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Chemical Properties of Carboxylic Acids

Reactivity involving O-H bond cleavage, C-OH bond cleavage, and Decarboxylation.

By chemca Team • Updated Jan 2026

Carboxylic acids are the most acidic organic compounds containing C, H, and O. Their reactions are classified based on bond cleavage: O-H cleavage (Acidity), C-OH cleavage (Derivative formation), and reactions involving the -COOH group or alkyl chain.

1. Reactions Involving Cleavage of O-H Bond (Acidity)

Acidic Nature

Carboxylic acids turn blue litmus red. They react with active metals, alkalis, and carbonates/bicarbonates.

Reaction with Sodium Bicarbonate ($NaHCO_3$): This is a distinguishing test for carboxylic acids. They evolve brisk effervescence of $CO_2$.

$$ R-COOH + NaHCO_3 \rightarrow R-COO^-Na^+ + H_2O + CO_2 \uparrow $$

Acidity Order: $RCOOH > H_2CO_3 > \text{Phenol} > H_2O > ROH$.
Electron withdrawing groups (EWG) increase acidity, while electron donating groups (ERG) decrease it.

2. Reactions Involving Cleavage of C-OH Bond

Formation of Acid Derivatives

The -OH group of the carboxylic acid is replaced by various nucleophiles.

A. Formation of Anhydride: Heating with dehydrating agents like $P_2O_5$ or Conc. $H_2SO_4$.

$$ 2CH_3COOH \xrightarrow{P_2O_5, \Delta} (CH_3CO)_2O + H_2O $$

B. Esterification: Reaction with alcohols in the presence of conc. $H_2SO_4$ (Fischer Esterification).

$$ R-COOH + R'-OH \xrightleftharpoons{H^+} R-COOR' + H_2O $$

C. Formation of Acyl Chlorides: Reaction with $PCl_5, PCl_3$ or $SOCl_2$.

$$ R-COOH + SOCl_2 \rightarrow R-COCl + SO_2 \uparrow + HCl \uparrow $$

D. Reaction with Ammonia (Amides): Forms ammonium salt which on heating gives Amide.

$$ R-COOH + NH_3 \rightleftharpoons R-COO^-NH_4^+ \xrightarrow{\Delta, -H_2O} R-CONH_2 $$

3. Reactions Involving -COOH Group

Reduction & Decarboxylation

A. Reduction: Reduced to primary alcohols by strong reducing agents like $LiAlH_4$ or $B_2H_6$.

$$ R-COOH \xrightarrow{1. LiAlH_4, 2. H_3O^+} R-CH_2OH $$

Note: $NaBH_4$ does not reduce carboxylic acids.

B. Decarboxylation: Sodium salts of carboxylic acids heated with Soda-lime ($NaOH + CaO$) lose $CO_2$ to form alkanes.

$$ R-COONa \xrightarrow{NaOH/CaO, \Delta} R-H + Na_2CO_3 $$

4. Substitution in Hydrocarbon Part

HVZ & Ring Substitution

A. Hell-Volhard-Zelinsky (HVZ) Reaction: Carboxylic acids having $\alpha$-hydrogen are halogenated at the $\alpha$-position on treatment with $Cl_2$ or $Br_2$ in the presence of Red Phosphorus.

$$ R-CH_2-COOH \xrightarrow{1. X_2/Red P, 2. H_2O} R-CH(X)-COOH $$

B. Electrophilic Ring Substitution (Aromatic Acids): The $-COOH$ group is deactivating and meta-directing.

$$ \text{Benzoic Acid} + HNO_3 \xrightarrow{H_2SO_4} \text{m-Nitrobenzoic Acid} $$ $$ \text{Benzoic Acid} + Br_2 \xrightarrow{FeBr_3} \text{m-Bromobenzoic Acid} $$

Note: Aromatic carboxylic acids do not undergo Friedel-Crafts reactions because the carboxyl group bonds with the catalyst ($AlCl_3$).

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