The reactivity of carboxylic acid derivatives towards nucleophilic acyl substitution depends on the electrophilicity of the carbonyl carbon and the leaving group ability of the substituent.

The order of reactivity is: Acyl Halides (RCOCl) > Acid Anhydrides ((RCO)₂O) > Esters (RCOOR') > Amides (RCONH₂)

Reasons:

1. Leaving Group Ability: The chloride ion ($Cl^-$) is a weaker base and thus a better leaving group than acetate ($RCOO^-$), alkoxide ($RO^-$), or amide ($NH_2^-$) ions.
2. Inductive Effect: Chlorine has a strong electron-withdrawing (-I) effect, which increases the positive charge density on the carbonyl carbon, making it more susceptible to nucleophilic attack.
3. Resonance: In amides and esters, strong resonance delocalization reduces the electrophilicity of the carbonyl carbon, making them less reactive.