Full Text
CHEMISTRY
V. A. PALM and M. P. KHЫRAK
INVESTIGATION OF THE ROLE OF ETHER IN THE REACTION OF FORMATION OF THE GRIGNARD REAGENT
(Presented by Academician V. N. Kondrat’ev, 10 X 1959)
The problems of the kinetics and mechanism of the reaction of formation of organomagnesium compounds are considered in a relatively small number of works \((^{1-6})\). It has been established that this reaction has a chain mechanism \((^{4-6})\) and that the rate of the reaction, after reaching its maximum value, is directly proportional to the concentration of alkyl halide and to the magnitude of the surface of the magnesium \((^3)\). There is also an indication that the observed induction period of this reaction can be eliminated if it is carried out in a medium in which, shortly before this, one portion of alkyl halide has already reacted with magnesium \((^3)\). This phenomenon has been explained on the assumption that the presence of an induction period follows from the necessity of preliminary cleaning of the magnesium surface. The same action is attributed to iodine as an activator, since the latter does not affect the rate of the reaction after the onset of the maximum rate, but only shortens the induction period \((^1, ^2)\).
Fig. 1. Solvent—abs. ether; the induction period is not eliminated. Solid lines—kinetic curves, dashed lines—the corresponding thermograms. 1—temperature \(20^\circ\), initial concentration of butyl bromide \(0.5\) mole/l, charge of magnesium \(1.5\) g; 2—\(20^\circ\), initial concentration of butyl bromide \(0.2\) mole/l, charge of magnesium \(1.5\) g; 3—\(15^\circ\), initial concentration of butyl bromide \(0.2\) mole/l, charge of magnesium \(4.5\) g.
It is known that the rate and the very possibility of the reaction between magnesium and alkyl halides depend strongly on the nature of the solvent \((^1)\). It is necessary that the solvent possess an unshared electron pair.
We investigated the kinetics of the reaction between butyl bromide and magnesium in a medium of abs. ether and in mixtures of abs. ether with n-hexane. Both solvents were previously carefully purified and dried. For monitoring the course of the reaction, a thermographic method was used. From the thermograms the corresponding kinetic curves were calculated in coordinates degree of conversion—time.
Two series of experiments were carried out. In the first series the induction period was not eliminated. In this case the thermograms and the kinetic curves obtained by their treatment have the form characteristic of this reaction. At first there is an induction period, during which it is not possible to detect any signs of the occurrence of the reaction. Then follows an S-shaped section of the kinetic curve, qualitatively resembling an autocatalytic curve (see Fig. 1).
In the second series of experiments the induction period was eliminated by carrying out the reaction in a medium in which one portion of butyl bromide had already reacted (Fig. 2).
The formal rate constants \(k_1\), in the case where the induction period was eliminated in anhydrous ether, could be calculated according to the usual monomolecular law. For the purpose of calculating the rate constant in our experimental procedure, it is also possible to use quantities characterizing the reaction rate and the degree of conversion at the thermal maximum of the reaction, where the rate of heat evolution, proportional to the reaction rate, is equal to the rate of heat removal owing to cooling. The values of the rate constant \(k_1\) calculated in this way agree well with those calculated from the kinetic curves either by the monomolecular law or from the initial reaction rates.
Fig. 2. Solvent—anhydrous ether; induction period eliminated. Thermostat temperature 20°. 1—initial concentration of butyl bromide 0.2 mole/liter, magnesium charge 1.5 g; 2—initial concentration of butyl bromide 0.2 mole/liter, magnesium charge (fresh portion) 1.5 g; 3—initial concentration of butyl bromide 0.1 mole/liter, magnesium charge 1.5 g.
When the reaction was carried out in anhydrous ether, the thermograms always had a single maximum. In mixtures of ether with \(n\)-hexane, 2 or, at higher concentrations of \(n\)-hexane, 3 maxima were observed (Fig. 3). On going from one maximum to the next along the time axis, the corresponding rate constant \(k_1\) increased noticeably.
Fig. 3. Solvent—a mixture of ether with hexane; thermostat temperature 20°. 1—ether concentration 50 mole %, initial concentration of butyl bromide 0.2 mole/liter, magnesium charge 1.5 g. Induction period eliminated; 2—ether concentration 25 mole %, initial concentration of bromide 0.5 mole/liter, magnesium charge 4.5 g.
