Carboxylic Acid Reactions

8.9 Chemical Reactions of Carboxylic Acids L3

The reactions of -COOH cluster into four groups depending on which bond breaks:

• O–H bond cleaved — salt formation, ester formation, anhydride formation, conversion to acyl chloride (with PCl₃, PCl₅, SOCl₂).
• Whole –COOH replaced — reduction to alcohol, decarboxylation.
• Substitution on the α-carbon — HVZ halogenation.
• Substitution on the ring (for aromatic acids) — electrophilic substitution.

8.9.1 Reactions Involving Cleavage of O–H Bond

(a) Salt formation — distinguishes acids from phenols

Phenols dissolve in NaOH but not in NaHCO₃ — so the NaHCO₃ "fizz" test confirms a carboxylic acid.

(b) Esterification with alcohols (Fischer esterification)

Acid-catalysed reversible reaction. To push equilibrium right, use excess alcohol or remove water (Dean-Stark trap).

(c) Acid anhydride formation

(d) Conversion to acyl halides with PCl₃, PCl₅, SOCl₂

SOCl₂ is preferred — the by-products are gases and escape, so the acyl chloride is easily purified.

8.9.2 Reactions Involving the –COOH (or –CO) Group

(a) Reduction to 1° alcohols

LiAlH₄ reduces –COOH all the way to –CH₂OH. Diborane (B₂H₆) is also effective and is selective — it leaves nitro, halo, ester groups intact.

(b) Decarboxylation — loss of CO₂

Sodium salts of carboxylic acids, on heating with soda-lime (NaOH+CaO), lose CO₂ to give an alkane (carbon count − 1):

Kolbe's electrolysis:

Example: electrolysis of CH₃COONa solution gives ethane.

8.9.3 Substitution on the α-Carbon — HVZ Reaction

Hell-Volhard-Zelinsky (HVZ) reaction: acids that have an α-H react with Cl₂ or Br₂ in the presence of a small amount of red phosphorus to give α-haloacids.

The α-haloacid is a synthetic gateway — KCN gives α-cyanoacids, NH₃ gives α-aminoacids, OH⁻ gives α-hydroxyacids.

8.9.4 Substitution in the Ring (Aromatic Acids)

The –COOH group is electron-withdrawing and meta-directing. So benzoic acid undergoes electrophilic substitution (nitration, sulphonation, halogenation) slowly and gives the meta product as the major isomer.

Friedel-Crafts alkylation/acylation does not work on benzoic acid — the COOH deactivates the ring strongly and also coordinates AlCl₃.

8.10 Uses of Carboxylic Acids L1

• Methanoic acid — leather processing, rubber coagulation, medicines for treating muscle pain.
• Ethanoic acid — vinegar (5–8 % in water), industrial solvent, manufacture of cellulose acetate fibres and white lead pigments.
• Hexanedioic acid — monomer for Nylon-6,6.
• Esters of benzoic acid — perfumes; sodium benzoate is a food preservative.
• Higher fatty acids — Na/K salts make soaps; Mg/Ca salts make greases and lubricants.
• Aspirin = acetylsalicylic acid — analgesic, anti-inflammatory, antipyretic.

Intext Practice L3