Abstract
Full Text
CHEMISTRY
V. A. KALYAVIN, T. A. SMOLINA, Corresponding Member of the Academy of Sciences of the USSR O. A. REUTOV
ON THE MONOMOLECULAR MECHANISM OF ISOTOPIC EXCHANGE OF BENZYLMERCURY HALIDES WITH RADIOACTIVE MERCURIC HALIDE
In the last decade interest has sharply increased in the study of electrophilic substitution reactions. It is obvious that the regularities obtained in this way may directly or indirectly contribute to the elucidation of a number of general questions concerning mechanisms of substitution reactions at a saturated carbon atom. A number of authors have described reactions of bimolecular electrophilic substitution \((S_E2)\) at a saturated carbon atom. At the same time, until quite recently \(S_E1\)-reactions were unknown.
In studying the influence of the nature of the solvent on the isotopic-exchange reaction:
\[ \mathrm{RR'R''C-HgX + Hg^{203}X_2 \rightleftharpoons RR'R''C-Hg^{203}X + HgX_2} \]
one of us, together with co-workers \((^{1,2})\), succeeded for the first time in carrying out an \(S_E1\)-reaction (first kinetic order with respect to the organomercury compound and zero order with respect to mercuric halide) for the case of the ethyl ester of \(\alpha\)-bromomercuriphenylacetic acid, using solvents with high ionizing power. Dimethyl sulfoxide (DMSO) proved especially suitable for this purpose.*
However, in studying the isotopic exchange in DMSO of benzylic organomercury compounds (less reactive in \(S_E\)-reactions than esters of \(\alpha\)-bromomercuriphenylacetic acid), we found \((^3)\) that the reaction proceeds not by a monomolecular but by a bimolecular mechanism
\[ \mathrm{ X{-}C_6H_4{-}CH_2{-}HgBr + Hg^{203}Br_2 \ \xrightleftharpoons[\ ]{\mathrm{DMSO}}\ X{-}C_6H_4{-}CH_2{-}Hg^{203}Br + HgBr_2 \quad S_E2 } \]
\[ \mathrm{X=(CH_3)_2CH,\ CH_3,\ H,\ Cl,\ F} \]
i.e., just as when quinoline is used as the solvent \((^4)\). Obviously, despite the high ionizing power of DMSO, in the present case it is insufficient to ionize the strong C—Hg bond with formation of an ion pair:
\[ \mathrm{>C:HgBr \rightleftharpoons >C^{(-)}:Hg^{(+)}Br} \]
It is natural to assume that the probability of an \(S_E1\)-mechanism for the reaction under consideration in DMSO should increase in the case where the carbanion formed as a result of ionization possesses greater stability owing to delocalization of the negative charge, for example.
* In DMSO it proved possible to carry out an \(S_E1\)-mechanism also for protolysis reactions (under the action of HX or DX) of organomercury compounds with a mercury atom attached to an olefinic or aromatic carbon atom:
\[ \mathrm{RHC{=}CH{-}HgX + DX \rightarrow RHC{=}CHD + HgX_2} \]
\[ \mathrm{Ar{-}HgX + HX \rightarrow Ar{-}H + HgX_2.} \]
All these reactions are first order with respect to the organomercury compound and zero order with respect to hydrogen halide.
structural factors:
\[ \mathrm{X{-}C_6H_4{-}\overset{(+)}{CH_2}\quad HgBr^{(-)}} \]
The most suitable object for carrying out an \(S_E1\)-mechanism should be substituted benzylmercury halides with strong electron-withdrawing substituents in the benzene ring.
Experimental verification confirmed this supposition.
We studied the kinetics of isotopic exchange of \(p\)-nitrobenzylmercury bromide with radioactive mercuric bromide in DMSO. As we had assumed, the reaction has an overall first order (first order with respect to the organomercury compound and zero order with respect to mercuric bromide).
Thus, we succeeded in carrying out an \(S_E1\)-reaction of the type under consideration for benzyl systems, purposefully using structural factors and the ionizing ability of the solvent.
