A radiochemical study of the pyrolytic formation of carbon suboxide in the diethyl oxaloacetate–acetic anhydride system: isotopic exchange between the products

Abstract

Diethyl [1,2-¹⁴C]–, [3-¹⁴C]–, [4-¹⁴C]–, [1,4-¹⁴C]–, [2,3-¹⁴C]–, [1′,1″-¹⁴C]–, and [2′,2″-¹⁴C]–oxaloacetate (1 mol.) have been pyrolysed in acetic anhydride (4 mol.) at 700°, and the position and extent of labelling in the resulting carbon suboxide traced. A radiochemical balance between the labelled esters and the carbon suboxide is not achieved; the deficiencies amount to 32–34% for the carbonyl carbons and 30% for the central carbon. Deficiencies are still more pronounced when a longer furnace is used. Pyrolysis of unlabelled diethyl oxaloacetate (1 mol.) in the presence of [1-¹⁴C]acetic anhydride (4 mol.) causes transference of label (27%) to each carbonyl group of carbon suboxide but not to the central carbon; [2-¹⁴C]acetic anhydride causes no transference of label to the carbonyl groups but 29% to the central carbon of carbon suboxide. This accounts for the deficiencies. [1-¹⁴C]Propionic anhydride, when similarly pyrolysed with diethyl oxaloacetate, labels (13%) of the carbonyl groups of carbon suboxide but not the central carbon; with [2-¹⁴C]propionic anhydride there is negligible labelling of all the carbon suboxide carbons. Carbon suboxide itself exchanges radioactivity with [1-¹⁴C]acetic anhydride at 700° and [1-¹⁴C]keten is implicated. A mechanism based on the reversible formation of two types of mixed ‘dimer’ is proposed.

These results indicate that earlier experimental work, which seemingly supported a cyclic heteryne intermediate in this system, was misinterpreted through failure to recognise the exchange reaction: C-1 is lost entirely during the formation of carbon suboxide from diethyl oxaloacetate. The mass spectrum of diethyl oxaloacetate is discussed.