Final answer:
Dimethyl sulfide ((CH₃)₂S) is a stronger nucleophile than methanethiol (CH₃SH) due to the additional electron-donating methyl group and sulfur's ability to accommodate larger substituents with less steric hindrance, resulting in increased electron density around sulfur.
Step-by-step explanation:
The reason why (CH₃)₂S is a stronger nucleophile than CH₃SH is influenced by the molecule's ability to donate electron density to an electrophile. In the case of (CH₃)₂S, the presence of two methyl groups provides more electron-releasing effects compared to CH₃SH, where only one such group is present. This additional electron-donating effect makes the sulfur atom in (CH₃)₂S more negatively charged and thus a stronger nucleophile. Furthermore, steric hindrance is less of an issue for sulfur, which is a third-row element, meaning it can accommodate larger substituents easier than a second-row element such as oxygen. As such, (CH₃)₂S can still effectively approach and react with electrophiles despite the presence of two bulky methyl groups.
Moreover, nucleophilicity is also affected by the solvent in which the reaction takes place. Nucleophiles tend to be more reactive in polar aprotic solvents that do not form hydrogen bonds with the nucleophile, allowing it greater freedom to react. Conversely, polar protic solvents could hinder the nucleophilicity of a molecule by solvating it too intensely.