The possibility of eliminating the induction period shows that the latter is not necessarily associated with the reaction mechanism, but is the result of the action of removable side factors. The fact that, when the induction period is eliminated, replacement of the portion of magnesium that had already participated in the reaction by a fresh portion practically does not affect the form of the kinetic curve or the reaction rate (Fig. 2) permits the conclusion that the presence of the induction period is not connected with the need for preliminary purification of the magnesium surface. This circumstance, as well as the form of the kinetic curves (the practical absence of signs of reaction during the induction period), permits the conclusion that the presence of an induction period is connected with the action of a powerful inhibitor present in the reaction mixture. Owing to its high reactivity, such an inhibitor “captures” all the free radicals that arise as a result of the chain-initiation reaction. As a result, the chain reaction cannot develop until practically all the induction-period inhibitor present in the reaction mixture has been consumed.
the inhibitor has not reacted with the radicals that arise as a result of the initiation reaction. Elimination of the induction period amounts to removal of the inhibitor. The proportionality of the reaction rate to the concentration of butyl bromide under these conditions indicates that the length of the reaction chains, in the absence of inhibitor, does not depend on the extent of the reaction.
Table 1
Solvent—abs. ether. Solution volume 40 ml. Thermostat temperature 15.0°
| Initial conc. of butyl bromide, mole/l | $g_{\mathrm{Mg}}$, (magnesium charge, g) | $k_1 \cdot 10^3$, sec$^{-1}$ (mean value) | $\dfrac{k_1}{g_{\mathrm{Mg}}} \cdot 10^3$ | Number of experiments |
|---|---|---|---|---|
| 0.2 | 1.5 | 4.9 | 3.3 | 9 |
| 0.2 | 4.5 | 14.1 | 3.1 | 2 |
| 0.2 | 9.0 | 28 | 3.1 | 1 |
The presence of several maxima on the thermograms when the reaction is carried out in a mixture of ether with $n$-hexane can be explained by the presence in the $n$-hexane of several less active inhibitors, which shorten the chains through reaction with free radicals. Therefore, in order to establish the role of ether in the reaction under study, only the values of the rate constant at the last thermal maximum can be used, when all inhibitors have already had time to react. The latter was also ensured by increasing, in these cases, the initial concentration of butyl bromide.
We checked for proportionality between the reaction rate and the magnitude of the magnesium surface, which was taken as proportional to the charge of magnesium used. The corresponding data are given in Table 1.
Table 2
Solution volume 40 ml. Thermostat temperature 20°
| Initial conc. of butyl bromide, mole/l | $g_{\mathrm{Mg}}$, g | $N_\text{э}$ | Ordinal number of the maximum on the thermogram | $k_1 \cdot 10^3$ (mean value) | $k_3 \cdot 10^8$ | Number of experiments |
|---|---|---|---|---|---|---|
| 0.1 and 0.2 | 1.5 | 1.00 | 1 | 11.0 | 7.3 | 3* |
| 0.2 and 0.5 | 1.5 | 1.00 | 1 | 7.9 | 5.3 | 3 |
| 0.2 | 1.5 | 0.50 | 1 | 4.3 | 5.7 | 1 |
| 0.5 | 3.5 | 0.50 | 2 | 9.4 | 5.5 | 2 |
| 0.5 | 3.5 | 0.25 | 3 | 4.5 | 5.1 | 2 |
| 2.0 | 7.0 | 0.10 | 3 | 3.8 | 5.4 | 1 |
* The induction period has been eliminated.
Table 2 gives the data we obtained on the dependence of the rate constant $k_1$ on the concentration of ether in ether—$n$-hexane mixtures. Table 2 gives the values of the constant $k_3$, calculated by the formula: $k_3 = k_1/g_{\mathrm{Mg}} N_\text{э}$, where $N_\text{э}$ is the mole fraction of ether in the ether—$n$-hexane mixture. The constancy of $k_3$ indicates that the reaction rate is proportional to the ether concentration in the first power. It follows from this that the order of the reaction is equal to three, and the reaction rate is:
\[ v = k_3 S_{\mathrm{Mg}} N_\text{э} [\mathrm{C_4H_9Br}], \tag{1} \]
where $S_{\mathrm{Mg}}$ is the effective surface of magnesium (it is assumed that $S_{\mathrm{Mg}} = g_{\mathrm{Mg}}$).
Expression (1), of course, is not applicable for describing the reaction rate during the induction period up to the attainment of the rate maximum.
