Chemical reactions of carboxylic acids

What reactions do carboxylic acids undergo?

In plain words: a carboxylic acid gives up its proton easily because the carboxylate ion left behind spreads its negative charge over two oxygen atoms. For otherwise comparable molecules, electron withdrawal generally favours acidity and electron donation opposes it. Compare the acid and conjugate base in the same solvent; position, solvation and steric effects can modify the trend.

Reactions with metals and alkalies

• Like alcohols, carboxylic acids give hydrogen with electropositive metals; like phenols, they form salts with alkalies (NCERT page 249).
• Unlike ordinary phenol in the hydrogencarbonate test, carboxylic acids give carbon dioxide with NaHCO₃. Sufficient acid also gives CO₂ from carbonate; carbonate itself can partly deprotonate phenol without that effervescence. Strongly electron-withdrawing substituents make some phenols exceptions. This reaction is used to detect a carboxyl group in an organic compound (NCERT page 249).

• In water, carboxylic acids dissociate into resonance-stabilised carboxylate ions and hydronium ions, RCOOH + H₂O ⇌ H₃O⁺ + RCOO⁻. In a dilute solution the concentration approximation to the acid dissociation constant is K_a = [H₃O⁺][RCOO⁻]/[RCOOH], and strength is usually given as pK_a = −log K_a (NCERT pages 249–250).

Acidity and the effect of substituents

At the same temperature and in the same solvent, the smaller the pK_a, the stronger the acid. Thermodynamic constants use activities; concentration expressions are dilute-solution approximations. NCERT’s values and its bands (page 250): strong acids have pK_a below 1, moderately strong acids 1 to 5, weak acids 5 to 15, and extremely weak acids above 15.

Typical simple carboxylic acids are weaker than strong mineral acids such as HCl and HNO₃ but stronger than alcohols and many simple phenols; NCERT calls them among the most acidic organic compounds met so far (page 250).

• ‘Moderately strong’ here, ‘weak’ in equilibrium. The four bands are NCERT’s for this section. In ionic equilibrium, an acid that ionises only partly in water is a weak acid, and acetic and benzoic acids are in NCERT Class 11’s table of weak acids (Unit 6, page 195). So in an equilibrium question call acetic acid a weak acid, and use the bands above only when a question asks for NCERT’s pK_a classes (reasoned).

• Why stronger than phenols. The carboxylate ion has two equivalent resonance structures, with the negative charge on the more electronegative oxygen atoms; the phenoxide ion has non-equivalent structures that put the charge on less electronegative carbon atoms. So the charge is spread over two oxygen atoms in carboxylate but less effectively over one oxygen and some carbon atoms in phenoxide, and the carboxylate ion is the more stable (NCERT page 250).

Effect of substituents

• Electron-withdrawing groups (EWG) stabilise the carboxylate ion by the inductive effect and/or resonance, and so strengthen the acid; electron-donating groups (EDG) destabilise it and weaken the acid (NCERT page 250).
• NCERT’s order of groups by the acidity they give (page 251): Ph < I < Br < Cl < F < CN < NO₂ < CF₃.
• Note on NCERT’s order (page 251): the order puts CF₃ above CN and NO₂, but CF₃ appears in NCERT’s acid list only as CF₃COOH, where its three fluorine atoms sit on the carbon next to –COOH; the list never compares it with CN or NO₂ at the same position. Beyond the NCERT text: compared on a benzene ring, CF₃ is the weakest of the three. Hammett’s substituent constant for a group in the 4-position is the drop it causes in the pK_a of benzoic acid in water: 0.54 for CF₃, 0.66 for CN and 0.78 for NO₂ (Hansch, Leo and Taft, Chemical Reviews, 1991, substituent-constant compilation). So 4-(trifluoromethyl)benzoic acid has pK_a about 4.19 − 0.54 = 3.65, while 4.19 − 0.78 = 3.41, NCERT’s own figure for 4-nitrobenzoic acid (reasoned). Do not treat the printed group sequence as a universal order. When two real acids are compared, such as 4-nitrobenzoic and 4-(trifluoromethyl)benzoic acid, the nitro acid is the stronger.
• NCERT’s list of acids (page 251), strongest first:

CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NC–CH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH

• Reading NCERT’s list. NCERT introduces it as “in order of increasing acidity” but writes it with > signs, strongest first, and draws arrows pointing to the left: acidity increases from right to left. Read each > as “is a stronger acid than”.
• Patterns in the list (reasoned): more halogen atoms, stronger acid (CCl₃COOH > CHCl₂COOH > ClCH₂COOH); with the same number of halogen atoms, F > Cl > Br, the order of electronegativity (CF₃COOH > CCl₃COOH, and FCH₂COOH > ClCH₂COOH > BrCH₂COOH); and the nearer the group, the bigger its effect. ClCH₂CH₂COOH, with chlorine one carbon further away, comes far below ClCH₂COOH, because the inductive effect “decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds” (NCERT Class 11, Section 8.7.5, page 274). Three bonds is a rule of thumb, not an exact physical cutoff (reasoned).
• Phenyl or vinyl attached directly to –COOH increases the acidity, contrary to the decrease the resonance effect would suggest, because the carboxyl carbon is then bonded to a more electronegative sp² carbon (NCERT page 251): C₆H₅COOH > CH₃COOH.
• Substituted benzoic acids. An EWG on the ring increases the acidity and an EDG decreases it: 4-nitrobenzoic acid (pK_a 3.41) > benzoic acid (4.19) > 4-methoxybenzoic acid (4.46) (NCERT page 251).

Reactions breaking the C–OH bond

• Anhydrides. Suitable dehydrating conditions, such as heating with P₂O₅, can join two acid molecules with loss of water. Heating any acid with any mineral acid is not a general guarantee of anhydride formation; water removal and the substrate matter: 2CH₃COOH → (CH₃CO)₂O + H₂O (H⁺, heat, or P₂O₅, heat), giving ethanoic anhydride (NCERT page 251; NCERT’s scheme leaves out the water).
• Esterification. Alcohols give esters in the presence of a mineral acid catalyst such as concentrated H₂SO₄ or HCl gas: RCOOH + R′OH ⇌ RCOOR′ + H₂O (H⁺) (NCERT page 251). Phenols are weaker nucleophiles and are not generally esterified efficiently by this ordinary Fischer procedure; acyl chlorides or anhydrides are usual alternatives. Special catalysed methods are a separate case.
• Mechanism of esterification (nucleophilic acyl substitution, NCERT page 252): (i) protonation of the carbonyl oxygen activates the C=O group; (ii) the alcohol adds as a nucleophile, giving a tetrahedral intermediate; (iii) a proton transfer turns an –OH into –OH₂⁺, a better leaving group; (iv) it leaves as a neutral water molecule; (v) the protonated ester loses a proton to give the ester.
• Acyl chlorides. The –OH is replaced by chlorine with PCl₅, PCl₃ or SOCl₂. Thionyl chloride is preferred, because the other two products are gases that escape, which makes the product easier to purify (NCERT page 252).

• Amides. With ammonia the acid first gives an ammonium salt, which on further heating at high temperature loses water to give the amide: CH₃COOH + NH₃ ⇌ CH₃COO⁻NH₄⁺ → CH₃CONH₂ + H₂O (heat); benzoic acid gives benzamide the same way. Phthalic acid gives ammonium phthalate, then phthalamide (losing 2H₂O), and on strong heating phthalimide (losing NH₃) (NCERT pages 252–253).
• NCERT slip (page 253): NCERT’s scheme shows phthalic acid + NH₃ ⇌ ammonium phthalate, but ammonium phthalate has two NH₄⁺ ions, one for each –COO⁻ group, so two molecules of ammonia are needed: C₆H₄(COOH)₂ + 2NH₃ ⇌ C₆H₄(COO⁻NH₄⁺)₂ (reasoned: count the nitrogen atoms). In an exam, write 2NH₃.

