7445
A Method for Determining Relative Rates of Reaction of Halogenating Agents with Enols N. C. Deno* and Richard Fishbein Contributionf r o m the Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802. Received M a y 21,1973 Abstract: The variation in ratios of 3-halo :I-halo product has been studied as a function of the per cent H2S04 for the reaction of 2-butanone with CI2,Br2,and two N-chloro amines. In dilute acid, all give the same ratio (-2.6) confirming that enolization of the 2-butdnone is rate determining. The ratio shifts to a new limiting value as the per cent H2SO4is increased because the reaction of enol with halogenating agent becomes rate determining. A method for determining relative rates of reaction of halogenating agents with enols is developed.
I
t is well established that the a-chlorination and a-bromination of ketones in dilute acids proceeds by a rate-determining en~lization.'-~ The rate law is eq 1. It is evident that the rate law could change to eq 2 (rate-determining reaction of enol with halogenating d[product]/dt = kl[ketone][H+] (1) d[product]dt = k2[ketone][halogenatingagent] ( 2 ) agent) if (1) an inactive halogenating agent were used whose rate of attack on enol was slow, (2) the concentration of halogenating agent was made very small,* and (3) the level of acidity was increased to make the keto-enol equilibration rapid. The changeover in mechanism from eq 1 to eq 2 would be signaled by a change in the ratio of the two a-halo products from one limiting value to a new limiting value. For the chlorination and bromination of 2-butanone with Cla and Bra, this phenomenon has been observed for changing halogen concentration and for varying H z S O ~concentration. Table I shows the variation in the ratio of 3-halo: Table I. Chlorination and Bromination of 2-Butanone at 25 O in Aqueous Sulfuric Acidsa
Z His01
3-CI :1-C1
Z His01
3-Br :1-Br
10 30 32 35 40 50 80 96
2.16 3.0 4.8 5.3 9.0 10.1 8.1 9.1
10 20 40 50 60 70
2.4c 2.4 4.0 7.3 11.5 11.5
a The initial concentration of 2-butanone was 0.025 M and the initial concentrations of Cl2 and Br2 were 0.0125 M . * A ratio of 2.6 was reported for chlorination in aqueous HCL3 A ratio of 2.7 was reported for bromination in dilute acid: H. M. Cardwell and A. E. Kilner, J. Chem. SOC.,2430 (1951).
I-halo product as the per cent HzSOr is varied. At low acidity (10% H2S04)the ratio of 3-C1: I-C1 and the ratio of 3-Br: I-Br were effectively equal (2.1 and 2.4), confirming that the reaction was a rate-determining (1) A.Lapworth, J . Chem. SOC.,85.30(1904). (2) R. P. Bell and I90% pure.
+
Acknowledgment. Grateful acknowledgment is made to the National Science Foundation and the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. (9) Applied Science Labs., State College, Pa. (10) F. Minisci, R . Bernardi, and R. Galli, J . Chem. Soc. B, 324 (1968). (1 1) R. S . Neale and N. L. Marcus, J . Org. Chrm., 34, 1808 ( 1 969).
1 95:22 1 October 31, 1973
