What are considered "good" leaving groups?

Consider a general nucleophilic substitution reaction.

The second arrow always shows a pair of electrons going toward the leaving group.

The best leaving groups "want" those electrons. They don't want to share them with other atoms.

Good leaving groups are weak bases.

Weak bases have strong conjugate acids. So we can identify weak bases by looking at a #"p"K_"a"# table.

Caution: The #"p"K_"a"# value measures the position of an equilibrium. But leaving group ability is based on reaction rates. So although the correlation is good, it's not perfect.

In general, the weaker the base, the better the leaving group.

Exception: Fluorine is a poor leaving group.

F⁻ is a small ion. Its high charge density makes it relatively unpolarizable. The leaving group needs to be polarizable to lower the energy of the transition state. You should never see F⁻ leave in an #"S"_"N"2# reaction.

Question: Why is water a good leaving group?
Answer#color(white)(ll)#: It isn't! Hydronium ion is the leaving group.

Explanation:

Consider the equilibrium

#"R-OH ⇌ R"^"+" + underbrace("OH"^"-")_color(red)("conj. base of H"_2"O")#

Water is a weak acid, so the hydroxide ion is a strong base. It "wants" to use its lone pair electrons to form a covalent bond.

Thus, the position of equilibrium lies far to the left.

If we protonate the alcohol, we get

#"R-"stackrelcolor(blue)(+)("O")"H"_2 ⇌ "R"^"+" + underbrace("OH"_2)_color(red)("conj. base of H"_3"O"^"+")#

Water is the conjugate base of hydronium ion, the strongest acid that can exist in water.Thus water is a weak base.

It has little tendency to share its electron pairs with another species.

Water in the form of hydronium ion is an excellent leaving group.

Here's a video on what makes a good leaving group.