Kinetic Studies of the Transesterification of Dimethyl Phosphite with Alcohols of the Guaiacyl and Veratryl Series
Abstract Generated abstract
This study examines the transesterification of dimethyl phosphite with aromatic alcohols of the guaiacyl and veratryl series, focusing on how ring substituents and hydroxyl position in the propyl side chain affect reactivity. Reactions were carried out without solvent or catalyst under helium at 100, 115, and 130°C, and kinetics were followed by measuring evolved methanol by gas liquid chromatography. The results indicate second order behavior and show a reactivity order of 1-guaiacyl-1-propanol > 1-veratryl-1-propanol > 1-guaiacyl-3-propanol > 1-phenyl-1-propanol > 1-veratryl-3-propanol. The authors conclude that guaiacyl derivatives are generally more reactive than veratryl analogs, and that alpha benzylic hydroxyl groups react more rapidly than corresponding primary hydroxyl groups.
Full Text
Kinetic Studies of the Transesterification Reaction of Dimethyl Phosphite with Alcohols of the Guaiacyl and Veratryl Series
V. P. Vasil’eva, G. G. Lykhina
(Presented by the joint scientific seminar of the organic departments and laboratories of the Faculty of Chemical Technology)
It is known that the transesterification reaction of dialkyl phosphates is readily carried out with alcohols of various structures [1,2].
The literature contains no data on the reactivity of alcohols of the guaiacyl and veratryl series in phosphorylation reactions with dialkyl phosphites; the present work is devoted to the study of the transesterification reaction of dimethyl phosphite with the above-mentioned alcohols, and to determining the reactivity of the OH groups of aromatic alcohols as a function of the nature of the substituents in the benzene ring and the position of the hydroxyl group in the side chain.
As the alcohols, we selected compounds of the guaiacyl and veratryl series containing primary and secondary OH groups, as well as phenylpropanol alcohols. The latter were taken in order to determine the influence of substituents located in the benzene ring on the reactivity of the hydroxyl groups at the α- and γ-carbon atoms of the side propyl chain.
The phosphorylating agent was dimethyl phosphite:
\[ \begin{array}{c} \mathrm{CH_3O}\backslash \quad \mathrm{O}\\ \qquad \mathrm{P} \\ \mathrm{CH_3O}/ \quad \backslash \mathrm{H} \end{array} \]
The kinetics of the phosphorylation reaction of aliphatic hydroxyl groups with dimethyl phosphite were studied without a solvent in a helium atmosphere, in the absence of a catalyst, and at an alcohol : dimethyl phosphite ratio equal to 3:1.
The reaction proceeds according to scheme [2]:
\[ \mathrm{(CH_3O)_2P(O)H} + 2\,\mathrm{R_1} \!\left( \begin{array}{c} \mathrm{R_2}\\[-2mm] \mathrm{\ \ \ \ \ \ \ } \\ \end{array} \right) \mathrm{C_6H_3{-}CH(OH){-}CH_2{-}CH_3} \longrightarrow \tag{I} \]
\[ \begin{aligned} &\longrightarrow \begin{gathered} \mathrm{Ar{-}CH(CH_2CH_3)O{-}P(=O)(H){-}OCH(CH_2CH_3){-}Ar} \end{gathered} + 2\,\mathrm{CH_3OH}, \end{aligned} \]
where
\[ \begin{aligned} R_1&=-\mathrm{OH},\ -\mathrm{OCH_3},\ -\mathrm{H},\\ R_2&=-\mathrm{OCH_3},\ -\mathrm{H}. \end{aligned} \]
Experimental Part
Dimethyl phosphite (0.0125 mole) and the aromatic alcohol (0.0375 mole) were placed in a Claisen flask equipped with a capillary for passing helium. The reaction was carried out at temperatures of 100, 115, and 130°C in an ultrathermostat, where the temperature was maintained with an accuracy of ±0.02°C. The methyl alcohol formed during the reaction was distilled off by a stream of helium and absorbed by n-propyl alcohol. The receiver containing n-propyl alcohol was cooled with ice. Samples were taken at definite time intervals by replacing the receiver. The amount of methyl alcohol formed according to reaction (1) was determined by gas–liquid chromatography on a KhL-4 chromatograph, previously calibrated with a series of standard solutions of methyl alcohol in n-propyl alcohol in the concentration range \(0.1 \div 1.0 \cdot 10^{-2}\) mole.
