[CONTRIBUTION FROM
2167
PROTODEBORONATION OF ARENEBORONIC ACIDS
May 5, 1961 THE
DEPARTMENT OF CHEMISTRY, UNIVERSITY
O F NEW
HAMPSHIRE, DURHAM, N. H.]
Electrophilic Displacement Reactions. XII. Substituent Effects in the Protodeboronation of Areneboronic Acids'" BY K. V. NAHABEDIAN AND HENRYG. KUIVILA RECEIVED NOVEMBER 2, 1960 Kinetic studies on the hydrolysis of nine areneboronic acids in aqueous sulfuric and phosphoric acids are described. Dependence of rate on acidity has been examined in each case, and activation parameters and solvent hydrogen isotope effects have been determined in certain cases, Conventional HOplots reveal the presence of two kinetically distinct regions separated by the HOrange 5.0-5.5. The behavior of activation parameters and solvent isotope effects bear out this dichotomy. Consideration of these facts, coupled with the effect of substituents on reactivity, leads t o an interpretation of the data in terms of the existence of a t least two mechanisms for the reaction.
In the preceding paperslav4it was established that the hydrolysis of p-methoxybenzeneboronic acid (eq. 1, X = p-OCH3) and of 2,6-dimethoxybenzeneboronic acid are subject to general acid catalysis, and i t was proposed that the reaction occurs by the A - S E ~mechanism. According t o this mechanism
the proton transfer occurs in the rate-determining step, and is followed by a rapid ionic cleavage of the boron-carbon bond. Because of its intrinsic interest and the likelihood that i t would provide further insights into the mechanism, a kinetic study of the hydrolysis of eight additional areneboronic acids in aqueous sulfuric acid has been made. For seven of these substrates (X = p-CH2, p-F, H , p-Br, m-F, m-C1 and m-h'02) the dependence of rate on the acidity function, Hc, has been determined, and for four (X = P-CH,, p-F, H and m-F) the dependence on temperature. The solvent hydrogen isotope effect has been measured for four of the substrates (X = p-CH,, p-OCH3, p-F and m-F). Rate measurements for the hydrolysis of p-tolueneboronic acid in aqueous phosphoric acid a t two temperatures have also been made. Experimental Reagents.-The preparation and properties of all but one of the areneboronic acids have been referred to previously.6 m-Trifluoromethylbenzeneboronic anhydride was prepared by the method of Bean and Johnsona in 30% yield; 1n.p. sinter 60°, m. 160-162' (acid), 160-162" (anhydride). Anal. Calcd. for C7HhBF80: C, 48.91; H, 2.35; neut. equiv., 171.9. Found: C, 49.03; H, 2.59; neut. equiv., 171.8. The preparation of deuteriosulfuric acid has been described." All of the other reagents were of the best grade available commercially. (1) (a) Preceding paper in this series: H. G. Kuivila and K. V. Nahabedian, J . A m . Cheiit. SOC.,83, 2164 (1961). (b) Presented in part at the 12th Meeting of the American Chemical Society, Chicago, Ill., September, 1958, Abstracts, p. 37p. (2) Based on the doctoral dissertation of K. V. Nahahedian, June, 1959. (3) This research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command, under Contract No. A P 49 (638)-312. Reproduction in whole or in part is permitted for any purpose of the United States Government. (4) H. G. Kuivila and K. V. Nahabedian, J . A m . Chem. Soc., 83, 2159 (1961). (5) (a) H.G. Kuivila and E. K. Easterbrook, ibid., 78, 4629 (1951); (b) H. G. Kuivila and A. R. Hendrickson, i b i d . , 74, 5068 (1952); H. G. Kuivila and A. G. Armour, i b i d . , 79, 5659 (1957). (6) F. R. Bean and J. R. Johnson, i b i d . , 64, 4415 (1932).
Kinetic Procedure.-Since each of the boronic acids has an ultraviolet absorption spectrum substantially different from that of its hydrolysis product, the concentration of unreacted boronic acid could be determined spectrophotometrically, a Beckman DU spectrophotometer being used. The absorptivities of the boronic acids and their hydrolysis products a t the wave lengths used for analysis are listed in Table I. I n all cases but one ( X = m-NOz) the absorptivities of boronic acids are much greater than those of the hydrolysis products. Therefore the absorbance of the kinetic sample could be taken as a direct measure of boronic acid concentration, C, of the sample. For X = m-NO2 the difference is small; therefore absorbances were converted to concentrations using the relationship C = (absorbance -2210Co)/(4250-2210), where COis the initial concentration of the substrate. Initial concentrations of boronic M. The procedure was acid were in the range lO-a-IO-* essentially that described previously.h
TABLE I SPECTRAL DATAUSEDFOR ANALYSISOF KINETICS SOLUTIONS X
Wave length, mp
-Absorptivity"XCaH4B(OH)z
m-NO2 p-Br m-F P-F
218 228 232 218 218
8450 4250 13'700 7300 7380
H
XCsHk
55* 2210 50 25 25
P-C& P-OCH,
226 10800 30 236 28200" 238 12000d 70d m-C1 228 3000 63 40 m-CFs 220 5200 In In 10-1470 sulfuric acid unless otherwise stated. 7670 sulfuric acid. I n water. I n 1%formic acid.
