# Carboxylic Acid Derivatives > Carboxylic acid derivatives for JEE Chemistry: reactivity order of acyl chlorides, anhydrides, esters and amides, hydrolysis, reductions and HVZ reaction. - Canonical URL: https://prepelephant.com/topics/jee/chemistry/carboxylic-acid-derivatives - Exam / course: JEE · Subject: Chemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Carboxylic Acid Derivatives", PrepElephant, https://prepelephant.com/topics/jee/chemistry/carboxylic-acid-derivatives ## Direct answer Down the acyl family, nucleophilic acyl substitution gets steadily harder: acid chloride > anhydride > ester > amide > carboxylate, an order that tracks leaving-group ability exactly in reverse of basicity (Cl- is the weakest base and best leaver; NH2- the worst). Every member interconverts toward the acid by hydrolysis and away from it by pairing with SOCl2, whose gaseous by-products (SO2 plus HCl) drive the acid chloride formation cleanly. Reduction splits the family: LiAlH4 takes acid chlorides and esters to primary alcohols but converts amides to amines with the carbonyl reduced to CH2; DIBAL-H halts at the aldehyde; and HVZ halogenation installs a halogen at the alpha carbon of the acid itself. ## What you must remember - **Reactivity order and its cause:** acyl chloride > anhydride > ester > amide; the better the leaving group (weaker base) and the more electron-withdrawing the acyl carbon's partner, the faster substitution. - **Best route to the acid chloride:** RCOOH + SOCl2 giving RCOCl with SO2 and HCl escaping as gases — equilibrium driven by departure, which PCl5 and PCl3 cannot match. - **Acid chloride signatures:** fumes of HCl in moist air, violent hydrolysis, Rosenmund reduction (H2 over Pd-BaSO4 poisoned with quinoline) stopping at the aldehyde. - **Ester chemistry both ways:** Fischer esterification (acid plus alcohol, H+ catalysed, reversible) versus saponification (OH- driven, irreversible because the carboxylate product is immune to nucleophilic attack). - **Reduction map:** LiAlH4 sends RCOCl and RCOOR to RCH2OH; RCONH2 with LiAlH4 gives RCH2NH2; DIBAL-H at low temperature converts esters and nitriles to aldehydes. - **HVZ reaction:** carboxylic acid plus Cl2/Br2 with red phosphorus installs an alpha-halogen — the route to alpha-amino acids via ammonia. - **Carbon arithmetic:** soda-lime decarboxylation (RCOONa with NaOH-CaO) removes one carbon as carbonate, giving RH; Hofmann bromamide does the same descent for amides. - **Ammonolysis ladder:** acid chloride plus ammonia gives the amide directly; ester plus ammonia needs heat; the amide is the least reactive and last stop. ## Why the order of acyl reactivity Ask why acetyl chloride fumes while acetamide sits quietly on the shelf. At the moment of attack, the nucleophile must push electrons onto the acyl carbon's partner; a partner that leaves as a stable, weakly basic species — chloride — makes that cheap, while an amide's NH2- would have to leave as the second-strongest base of the series, so it refuses and instead donates its lone pair into the carbonyl, choking the attack site with resonance. That same donation explains amide stability in water and their anomalous reduction: LiAlH4 delivers hydride not to the sluggish carbonyl but through the iminium stage after oxygen departure, so the product is an amine, not an alcohol — the single most examined surprise of the chapter. Now exploit the order commercially: to make an anhydride, let an acid chloride meet the acid's sodium salt; to make an ester, add the alcohol to the chloride with base; never attempt the reverse, because the arrow of reactivity only points downhill. ## How the exam frames it Acid derivatives sit inside the JEE Main unit on Organic Compounds Containing Oxygen, tested as reagent-product matching, reactivity-order ranking and one-step conversions; JEE Advanced adds mechanism tracing through the tetrahedral intermediate and reagent-selectivity puzzles. The dependable traps: predicting RCOOH from amide plus LiAlH4 (it is RCH2NH2), expecting NaBH4 to touch any acyl derivative (it reduces only aldehydes and ketones), and forgetting that saponification is irreversible precisely because the carboxylate ion cannot re-esterify. Also remember the smell clue the examiners like: fruity esters versus the acrid acid chloride. ## Frequently asked questions ### Why is thionyl chloride preferred for making acid chlorides? Its by-products, sulphur dioxide and hydrogen chloride, are both gases that leave the mixture, driving the equilibrium to completion and leaving a clean product. ### Why does reactivity fall from acid chloride to amide? Leaving-group ability falls and resonance donation rises along that order — chloride departs easily while NH2- cannot, and the amide nitrogen's lone pair deactivates the carbonyl toward attack. ### What does LiAlH4 do to an amide? It reduces the carbonyl all the way to CH2, giving a primary amine with the same carbon skeleton — an amine synthesis, not an alcohol synthesis. ### Why is ester saponification irreversible? The product is the carboxylate anion, whose negative charge repels the alkoxide leaving group and blocks re-esterification; acidification is needed merely to recover the free acid. ### How does DIBAL-H stop at the aldehyde? Its bulky aluminium centre and low-temperature stoichiometric use allow only one hydride delivery and fast breakdown of the intermediate, so the aldehyde is released before further reduction.