The correct order of leaving group ability in a nucleophilic substitution reaction is:
1. If we move from left to right on the periodic table, electronegativity increases. With an increase in electronegativity, basisity decreases, and the ability of the leaving group to leave increases.
2. This is because an increase in electronegativity results in a species that wants to hold onto its electrons rather than donate them.
Thus, Br−>Cl−>CH3CO−2>OH−>H−