98 per cent in this stage, and the remaining few per cent of acetylene can be recovered from the oxygen-cooled trap.