The effects of solvation on the pairing of electrons in molecules have been analyzed in a series of molecules and in the Menshutkin reaction between ammonia and methyl chloride. Solvation in water and chloroform has been modeled by means of the polarizable continuum model. A comparative analysis of the electron-pair structure of all the molecules studied has been carried out in the framework of the atoms in molecules theory. In particular, atomic populations and localization and delocalization indices have been used for describing the electron-pair characteristics of all the molecules. In general, this analysis shows that solute-solvent interactions modify the electron-pair distribution of the solute increasing the polarization of the molecular bonds. The electron-pair characteristics of the different stationary points found in the Menshutkin reaction have been also analyzed in detail. The evolution of electron pairing along the reaction path has also been followed in vacuo and in water, Comparison of the results obtained reveal that the main difference between the reaction in gas phase and in water is the structural and electronic advance of the transition state towards the reactant side. Finally, the Menshutkin reaction with explicit representation of the solvent molecules has been studied to discuss the relevance of charge-transfer and specific interactions between the solvent and the solute.