Question

How do polar protic solvents stabilize the chloride and bromide ions?

Answer 1

In addition to dipole–dipole interactions, polar protic solvents are capable of intermolecular hydrogen bonding.

Explanation:

In addition to dipole–dipole interactions, polar protic solvents are capable of intermolecular hydrogen bonding, because they contain an O – H or N – H bond. The most common polar protic solvents are water and alcohols (ROH), as seen in the examples in the Figure below. Polar protic solvents solvate both cations and anions well.

How do polar protic solvents affect nucleophilicity? In polar protic solvents, nucleophilicity increases down a column of the periodic table as the size of the anion increases.

This is opposite to basicity. A small electronegative anion like `F^-` is very well solvated by hydrogen bonding, effectively shielding it from reaction. On the other hand, a large, less electronegative anion like `I^–` does not hold onto solvent molecules as tightly. The solvent does not “hide” a large nucleophile as well, and the nucleophile is much more able to donate its electron pairs in a reac- tion.

Thus, nucleophilicity increases down a column even though basicity decreases, giving rise to the following trend in polar protic solvents:

This was taking from the text book: Organic Chemistry-Janice Gorzynski Smith 3rd Ed