NOTES
1323
+
OH
/I
OH
I I +
R-C-C-R+R-C-C-Rz
II
HO
+
HO
+
OH
Figure 2. First-derivative esr spectrum of t h e paramagnetic reduction p r o d w t of 5 mg of biacetyl in 0.75 ml of concentrated sulfuric acid solution to which had been added 22 mg of sodium dithionite. T h e low-field multiplet is attributed t o t h e biacetyl radical cation. T h e high-field multiplet increases in relative intensity as the sulfuric mid is diluted with water.
I R-e-C-RfR-C-C-R I + HO
OH
I/ 11
HO
+
paramagnetic species is not the time-averaged mixture
vb-vc.
Empirical spin densities can be calculated for the carbonyl carbon atoms of the biacetyl radical anion and cation by the equation5 aH = pcB cos2 8, wherein dithionite. In 98% sulfuric acid, the esr signal shows cos2% is taken as equal to 0.5 for a freely rotating methyl little decay in 30 min. group. A value of B = 40 seems most appropriate A biacetyl radical cation has been reported by for radical anions.6 For radical cations a larger value Steven and Ward from oxidation of acetoin by titanous of B is indicated.' This leads to empirical estimates of ion in acidified hydrogen peroxide (pH ~ l ) In. ~ pc = 0.28 for the carbonyl carbon atom for the trans view of the present results it seems most likely that radical anion in dimethyl sulfoxide and pc
