a. Which is a stronger nucleophile ? `C_(2)H_(5)SH` (ethane thiol or ethyl mercaptan) and `C_(2)H_(5)S^(-)` (ethane thiol or ethyl mercaptide) in a protic solved ?
b. Which of the following has high water solubility: `C_(2)H_(5)SH` and `C_(2)H_(5)OH` ?

Explanation

a. Nucleophilic character: `C_(2)H_(5)S^(-) gt C_(2)H_(5)SH`. `C_(2)H_(5)S^(-)` is negatively charged, has high `bar e`-density, and is a stronger nucleophile than uncharged conjugate acid, `C_(2)H_(5)SH`.
b. `C_(2)H_(5)SH` is a stronger acid than `C_(2)H_(5)OH`.
Acidic character: `C_(2)H_(5)SH gt C_(2)H_(5)OH`
Basic character : `C_(2)H_(5)S^(-) lt C_(2)H_(5)O^(-)`
Nucleophilic character: `C_(2)H_(5)S^(-) gt C_(2)H_(5)O^(-)`
When nucleophilic centes are different and belong to the same group of periodic table (16 groups), nucleophilicity and basic character are reversed. In protic solvent, the nucleophilicity of anion increases down the group in the periodic table. The `bar e` cloud on the larger S is more easily polarised, which makes easier for it to attack an electrophile. However, the smaller `C_(2)H_(5)O^(-)` has a high `bar e`-density on the O atom per unit surface area, causing it to form H-bonding with protic solvent and thereby decreases the avaiabilityy of its `bar e`'s pairs in SN reaction.
c. Alcohols `(C_(2)H_(5)OH)` have boiling points than the corresponding thiols, although thiols have high molecular masses than alcohols because alcohols are more polar and form H-bonding with each other.
d. Alcohols `(C_(2)H_(5)OH)` have high water soulbilities than thiols because alcohols are more polar and form H-bonding with `H_(2)O`. S is less EN than O, and thus does not form H-bonding. Only high EN elements such N,O,F, and sometimes CI form H-bonding.