Reactions of the –COOH group

• Reduction. Lithium aluminium hydride or diborane reduces carboxylic acids to primary alcohols: RCOOH → RCH₂OH ((i) LiAlH₄/ether or B₂H₆, (ii) H₃O⁺). NCERT prefers diborane (“or better with diborane”), which “does not easily reduce functional groups such as ester, nitro, halo, etc.”, and states: “Sodium borohydride does not reduce the carboxyl group” (NCERT page 253).
• Decarboxylation. Sodium salts of carboxylic acids heated with soda lime (NaOH and CaO in the ratio 3 : 1) give hydrocarbons with one fewer carbon; the carboxyl carbon ends up in carbonate, not free CO₂ gas in this overall equation: RCOONa + NaOH → RH + Na₂CO₃ (CaO, heat) (NCERT page 253).
• Kolbe electrolysis. Alkali-metal salts of carboxylic acids decarboxylate on electrolysis of their aqueous solutions, giving a hydrocarbon with twice the number of carbon atoms in the alkyl group (NCERT page 253). CO₂ is released at the anode; side reactions and yields depend on the substrate and conditions (reasoned): 2CH₃COONa + 2H₂O → CH₃–CH₃ + 2CO₂ + H₂ + 2NaOH, as in the preparation of alkanes (NCERT Class 11, Unit 9, page 301).

Substitution in the hydrocarbon part

Hell–Volhard–Zelinsky reaction

• Carboxylic acids with an α-hydrogen are halogenated at the α-position by chlorine or bromine with a small amount of red phosphorus, giving α-halocarboxylic acids (NCERT page 253):

Ring substitution in aromatic acids

• When –COOH is directly bonded to an aromatic ring, it is deactivating and meta-directing in electrophilic substitution (NCERT page 254; see directive influence): benzoic acid with concentrated HNO₃ and concentrated H₂SO₄ gives m-nitrobenzoic acid, and with Br₂/FeBr₃ m-bromobenzoic acid.
• No Friedel–Crafts reaction: the carboxyl group deactivates the ring, and the catalyst, aluminium chloride (a Lewis acid), gets bonded to the carboxyl group (NCERT page 254).

Solved examples

Common mistakes to avoid

Quick revision summary

• O–H cleavage: H₂ with Na; salts with NaOH; CO₂ with NaHCO₃ (supports an acidic group; ordinary phenol is negative, with exceptions for strongly acidic phenols).
• Acidity: pK_a = −log K_a, smaller pK_a stronger; CF₃COOH 0.23, benzoic 4.19, acetic 4.76, phenol 10, ethanol about 16. EWG raise acidity, EDG lower it; the effect falls off with distance.
• C–OH cleavage: anhydrides (H₂SO₄ or P₂O₅, heat); esters (alcohol, H⁺; nucleophilic acyl substitution); acyl chlorides (PCl₅, PCl₃, SOCl₂, SOCl₂ preferred); amides (heating ammonium salts).
• –COOH group: reduced to –CH₂OH by LiAlH₄ or B₂H₆ (not NaBH₄); removed into carbonate with soda lime; released as CO₂ during Kolbe electrolysis.
• Hydrocarbon part: Cl₂ or Br₂ with red phosphorus gives α-halo acids (HVZ); –COOH is meta-directing and deactivating; no Friedel–Crafts.

Questions students ask

Why are carboxylic acids stronger acids than alcohols?

Carboxylic acids are far stronger acids than alcohols because the carboxylate ion spreads its negative charge over two oxygen atoms through two equivalent resonance structures (NCERT page 250), while an alkoxide ion such as ethoxide has no resonance and keeps the charge on one oxygen (reasoned). The gap is large: pK_a about 16 for ethanol against 4.76 for acetic acid (NCERT page 250), so acetic acid’s K_a is roughly 10¹¹ times greater (reasoned). So carboxylic acids react with alkalies and even hydrogencarbonates, while alcohols are much weaker acids and can react with active metals such as sodium or sufficiently strong bases such as NaH (NCERT pages 205 and 249).