Results and Discussion
A. Kinetic Order of the Reaction.-Except
for the one run mentioned below, all of the rate experiments reported here showed first-order kinetics; that is, the data fit the rate equation kt = 2.303 log C
+- constant
where k is the pseudo-first-order rate coeficient and C is the concentration of areneboronic acid at time t . Figure 1 shows a typical rate plot for nznitrobenzeneboronic acid and Tables I1 and 111 list experimental values of log k obtained in aqueous sulfuric and phosphoric acids, respectively. B. Course of the Reaction.-In aqueous sulfuric acid, especially in the more concentrated solutions, sulfonation is a possible side reaction. Gold and Satchell' have reported that a t 25' the pseudofirst-order rate coefficient for the sulfonation of benzene in 77.5'35 H2S04 is 2.6 X set.-'. (7) V. Gold and D. P. N. Satchell, J . Chcm. SOC.,1635 (1956).
K. V. NAHABEDIAN AND HENRY G. KUIVILA
2168
TABLE I1 PSEUDO-FIRST-ORDER RATE COEFFICIEKTS, k, OUS
SULFURIC ACID
18-21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 39 40 41 42
60
43 44 45
25
46 47 48
40
11,- 3 4 5 6
60
I
8 9 10 11 12 13 14 15 16
60
111,- 1 2 3 5 6 7 8
60
IVr 1 2 3
60
4 5 6 19 20
k 4- 7
7 8 9
25
83.3 80.9 79.0
7.61 7.29 7.00
2.817 2 318 1.960
10 11 12
40
84.2 82.4 80.4
7.67 7.42
7.11
3.520 3.180 2.884
13 14 15
69.4
64.9 61.0 56.9
4.76 4.23 3.80
2.068 1,720 1.399
16 17 18
79.4
55.4 11.3 < 7.4
4.78 4.24 3.83
2.499 2.136 1.821
-CFj (V) E 5.4 3.66
0.499
% HzS04
-HO
X = H (I) 71.1 70.9 60.0 60.0 67.4 67.5 74.4 74.6 62.6 62.6 54.7 54.4 49.8 50.3 44.8 44.8 46.6 45.3 42.9 41.0
5.65 5.64 4.14 4.14 5.15 5.16 6.11 6.13 4.46 4.46 3.59 3.57 3.15 3.19 2.72 2.72 2.87 2.76 2.56 2.40
3.842 3.864 2.558 2.558 2.399 3.407 4.434 4.427 2.849 2.843 2.076 2.046 1.657 1.674 1.383 1.355 1.466 1.303 1.123 0,967
74.8 72.4 70.3
6.34 6.00 5.70
3.067 2.674 2.316
75.0 72.6 70.8
6.28 5.96 5.72
3.632 3.310 3.014
X = m-NOn (11) 92.1 8.30 91.6 8.26 8.01 89.3 89.4 8.05 97.0 8.86 96.7 8.83 88.5 7.97 88.5 7.97 83.0 7.37 82.7 7.33 78.8 6.74
VIL 1 2 3 4
2 783 2.772 2.601 2.601 3.138 3.162 2 515 2 508 1.887 1.874 1.458
8 9 10
Temp., "C.
Run
FOR AQUE-
78.5 73.7 73.6
6.69 6.00 5.99
X = p-Br (111) 74.5 6.12 69.4 5.43 65.2 4.83 77.0 6.47 61.2 4.27 53.4 3.47 47.7 2.96 X = m-F (IV) 83.8 7.48 78.5 6.70 74.1 6.06 68.5 5.30 62.2 4.41 55.2 3.64 69.4 5.43 70.3 5.55
log
1.423 0.839 0.883 3.789 3.085 2.538 4.111 2.122 1.375 0.953 4.196 3.423 2.739 2.007 1.403 0.821 2.152 2.193
Vol. 83
X =
fi
6
60
VI,- 1
60
6 L.7 6 1.1 5 :.9 4 1.5
4.76 4.14 3.62 3.11
3.455 2.938 2.492 2.039
8 9 10
40
5 1.3 5 1.0 6 .7
3.49 4.00 4.48
1.462 1.826 2.236
5 6
25
5 1.1 5 i.4 6 i.0
3.48 4.02 4.63
0.641 1.148 1.629
X = p-1 :HO ( V I I ) 5 ).5 3.67 60 4 1.6 3.13 4 1.7 2.38 2 r.6 1.53
3.641 3,100 2.363 1.455
40
4 '.9 4 '.8 5 '.9
2.55 2.98 3.45
1.612 2.042 2.574
5 6 7
25
4 '.l 4 .3 5 .5
2.57 3.02 3 52
0.923 1.419 1.916
IL- 1 2 3 4
