Ethyl alcohol, atom polarization

The carbonyl (C=0) is protonated because of its polarization, which results in a partially negatively charged oxygen atom. Next, the protonated acid is attacked by a nucleophile, in this case an alcohol (ethyl alcohol) due to the fact that the carboxyl carbon now has significant positive character. 

The solvent used in these dimerization reactions dictates the activity of the catalyst. Besides benzene and dioxane, acetic acid [246,247], halogenated hydrocarbons [245,247], nitroderivatives [247], sulfones, tetrahydrofuran (THF), ethyl acetate, phenol, dimethylformamide, hydroquinone, catechol, benzyl alcohol, salicylic acid, anisole, and acetone have been used [247]. In solvents like benzene and chlorohydrocarbons and in highly polar solvents like dimethylsulfoxide and dimethylformamide, dimerization does not occur. The reaction proceeds in solvents containing oxygen atoms. The dissociative solvents like phenol and acetic acid show high solvent effect. In carboxylic acids the rate of dimerization decreases with an increase in pK . 

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