The only possible scheme, in our opinion, for the reaction mechanism from which all the above-mentioned features of the reaction under study follow can be written in the following form:
\[ \text{1.}\quad \mathrm{C_4H_9Br} + \mathrm{Mg} \xrightarrow{k_1} \mathrm{C_4H_9\cdot} + \mathrm{MgBr}, \]
\[ \text{2.}\quad \mathrm{C_4H_9\cdot} + \mathrm{Mg} \xrightarrow{k_2} \mathrm{C_4H_9\dot{M}g}, \]
-
\[ \mathrm{C_4H_9Mg} + :\mathrm{S} \xrightarrow{k_3} \mathrm{C_4H_9Mg:S}, \]
-
\[ \mathrm{C_4H_9Mg:S} + \mathrm{C_4H_9Br} \xrightarrow{k_4} \mathrm{C_4H_9MgBr} + :\mathrm{S} + \mathrm{C_4H_9}\cdot, \]
-
\[ \mathrm{C_4H_9Mg} \xrightarrow{k_5} x, \]
-
\[ \mathrm{C_4H_9Mg} + :i \xrightarrow{k_6} y, \]
where \(:\mathrm{S}\) is ether; \(:i\) is an active inhibitor.
Reaction 1 of chain initiation is a slow, rate-limiting stage; reactions 2–6 are relatively fast. The development of the chain cycle (reactions 2–4) is limited by competition between reaction 3 and reactions 5 and 6. Chain termination occurs practically only as a result of the disappearance of the radicals \(\mathrm{C_4H_9Mg}\), which takes place as a result of the “normal” reaction \({}^{5}\) of chain termination and reaction 6, in which the active inhibitor participates. The role of ether consists in its giving complexes \(\mathrm{C_4H_9Mg:S}\) with the radicals \(\mathrm{C_4H_9Mg}\), which readily react with butyl bromide, while at the same time taking practically no part in chain-termination reactions. If it is assumed that \(v_6 \gg v_5\), \(v_6 \gg v_3\), and \(v_3 \gg v_5\), then from scheme 1–6 there follow qualitatively the principal experimentally observed features of the reaction under study. The kinetic differential equation is then written as follows:
\[ -\,d[\mathrm{C_4H_9Br}]/dt = k_1 k_3 \{k_5 + k_6 ([i]_0 - k_1 S_{\mathrm{Mg}}[\mathrm{C_4H_9Br}]_0 t)\}^{-1} \cdot S_{\mathrm{Mg}}N_{\!s}[\mathrm{C_4H_9Br}], \]
if
\[ t \leq [i]_0/k_1 S_{\mathrm{Mg}}[\mathrm{C_4H_9Br}]_0, \]
and
\[ -\,d[\mathrm{C_4H_9Br}]/dt = (k_1 k_3/k_5)\cdot S_{\mathrm{Mg}}N_{\!s}[\mathrm{C_4H_9Br}], \]
if
\[ t \geq [i]_0/k_1 S_{\mathrm{Mg}}[\mathrm{C_4H_9Br}]_0, \]
where \([i]_0\) and \([\mathrm{C_4H_9Br}]_0\) are the initial concentrations of the inhibitor and butyl bromide, and \(t\) is time.
The kinetic curves obtained by integrating this equation and substituting reasonably selected values of the parameters entering into it are, in form, analogous to those obtained experimentally. On the basis of what has been said, the catalytic activity of diethyl ether in the reaction of formation of butylmagnesium bromide from butyl bromide and magnesium is determined by two factors. The first of them is the “kinetic basicity” (nucleophilicity) of the ether, on which the rate of reaction 3 and the chain length depend. The second factor is the increased reactivity of the complex \(\mathrm{C_4H_9Mg:S}\), ensuring a situation in which each such complex reacts only with butyl bromide, as a result of which \(v_4 = v_3\). It is possible that the reactivity of this complex is connected with the thermodynamic basicity of the ether.
As for the nature of the inhibitor responsible for the induction period, on the basis of the available data this question cannot be resolved. It is possible that traces of dissolved oxygen remaining in the reaction mixture may act as such, despite the fact that all experiments were carried out in an atmosphere of nitrogen.
Tartu State University
Received
7 X 1959
CITED LITERATURE
\({}^{1}\) M. S. Kharasch, O. Reinmuth, Grignard Reactions of Nonmetallic Substances, N. Y., 1954.
\({}^{2}\) M. Kilpatrick, H. P. Simon, J. Org. Chem., 2, 459 (1938).
\({}^{3}\) J. Ozenski, M. Kilpatrick, J. Org. Chem., 5, 264 (1940).
\({}^{4}\) M. Gomberg, W. E. Bachuba, J. Am. Chem. Soc., 49, 236 (1927).
\({}^{5}\) H. Gilman, R. E. Fothergill, J. Am. Chem. Soc., 50, 3334 (1928).
\({}^{6}\) H. Gilman, J. E. Kirby, J. Am. Chem. Soc., 51, 1571 (1928).