Reactivity order for nucleophilic acyl substitution: Acid chloride > Anhydride > Ester > Amide

Key interconversions (downhill in reactivity):

• RCOCl + R'OH → RCOOR' (ester) + HCl
• RCOCl + R'NH2 → RCONHR' (amide) + HCl
• RCOCl + RCOO-Na+ → (RCO)2O (anhydride)
• (RCO)2O + R'OH → RCOOR' + RCOOH
• (RCO)2O + R'NH2 → RCONHR' + RCOOH
• RCOOR' + R"NH2 → RCONHR" + R'OH (aminolysis)

Going uphill (more reactive derivative from less reactive):

• RCOOH + SOCl2 → RCOCl + SO2 + HCl
• RCOOH + PCl5 → RCOCl + POCl3 + HCl
• RCOOR' + OH- → RCOO- + R'OH (saponification — breaks ester)

JEE tip: You can always convert a more reactive derivative to a less reactive one (downhill). Going uphill requires special reagents (SOCl2, PCl5).