Ants Tuulmets

Effect of ultrasound on ester hydrolysis in aqueous ethanol

Kinetics of the acid-catalyzed hydrolysis of ethyl acetate in ethanol-water binary solutions were investigated without sonication and under ultrasound at 22 kHz. Rate enhancements by 1.03-2.4 times were found with a minimum at 18 wt.% and a maximum at 45 wt.% of ethanol. The results suggest that ultrasonic acceleration of the reaction may be interrelated to the perturbation of the molecular structure of the binary solvent.

Solvation effects in partially solvated Grignard reagents

Abstract Grignard reagents were prepared from alkyl chlorides ( n -BuCl, i -PrCl, s -BuCl and t -BuCl) in toluene in the presence of one or less equivalents of various organic bases (ethers, triethylamine). Ultrasound was used to promote the process. Diethyl ether proved to be the most effective donor affording Grignard reagents with a very small content of the base. Some of the reagents are disproportionate to magnesium chloride and the dialkylmagnesium. Stronger solvation of the species is favourable for the dismutation process. Increasing bulkiness of the alkyl group in the Grignard reagent hinders the complexing with the donor and shifts the equilibrium to the formation of unsymmetrical species. The extent and the rate of the reaction are also governed by the solvating power of donors. The importance of specific solvation in the Grignard chemistry and the decisive role of the steric requirements of the reagents over their intrinsic acid–base properties were demonstrated.

Partially solvated alkylmagnesium chlorides in toluene

Abstract Toluene solutions of alkylmagnesium chlorides partially solvated by diethyl ether were investigated. Primary and secondary alkyl chlorides can be converted into Grignard reagents in good yields in the presence of small amounts of ether (less than one mole per mole of halide). Tertiary chlorides from only monosolvated organomagnesium compounds. Ultrasound accelerates the process. The reagents obtained are heterogeneous, but the solubilities of the partially solvated complexes in toluene are fairly high. Some of the reagents disproportionate to magnesium chloride and the dialkylmagnesium. The extent of disproportionation decreases with an increase in the concentration of the reagent or in the steric requirements of the alkyl moiety.

Ultrasonic acceleration of ester hydrolyses.

Kinetics of acid-catalyzed hydrolysis of ethyl acetate in 1M HCl solution was investigated without sonication and under ultrasound at 22 kHz. An average 6.2-fold ultrasonic rate enhancement for the temperature range of 10-28 degrees C was found. This is a considerably greater acceleration than previously reported for ester hydrolyses. It was suggested that a significant part of the sonochemical effect on polar reactions may be due to temperature gradients in the cavitating medium.

REACTIONS OF PARTIALLY SOLVATED GRIGNARD REAGENTS WITH A KETONE

Abstract Ratios of the yields of addition and reduction products for the reactions of n-butylmagnesium chloride and bromide with diisopropyl ketone in toluene were determined at different THF, diethyl ether and methyl tertbutyl ether (MTBE) contents in the Grignard reagent. The reduction reaction yield is a maximum at the molar ratio 0.1 of THF or diethyl ether to the Grignard reagent. The addition reaction has a maximum in the region of 0.3–0.4 for the same ethers and about 1.5 for MTBE. The ratio Add/Red for conventional Grignard reagents is lower than that for partially solvated reagents. The results were discussed in terms of the solvation of the species in the reaction mixture. The decisive role of the steric requirements of the reagents over their intrinsic acid-base properties was demonstrated. Partially solvated Grignard reagents can serve as tools for the investigation of solvent effects in the Grignard reaction.

Influence of sonication on Grignard reagent formation

Abstract The effect of ultrasonic irradiation on the kinetics of the reaction of alkyl bromides with magnesium in toluene in the presence of catalytic amounts of organic bases (diethyl ether, tetrahydrofuran) was investigated. Sonication exerts an accelerating effect on all steps of the reaction. The initiation constants of the reaction were determined and used as characteristics of the induction period of the process.

On the mechanism derived from kinetic solvent effects of Grignard reactions with silanes

Abstract In this communication we present the results of initial kinetic studies in which we have established that alkoxysilanes and chlorosilanes react with Grignard reagents in entirely different ways. The Grignard reaction with alkoxysilanes consists of replacement of a donor molecule at the magnesium centre by silane, followed by a subsequent rearrangement of the complex to the products. Chlorosilanes react without solvent molecule replacement.

KINETICS AND MECHANISM OF AMINOLYSIS OF ALIPHATIC ESTERS IN APROTIC SOLVENTS

Kinetic studies were carried out on the aminolysis reactions of substituted aliphatic esters in a variety of aprotic solvents. The reaction rate is strongly affected by inductive and steric effects of substituents in the acyl group, rising more than 104-fold from cyanoacetate to trifluoroacetate. The quantitative treatment of solvent effects revealed a rate decrease by the polarity and π-basicity of the solvents, and also an accelerating effect of the polarizability of solvents. Cyclic transition states were assumed for both the first and second-order (in amine) reactions. Copyright

Steric Effects of Alkylmagnesium Chlorides in the Grignard Reaction with Silanes

Rate constants for the reactions of methylvinyldichlorosilane and tetraethoxysilane with alkylmagnesium chlorides RMgCl (R = Et, n-Bu, i-Bu, i-Pr, s-Bu, t-Bu) in diethyl ether were determined. Excellent correlations of rate data with steric constants ES(Si) by Cartledge and v′ by Charton were found for the reaction of methylvinyldichlorosilane. Linear correlations with break points were obtained for the tetraethoxysilane reaction. It was assumed that this could be referred to a change in the reaction mechanism.

Mechanism of the reaction of acetylene with Grignard reagents

Abstract Kinetics of the reaction between acetylene and phenylmagnesium bromide were investigated in diethyl ether in the presence of small additions of triethylamine and without a catalyst. The mechanistic scheme suggested by Grignard et al. was supplemented with the reaction of bromomagnesiumacetylene with the Grignard reagent. The rate constants for individual reactions were determined. Triethylamine catalyzes the reactions to different extents, the conversions of bromomagnesiumacetylene being the most susceptible to the catalysis. The possible ways of the action of the catalyst were discussed and the importance of nucleophilic assistance was stressed.