Is formic acid a stronger acid than acetic acid?

Formic acid, HCOOH, is about ten times as strong an acid as acetic acid, CH₃COOH: NCERT Class 11 gives K_a = 1.8 × 10⁻⁴ and 1.74 × 10⁻⁵ (Unit 6, Table 6.6, page 195), a ratio of 10.3, or pK_a 3.74 against 4.76 (reasoned). The difference is the methyl group: it donates electrons by its inductive effect and so destabilises the acetate ion, while formate has only a hydrogen on its carboxyl carbon (reasoned from NCERT page 250). NCERT’s acid list ranks HCOOH above CH₃COOH, with CH₃CH₂COOH weakest (page 251).

What is the order of reactivity of carboxylic acid derivatives?

Carboxylic acid derivatives react with nucleophiles in the order acyl chloride > anhydride > ester > amide, a point beyond the NCERT text (Wikipedia, “Acyl group”) that fits NCERT’s conditions: water alone hydrolyses acyl chlorides and anhydrides, esters are hydrolysed with acid or base (page 247), and amides with H₃O⁺ and heat (page 246). Two effects set the order (Wikipedia): Cl⁻, the conjugate base of the strong acid HCl, leaves far more easily than RCOO⁻, RO⁻ or NH₂⁻, and the more the atom next to C=O shares its lone pair, as an amide’s nitrogen does most, the less electrophilic the carbon.

Why does sodium borohydride not reduce carboxylic acids when LiAlH₄ does?

Under ordinary conditions NaBH₄ reduces aldehydes and ketones but is not an effective reagent for converting –COOH to –CH₂OH. An acid can consume hydride by proton transfer, generating H₂ and carboxylate, and reduction also depends on electrophilicity and coordination. LiAlH₄ or suitable borane reagents can complete the reduction after appropriate work-up. The difference is not explained by bond strengths alone, and modified NaBH₄ systems should not be confused with the standard textbook reagent.

What does HVZ stand for, and what does the red phosphorus do in the HVZ reaction?

HVZ stands for Hell–Volhard–Zelinsky, the three chemists the reaction is named after, Carl Magnus von Hell and Jacob Volhard, both German, and Nikolay Zelinsky, a Russian, a point beyond the NCERT text (Wikipedia, “Hell–Volhard–Zelinsky halogenation”). The red phosphorus and bromine make PBr₃ in the flask, as NCERT describes for making haloalkanes (Unit 6, page 164); PBr₃ replaces the acid’s –OH by Br, the acyl bromide formed changes into its enol, bromine attacks the enol at its α-carbon, and water finally turns the –COBr group back into –COOH (Wikipedia). This route through the enol is why only acids with an α-hydrogen react (NCERT page 253; reasoned).

Where is the HVZ reaction taught, and how should I check examination coverage?

The Hell–Volhard–Zelinsky reaction is taught in NCERT Class 12 Chemistry Part II, Unit 8, Section 8.9.4, page 253, under substitution in the hydrocarbon part of carboxylic acids. It is useful for connecting an α-hydrogen to α-halogenation. For examination scope, consult the official syllabus for your year; a broad heading about general reactions does not guarantee inclusion of every named reaction.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why are carboxylic acids stronger acids than phenols?

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   The carboxylate ion is stabilised by two equivalent resonance structures with the negative charge on the electronegative oxygen atoms; the phenoxide ion’s structures put the charge partly on carbon atoms, so it is less stable. The more stable the conjugate base, the stronger the acid (NCERT page 250).

2. Q2. Why is thionyl chloride preferred to PCl₅ for making acyl chlorides?

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   With SOCl₂ the other two products, SO₂ and HCl, are gases that escape from the reaction mixture, so the acyl chloride is easier to purify (NCERT page 252).