The reaction mechanism may be expressed by the following scheme:
\[ \begin{aligned} &\mathrm{O_2N{-}C_6H_4{-}CH_2{-}HgBr} \ \xrightleftharpoons[\text{fast}]{\text{slow}}\ \mathrm{O_2N{-}C_6H_4{-}\overset{(+)}{CH_2}\ Br^{(-)}Hg} \\[4pt] &\hspace{8.5cm} \begin{gathered} \mathrm{HgBr_2}\\ \text{fast} \end{gathered} \ \Bigg\downarrow\Bigg\uparrow\ \begin{gathered} \mathrm{Hg^{203}Br_2}\\ \text{fast} \end{gathered} \\[4pt] &\mathrm{O_2N{-}C_6H_4{-}CH_2{-}Hg^{203}Br} \ \xrightleftharpoons[\text{slow}]{\text{fast}}\ \mathrm{O_2N{-}C_6H_4{-}\overset{(+)}{CH_2}\ Br^{(-)}Hg^{203}} \end{aligned} \]
Experimental Part
The kinetics of the reaction was studied in the reagent concentration range \(0.015\)—\(0.18\) mole/l at temperatures of 40, 50, 60, and 70°. Kinetic measurements were carried out by the previously developed method of paper radiochromatography \((^5)\).
Fig. 1. Isotopic exchange of \(p\)-\(\mathrm{NO_2C_6H_4CH_2HgBr}\) with \(\mathrm{Hg^{203}Br_2}\) in DMSO at 70° and at different equimolecular concentrations.
\(1\)—0.03; \(2\)—0.06; \(3\)—0.09; \(4\)—0.12 mole/liter.
Fig. 2. Dependence of the exchange rate constant on the concentration of one of the components.
\(1\)—\(\lg R = f(C_{\mathrm{NO_2C_6H_4CH_2HgBr}})\); \(2\)—\(\lg R = f(C_{\mathrm{HgBr_2}})\)
Weighed portions of mercuric bromide and \(p\)-nitrobenzylmercury bromide* were dissolved in 1–2 ml of DMSO, and the resulting solutions were placed in a thermostatted system. At definite time intervals, samples of the ra—
* \(p\)-Nitrobenzylmercury bromide was obtained by shaking an acetone solution of \(p\)-nitrobenzyl bromide with metallic mercury under ultraviolet irradiation. M.p. 181–182° (from dioxane). Analytical results:
Found, %: C 20.50; H 1.50; Hg 47.84; Br 19.09
\(\mathrm{C_7H_6BrHgNO_2}\). Calculated, %: C 20.18; H 1.45; Hg 48.14; Br 19.18
The preparation of mercuric bromide labeled with the radioactive isotope \(\mathrm{Hg}^{203}\) and the purification of DMSO were described by us earlier \((^3)\).
Table 1
| \(C_{\mathrm{HgBr_2}}\), mol/l | \(C_{\mathrm{NO_2C_6H_4CH_2HgBr}}\), mol/l | Exchange time, h | \(A_{\mathrm{HgBr_2}}\), imp/min | \(A_{\mathrm{NO_2C_6H_4CH_2HgBr}}\), imp/min | Degree of exchange \(F\), % | Initial rate \(R\cdot10^3\), mol·l\(^{-1}\)·h\(^{-1}\) | \(K_1\), h\(^{-1}\) | \(K_2\), l·mol\(^{-1}\)·h\(^{-1}\) | \(C_{\mathrm{HgBr_2}}\), mol/l | \(C_{\mathrm{NO_2C_6H_4CH_2HgBr}}\), mol/l | Exchange time, h | \(A_{\mathrm{HgBr_2}}\), imp/min | \(A_{\mathrm{NO_2C_6H_4CH_2HgBr}}\), imp/min | Degree of exchange \(F\), % | Initial rate \(R\cdot10^3\), mol·l\(^{-1}\)·h\(^{-1}\) | \(K_1\), h\(^{-1}\) | \(K_2\), l·mol\(^{-1}\)·h\(^{-1}\) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.03 | 0.03 | 0.5 | 3349 | 147 | 8 | 0.41 | 0.136 | 4.5 | 0.06 | 0.12 | 0.5 | 5518 | 1077 | 24 | 0.71 | 0.143 | 2.3 |
| 0.03 | 0.03 | 1 | 3098 | 310 | 18 | 0.41 | 0.136 | 4.5 | 0.06 | 0.12 | 1 | 4264 | 1520 | 39 | 0.71 | 0.143 | 2.3 |
| 0.03 | 0.03 | 2 | 2832 | 740 | 41 | 0.41 | 0.136 | 4.5 | 0.06 | 0.12 | 1.5 | 2650 | 1325 | 50 | 0.71 | 0.143 | 2.3 |
| 0.03 | 0.03 | 3.5 | 2474 | 1072 | 61 | 0.41 | 0.136 | 4.5 | 0.06 | 0.12 | 2.5 | 1873 | 1321 | 62 | 0.71 | 0.143 | 2.3 |
| 0.03 | 0.03 | 5.5 | 2306 | 1465 | 78 | 0.41 | 0.136 | 4.5 | 0.06 | 0.12 | 3.5 | 1920 | 2012 | 77 | 0.71 | 0.143 | 2.3 |
| 0.03 | 0.03 | 8 | 1886 | 1466 | 88 | 0.41 | 0.136 | 4.5 | |||||||||
| 0.06 | 0.06 | 0.0 | 4312 | 73 | 3 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 0.5 | 1382 | 123 | 12 | 0.75 | 0.124 | 4.1 |
| 0.06 | 0.06 | 0.5 | 4904 | 321 | 12 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 1 | 1333 | 268 | 25 | 0.75 | 0.124 | 4.1 |
| 0.06 | 0.06 | 1.5 | 3369 | 659 | 33 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 1.5 | 2209 | 738 | 38 | 0.75 | 0.124 | 4.1 |