Results and Discussion
The kinetic curves for the evolution of methyl alcohol in the transesterification reaction of dimethyl phosphite with aromatic alcohols are shown in Figs. 1 and 2.
Fig. 1. Kinetic curves for the evolution of methyl alcohol in the transesterification reaction of dimethyl phosphite with 1-veratryl-1-propanol: 1 — at 100°C, 2 — 115°C, 3 — 130°C.
The rate constants of the reaction were calculated from the equation for second-order reactions [3]:
\[ K=\frac{2.3}{t(CO_1-CO_2)}\lg\frac{CO_2(CO_1-x)}{CO_1(CO_2-x_2)}\ \text{mol/min}, \tag{2} \]
where
\(CO_1\) — initial concentration of the alcohol, mol;
\(CO_2\) — initial concentration of dimethyl phosphite, mol;
\(x\) — amount of methyl alcohol formed at time \(t\), mol.
Fig. 2. Kinetic curves for the liberation of methyl alcohol in the transesterification reaction of dimethyl phosphite with aromatic alcohols at 130°C: 1, 1-veratryl-3-propanol; 2, 1-phenyl-1-propanol; 3, 1-guaiacyl-3-propanol; 4, 1-guaiacyl-1-propanol; 5, 1-veratryl-1-propanol.
The rate constants found for the transesterification reaction of dimethyl phosphite with 1-veratryl-1-propanol were as follows:
\[ \begin{aligned} T\ 100^\circ\mathrm{C} \qquad &K_{\mathrm{av}}=0.875\cdot 10^{-2}\ \text{mol/min},\\ T\ 115^\circ\mathrm{C} \qquad &K_{\mathrm{av}}=4.55\cdot 10^{-2}\ \text{mol/min},\\ T\ 130^\circ\mathrm{C} \qquad &K_{\mathrm{av}}=20.3\cdot 10^{-2}\ \text{mol/min}. \end{aligned} \]
To compare the reactivity of the aliphatic OH groups of aromatic alcohols, the kinetics of the transesterification reaction of dimethyl phosphite with 1-guaiacyl-1-propanol, 1-guaiacyl-3-propanol, and 1-phenyl-1-propanol were studied at 130°C. The rate constants were found to be: for 1-guaiacyl-3-propanol—\(1.86\cdot 10^{-2}\) mol/min, and for 1-phenyl-1-propanol—\(1.85\cdot 10^{-2}\) mol/min.
It was not possible to calculate the rate constant for the reaction with 1-guaiacyl-1-propanol, since the liberation of methyl alcohol proceeded very vigorously and the reaction was practically complete after 5–7 min. The method of investigation used did not allow samples to be taken at short time intervals. Comparison of the rate constants shows that the reaction rate depends on the nature of the substituents in the aromatic ring and on the position of the OH group in the side chain. It is known that transesterification of dimethyl phosphite with alcohols proceeds the faster, the higher the nucleophilicity of the oxygen atom in the hydroxyl group.
[5]; in other words, the higher the basicity of the alcohol, the greater its reactivity in this reaction. This is confirmed by the results obtained.
It was found that the reactivity of the alcohols studied decreases in the following order: 1-guaiacyl-1-propanol > 1-veratryl-1-propanol > 1-guaiacyl-3-propanol > 1-phenyl-1-propanol > 1-veratryl-3-propanol. The guaiacyl derivatives are somewhat more reactive than the corresponding veratryl derivatives, which is consistent with theoretical expectations based on the fact that the OH group is a stronger electron donor than the OCH₃ group [4, 5].
It was established that the α-benzyl alcohol groups in the veratryl and guaiacyl derivatives are more reactive than the primary OH groups of the corresponding compounds. Thus, the rate constant of the reaction with 1-veratryl-1-propanol is 15 times greater than that with 1-veratryl-3-propanol.
The results of the study showed that the absence of substituents in the benzene ring deactivates the OH group located in the α-position of the side propyl chain. This accounts for the low reactivity of phenylpropyl alcohols.
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