acyl reactivity order
Picture the carboxylic-acid family arranged on a staircase, with the twitchy, hyper-reactive members at the top and the calm, stable ones at the bottom. That staircase is the acyl reactivity order, and it tells you which derivatives can be turned into which — a single idea that organizes a large slice of organic chemistry.
From most to least reactive, the common acyl derivatives rank: acyl chlorides > anhydrides > esters (and carboxylic acids, similar) > amides. Two factors set the order. First, the leaving group: chloride leaves easily (stable conjugate base of strong HCl), a carboxylate leaves fairly well, an alkoxide leaves poorly, and an amide's nitrogen leaving group is terrible. Second, how much the attached atom's lone pair stabilizes the carbonyl by donating electron density into it: nitrogen donates strongly (so amides are very stable and unreactive), oxygen donates moderately (esters), and chlorine donates poorly (so acyl chlorides keep a hungry, electrophilic carbonyl). Both factors line up to give the same ranking.
The practical rule is that you can move down the staircase freely but not up. A more reactive derivative can be converted into any less reactive one — an acyl chloride can become an anhydride, ester, or amide; an ester can become an amide. Going uphill (say, ester back to acyl chloride) does not happen by simple substitution because it would require expelling a worse leaving group and creating a less stable product. This one-way logic is why acyl chlorides are the universal starting reagents and why the robust amide bond is nature's choice for proteins.
An acyl chloride + ammonia gives an amide easily (going down the staircase). But an amide + chloride will not give back an acyl chloride by simple substitution (going up is forbidden).
Down the reactivity staircase is allowed; up is not, by simple acyl substitution.
Nitriles are sometimes folded into this family too, but they react by addition rather than acyl substitution. The order is about ground-state stability and leaving-group ability, not about how fast any single reaction runs in a given setup.