| 0.06 | 0.06 | 3 | 3000 | 1308 | 61 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 2 | 1895 | 1119 | 52 | 0.75 | 0.124 | 4.1 |
| 0.06 | 0.06 | 4 | 2375 | 1205 | 67 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 2.5 | 1727 | 1181 | 61 | 0.75 | 0.124 | 4.1 |
| 0.06 | 0.06 | 6 | 2236 | 1534 | 81 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 3 | 1613 | 1516 | 73 | 0.75 | 0.124 | 4.1 |
| 0.06 | 0.06 | 7.7 | 2285 | 1793 | 88 | 0.82 | 0.136 | 2.3 | 0.03 | 0.06 | 4 | 1466 | 1595 | 78 | 0.75 | 0.124 | 4.1 |
| 0.03 | 0.06 | 4.5 | 1408 | 1692 | 82 | 0.75 | 0.124 | 4.1 | |||||||||
| 0.03 | 0.06 | 5 | 1230 | 1528 | 83 | 0.75 | 0.124 | 4.1 | |||||||||
| 0.09 | 0.09 | 0.5 | 4527 | 284 | 12 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 0.5 | 6925 | 195 | 8 | 0.79 | 0.132 | 1.1 |
| 0.09 | 0.09 | 1.5 | 3367 | 685 | 34 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 1 | 6516 | 343 | 15 | 0.79 | 0.132 | 1.1 |
| 0.09 | 0.09 | 2 | 3247 | 889 | 43 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 1.5 | 6301 | 572 | 25 | 0.79 | 0.132 | 1.1 |
| 0.09 | 0.09 | 3 | 3180 | 1237 | 56 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 2 | 6144 | 735 | 32 | 0.79 | 0.132 | 1.1 |
| 0.09 | 0.09 | 4.5 | 2911 | 1569 | 70 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 2.5 | 6067 | 920 | 39 | 0.79 | 0.132 | 1.1 |
| 0.09 | 0.09 | 6 | 2650 | 1696 | 78 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 3 | 5878 | 1098 | 47 | 0.79 | 0.132 | 1.1 |
| 0.09 | 0.09 | 7 | 2707 | 2074 | 87 | 1.22 | 0.136 | 1.5 | 0.12 | 0.06 | 3.5 | 4464 | 987 | 54 | 0.79 | 0.132 | 1.1 |
| 0.12 | 0.06 | 4.5 | 5117 | 1233 | 58 | 0.79 | 0.132 | 1.1 | |||||||||
| 0.12 | 0.06 | 5 | 4595 | 1170 | 61 | 0.79 | 0.132 | 1.1 | |||||||||
| 0.12 | 0.06 | 5 | 4859 | 1296 | 63 | 0.79 | 0.132 | 1.1 | |||||||||
| 0.12 | 0.12 | 0.0 | 2692 | 30 | 2 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 0.5 | 10000 | 242 | 9 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 0.5 | 1564 | 94 | 11 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 1 | 9496 | 364 | 15 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 1 | 2875 | 409 | 25 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 1.5 | 8800 | 565 | 24 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 1.5 | 3142 | 659 | 35 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 2 | 8156 | 672 | 30 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 2 | 2267 | 587 | 41 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 2.5 | 7970 | 735 | 34 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 2.5 | 2862 | 987 | 51 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 3.5 | 7763 | 976 | 45 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 3 | 1906 | 826 | 58 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 4 | 7907 | 1146 | 51 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 4 | 2274 | 1083 | 65 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 4.5 | 6383 | 902 | 50 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 5 | 1906 | 1059 | 72 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 5 | 7539 | 1340 | 60 | 0.81 | 0.135 | 0.7 |
| 0.12 | 0.12 | 7 | 2054 | 1414 | 82 | 1.63 | 0.136 | 1.1 | 0.18 | 0.06 | 6 | 8293 | 1592 | 64 | 0.81 | 0.135 | 0.7 |
| 0.06 | 0.015 | 0.5 | 8476 | 162 | 9 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.015 | 1 | 8922 | 326 | 18 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.015 | 1.5 | 10488 | 560 | 25 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.015 | 2.5 | 9030 | 658 | 34 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.015 | 4.5 | 9248 | 1183 | 56 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.015 | 5 | 7842 | 1050 | 59 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.015 | 7 | 8839 | 1436 | 70 | 0.21 | 0.140 | 2.3 | |||||||||
