Paramagnetic Resonance Study of Liquids during Photolysis. IV. Free Radicals from Acetaldehyde, Diacetyl, and Acetoin

Paramagnetic resonance spectra in solutions at room temperature were studied during irradiation with ultraviolet light. The radical CH3ĊOHCOCH3, the acetoin radical, was produced from diacetyl in the presence of good H-atom donors. Using p-dioxane as the donor, the radical produced from the donor was also seen. The acetoin radical and CH3ĊHOH were produced from acetaldehyde in the absence of a good H donor. By adding a good donor to acetaldehyde solutions, the yield of the acetoin radical was suppressed, and the radical produced from the H-atom donor appeared. Reasonable reactions are proposed. In nearly neutral aqueous solutions or in slightly alkaline isopropyl alcohol solution, the acetoin radical was converted to the anion CH3COCOCH3−. The parameters of the acetoin radical and of its anion were measured in a number of solutions. The radical CH3ĊHOH was produced from acetoin in a variety of solutions, showing that cleavage of the acetoin molecules takes place. The spectrum of the acetoin radical underwent striking changes as acid was added to the solution. Spectra were calculated for a simple, reasonable exchange scheme. Calculated and observed spectra agreed very well for a large range of acid concentrations.