Question

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

Answer 1

Protic solvents are the solvents which contain hydrogen atoms attached to a heteroatom like nitrogen, oxygen or fluorine. These solvents involve intermolecular hydrogen bonding wherever required.

Complete step by step answer:
Polar protic solvents are a class of solvents which have sufficient polar groups attached. Examples are alcohols like methanol or ethanol and amines like methyl amine, ethyl amine etc.
These compounds possess considerable dipole moments which are induced by the presence of active polar functional groups. The term protic refers to the hydrogen atom. The polar protic solvents contain hydrogen atoms which are available for donation or bonding.
Besides having dipole-dipole interactions these solvents form intermolecular hydrogen bonding with the compounds dissolved in it. The hydrogen atom in these solvents is bound to heteroatoms like $O - H$ and $N - H$. The charge of the solvents are expressed as
$C{H_3}{O^{\delta - }}{H^{\delta + }}$
The partial dipolar nature by virtue of $\delta +$ and $\delta -$ charges arises because of electronegativity difference which accounts for these solvents to solve cations and anions.
$C{H_3}O{}^{\delta - }{H^{\delta + }} \cdot \cdot \cdot \cdot \cdot C{l^ - } \cdot \cdot \cdot \cdot \cdot {H^{\delta + }}{O^{\delta - }}C{H_3}$
Thus the polar protic solvents affect the nucleophilicity of the functional groups attached to the compound. Hence the anions like chloride and bromide ions are stabilized by dipole-dipole interactions and intermolecular hydrogen bonding.

Note: The nucleophilicity of the anions increases down the column. The smaller anions are solvated more than the larger anions. Thus a fluoride ion is well solvated by hydrogen bonding while the iodide ion was not solvated tightly. This also leads to the availability of the corresponding anions towards various reactions. Unlike polar protic solvents, polar aprotic solvents do not donate protons and only solvate the cations and anions by dipole-dipole interactions.