| 0.06 | 0.03 | 0.5 | 6529 | 233 | 10 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 1 | 4781 | 330 | 19 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 1.5 | 8904 | 825 | 25 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 2 | 5875 | 838 | 37 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 2.5 | 5462 | 933 | 44 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 3 | 5626 | 1193 | 53 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 3.5 | 4770 | 1127 | 58 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 4 | 5089 | 1308 | 61 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 5.5 | 4821 | 1578 | 74 | 0.46 | 0.154 | 2.6 | |||||||||
| 0.06 | 0.03 | 7 | 4728 | 1727 | 81 | 0.46 | 0.154 | 2.6 |
...of the solution by means of a capillary and applied to strips of chromatographic paper 1.3 cm wide. The DMSO was evaporated from the paper in a stream of air heated to 40°, after which the paper was impregnated with a 10% solution of ethylene glycol in acetone. Chromatography was carried out with an octane—benzene mixture in a ratio of 1 : 4. The chromatograms were divided into equal parts, the activities of which were measured on a B-2 apparatus with an MST-17 counter. The degree of exchange was calculated from the formula:
\[ F= \frac{A_{\mathrm{NO_2C_6H_4CH_2HgBr}}} {A_{\mathrm{NO_2C_6H_4CH_2HgBr}}+A_{\mathrm{HgBr_2}}} \cdot \frac{C_{\mathrm{NO_2C_6H_4CH_2HgBr}}+C_{\mathrm{HgBr_2}}} {C_{\mathrm{NO_2C_6H_4CH_2HgBr}}}, \]
where \(C\) is the concentration in mol/l, and \(A\) is the activity in imp/min.
Table 1 gives experimental data* on the dependence of the degree of exchange on the concentration of the reagents at a temperature of 70°.
From the data of Table 1, for various equimolecular concentrations of mercuric bromide and the organomercury compound, a plot was constructed in the coordinates \(-\lg(1-F)\)—\(t\) (Fig. 1). As can be seen, the half-exchange period does not depend on
* The activities were measured in parallel in two samples; the error in measuring the exchange did not exceed 7% on average. Table 1 gives the data of one measurement.
concentrations and remains constant. It follows from this that the reaction has an overall order equal to 1.
On the basis of a study of the dependence of the reaction rate on the concentration of one of the components (the concentration of the other component being kept constant), the partial orders of the reaction were determined. Figure 2 gives the graphical dependence of the logarithm of the initial rate of the isotope-exchange reaction on the concentration of each of the components.
Fig. 3. For determining the activation energy of the isotope-exchange reaction of \(n\)-NO\(_2\)C\(_6\)H\(_4\)CH\(_2\)HgBr with Hg\(^{203}\)Br\(_2\) in DMSO
At a constant concentration of the organomercury compound, the initial reaction rate is practically independent of the concentration of mercuric bromide (straight line 2 is parallel to the abscissa axis, Fig. 2). However, at a constant concentration of mercuric bromide the initial reaction rate increases with increasing concentration of the organomercury compound, the tangent of the angle of inclination of straight line 1 being equal to unity. These data indicate that the reaction under study is of zero order with respect to mercuric bromide and of first order with respect to the organomercury compound, i.e., it is a monomolecular reaction. The first-order rate constants given in Table 1 remain quite constant, which is not observed for the second-order rate constants.
