Carboxylic Acids
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Direct answer
Carboxylic acids contain the -COOH group, whose acidity comes from resonance delocalisation of the carboxylate's negative charge over two oxygens. Their reactions split between the O-H bond (acid-base behaviour, salt formation) and the C-O bond (esterification, acyl chloride formation), while the alpha carbon carries the Hell-Volhard-Zelinsky halogenation. The acid derivatives — acyl chlorides, anhydrides, esters, amides — react by nucleophilic acyl substitution at rates set by leaving-group ability and resonance donation.
What you must remember
- Acidity: formic acid > benzoic acid > acetic acid, all above phenol and water; electron-withdrawing substituents strengthen — CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH — and the effect fades with distance from the carboxyl.
- Ortho effect: almost any ortho substituent makes benzoic acid stronger than benzoic acid itself, whatever its electronic nature.
- Separation test: carboxylic acids effervesce with NaHCO3 (CO2 released); phenols do not.
- Fischer esterification: RCOOH + R'OH with concentrated H2SO4, reversible and acid-catalysed; esters smell fruity.
- C-O conversions: SOCl2 gives acyl chlorides cleanly (gaseous SO2 and HCl leave the product pure); PCl5 also works; ammonia gives ammonium salts that dehydrate to amides on heating.
- HVZ reaction: acids with alpha hydrogen plus Cl2 and red phosphorus give alpha-chloro acids — the gateway to alpha-amino acids.
- Hofmann bromamide degradation: RCONH2 + Br2 + 4KOH → RNH2 + K2CO3 + 2KBr — the amine one carbon shorter.
- Decarboxylation: sodium salts with soda lime give RH — RCOONa + NaOH → RH + Na2CO3.
- Reduction: LiAlH4 takes RCOOH to RCH2OH with the chain intact.
- Derivative reactivity: acyl chloride > anhydride > ester > amide, set by leaving-group basicity and resonance donation from the adjacent heteroatom.
Common confusion
The recurring error is ranking acidity by intuition rather than carboxylate stabilisation: students expect benzoic acid to beat formic acid, but the ring's donation within its framework leaves benzoate less stabilised than formate. The second confusion is reduction scope — LiAlH4 reduces the acid fully to the primary alcohol. In derivative interconversions, students forget the ladder is one-way: acyl chloride converts to everything, but an amide cannot climb to an acid chloride directly.
Exam-focused takeaway
JEE Main tests this chapter as facts: pick the strongest acid, name the esterification product, identify HVZ or Hofmann products. JEE Advanced weaves derivatives into synthesis — convert an acid to the amine one carbon shorter, pick reagents that reduce the acid without touching an ester, reason through derivative reactivity ladders in multi-step sequences. Anchor acidity questions on carboxylate stabilisation and derivative questions on the leaving group; those two handles solve most of the chapter.
Frequently asked questions
Why is formic acid stronger than acetic?
Acetate carries a methyl group donating electron density into the anion; formate does not, so formate — and its acid — is stabilised better.
What is the ortho effect?
Almost any group at the ortho position of benzoic acid raises its acidity above the parent acid, through steric and electronic disruption of the carboxyl's resonance with the ring.
What separates a carboxylic acid from a phenol?
Sodium bicarbonate: the acid liberates CO2 briskly; phenol, weaker than carbonic acid, gives no reaction.
What is the HVZ reaction?
Alpha-halogenation of a carboxylic acid with Cl2 or Br2 and red phosphorus, giving alpha-halo acids — the route to alpha-amino acids.
Why are amides the least reactive derivatives?
The nitrogen lone pair donates strongly into the carbonyl, stabilising the amide and resisting nucleophilic acyl substitution.
What does soda-lime decarboxylation give?
The alkane one carbon shorter: RCOONa with NaOH/CaO expels the carboxyl carbon as carbonate, leaving RH.