polar protic solvent

Some solvents have a hydrogen they can offer up — an O–H or N–H bond — and these hydrogens let them form hydrogen bonds with whatever is dissolved in them. Water, methanol, ethanol, and acetic acid are the everyday examples. A solvent that is both polar (it has positive and negative ends) and protic (it has these donatable H's) is a polar protic solvent.

What matters for substitution is how such a solvent treats ions. Polar protic solvents are wonderful at surrounding both cations and anions. The solvent's positive (H) ends cluster around negative ions, and its negative (O) ends cluster around positive ions, wrapping each in a stabilizing shell — this is solvation. This makes polar protic solvents the natural home for SN1 reactions: they stabilize the carbocation and the departing anion as the substrate ionizes, easing the otherwise hard ionization step and speeding SN1 along.

But that same wrapping has a cost for SN2. By hydrogen-bonding tightly around a small anionic nucleophile, a protic solvent cages it, making it sluggish — the nucleophile must shrug off its solvent shell before it can attack, which slows it down. This is exactly why nucleophilicity trends down a group in protic solvents (small, hard F- is tightly caged and slow; big, soft I- is loosely held and fast). So the rule of thumb: polar protic solvents favor SN1 and tend to suppress SN2; polar aprotic solvents do the reverse.