Table 2
| Temp., °C | Exchange time, \(t\), h | \(A\)H\(_2\)gBr\(_2\), imp/min | \(A\)NO\(_2\)C\(_6\)H\(_4\)CH\(_2\)HgBr, imp/min | Extent of exchange, % | \(K_1\), h\(^{-1}\) | \(\lg K_1\) | Temp., °C | Exchange time, \(t\), h | \(A\)HgBr\(_2\), imp/min | \(A\)NO\(_2\)C\(_6\)H\(_4\)CH\(_2\)HgBr, imp/min | Extent of exchange, % | \(K_1\), h\(^{-1}\) | \(\lg K_1\) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 40 | 2 | 3342 | 78 | 5 | 0.011 | −1.97 | 60 | 0.5 | 3725 | 179 | 9 | 0.062 | −1.21 |
| 40 | 5.5 | 2984 | 178 | 11 | 0.011 | −1.97 | 60 | 1.5 | 3500 | 241 | 13 | 0.062 | −1.21 |
| 40 | 10 | 3102 | 317 | 19 | 0.011 | −1.97 | 60 | 2.5 | 3046 | 381 | 22 | 0.062 | −1.21 |
| 40 | 21.5 | 2503 | 632 | 40 | 0.011 | −1.97 | 60 | 3.5 | 2975 | 622 | 35 | 0.062 | −1.21 |
| 40 | 15.5 | 2760 | 471 | 29 | 0.011 | −1.97 | 60 | 5 | 2685 | 881 | 49 | 0.062 | −1.21 |
| 40 | 23 | 2735 | 624 | 37 | 0.011 | −1.97 | 60 | 9 | 2568 | 1290 | 67 | 0.062 | −1.21 |
| 40 | 29 | 2191 | 641 | 45 | 0.011 | −1.97 | 70 | 0.5 | 4904 | 321 | 12 | 0.136 | −0.87 |
| 50 | 1.5 | 3739 | 141 | 7 | 0.026 | −1.58 | 70 | 1.5 | 3369 | 659 | 33 | 0.136 | −0.87 |
| 50 | 3 | 3097 | 176 | 11 | 0.026 | −1.58 | 70 | 3 | 3000 | 1308 | 61 | 0.136 | −0.87 |
| 50 | 5 | 3274 | 379 | 21 | 0.026 | −1.58 | 70 | 4 | 2375 | 1205 | 67 | 0.136 | −0.87 |
| 50 | 7.3 | 2519 | 514 | 34 | 0.026 | −1.58 | 70 | 6 | 2236 | 1534 | 81 | 0.136 | −0.87 |
| 50 | 10 | 2582 | 674 | 44 | 0.026 | −1.58 | 70 | 7.7 | 2285 | 1793 | 88 | 0.136 | −0.87 |
| 50 | 13 | 2193 | 750 | 51 | 0.026 | −1.58 | |||||||
| 50 | 16.5 | 2471 | 1005 | 58 | 0.026 | −1.58 | |||||||
| 50 | 20 | 2196 | 1113 | 67 | 0.026 | −1.58 |
Table 2 gives the dependence of the extent of exchange at temperatures of 40, 50, 60, and 70° and constant reagent concentrations of 0.06 mol/l. From these data a plot of the dependence
\[ \lg K_1 = f(1/T) \]
was constructed (Fig. 3), from which the parameters of the Arrhenius equation were calculated:
\[ E = 18 \pm 1 \text{ kcal/mol}, \qquad \lg A = 10.65 \]
Moscow State University
named after M. V. Lomonosov
Institute of Organoelement Compounds
Academy of Sciences of the USSR
Received
31 I 1964
CITED LITERATURE
- O. A. Reutov, V. I. Sokolov, I. P. Beletskaya, DAN, 136, No. 3, 631 (1961).
- O. A. Reutov, B. P. Fisher et al., Izv. AN SSSR, OKhN, 1963, 970.
- V. A. Kalyavin, T. A. Smolina, O. A. Reutov, DAN, 155, No. 3 (1964).
- O. A. Reutov, T. A. Smolina, V. A. Kalyavin, DAN, 139, No. 2, 389 (1961); 35, 119 (1962).
- O. A. Reutov, V. I. Sokolov, DAN, 136, No. 2, 366 (1961).