Influence of solvents

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The influence of solvents is understood to mean the effects of a solvent on chemical reactions, in particular the influence on the reaction rate and the chemical equilibrium .

history

Marcelin Berthelot found in 1862 that the esterification of acetic acid and ethanol in the two solvents benzene and diethyl ether proceeded at different rates and that the reaction rate was greatest without solvents. From 1887 Nikolai Alexandrowitsch Menschutkin investigated esterification reactions and the influence of 23 different solvents on the reaction between triethylamine and ethyl iodide . Simply changing the solvent from n- hexane to benzyl alcohol accelerated the Menschutkin reaction by a factor of 742. An explanation for the influence of different solvents on the reactions could not be provided at that time.

function

Intermolecular interactions between the solvent and the substances involved and formed in the reaction are decisive for influencing the reaction rate and setting the chemical equilibrium . It is particularly important here whether the solvent is polar or non-polar. In the case of polar solvents, a distinction is also made between polar protic and polar aprotic . The interactions lead to a more or less pronounced solvation and to different solvation enthalpies .

For the influence on the position of homogeneous chemical equilibrium, the difference in the solvation enthalpy of the starting and end materials is of decisive importance. The influence on the rate of homogeneous reaction depends on whether the activation enthalpy of the activated complex in the transition state is increased or decreased by the solvation enthalpy. A decrease leads to an accelerated and an increase to a delayed response. According to the rule established by Hughes and Ingold in 1935, reactions that generate charges during the activation process are accelerated by increasing the solvent polarity. One example is the hydrolysis of tert-butyl chloride . In the intermediate step, a tert-butyl cation is formed which, depending on the solvent, further reacts to tert- butanol more or less quickly . The reaction rate in water is 335,000 times faster than in ethanol. The Hughes-Ingold rule is, however, limited in its general applicability because additional specific solvent effects often play a role.

literature