Chemical Reactions of Carboxylic Acids

Acidity

• Like alcohols, carboxylic acids evolve hydrogen gas when reacting with metal and form salts with alkalis. Unlike phenols, they require weaker base like sodium carbonate to evolve carbon dioxide

• Smaller pKa value, stronger the acid

• Trifluroacetic acid is the strongest organic acid (pKa = 0.23)

• Carboxylic acids tend to be weaker than mineral acids but stronger than alcohols and phenols

• Acidic nature of carboxyl group is due to the breaking of O-H bond, due to positively charged O weakening the bond as well as resonance stabilization of carboxylate ion

Effect of Substituents on Acidity of Carboxylic Acid

• Electron withdrawing groups increase stability by dispersing negative charge via resonance effect

• Electron donating groups decrease stability by intensifying negative charge

• Directly substituted phenyl and vinyl actually increase stability despite decrease in resonance effect

• Order of acidity for hybridization

  • sp3 > sp2 > sp

• Order of increasing acidity

  • Ph < I < Br < Cl < F < CN < NO2 < CF3

Esterification

• Carboxylic acids react with alcohol/phenol in the presence of mineral acids like conc. H2SO4 and HCl as a gas (as catalyst) to produce esters

• Mechanism

  • Protonation of carbonyl oxygen

  • Nucleophilic addition of alcohol

  • Loss of water molecule and proton

Anhydride Formation

• Carboxylic acids heat with mineral acids like conc. H2SO4 or P2O5 to give us anhydrides

• Acid anhydrides are formed by the elimination of a water molecule

Reaction with Phosphorus Trihalides, Phosphorus Pentahalides and Thionyl Chloride

• Like alcohols, hydroxyl group is replaced with Cl using reagents like PCl5, PCl3, SOCl2

• SOCl2 is the preferred reagent because the other two products obtained are gaseous giving easy purification

Reaction with Ammonia

• Carboxylic acids react with ammonia to give ammonium salts that on further heating give amides

Reduction

• Carboxylic acids can be reduced to primary alcohols using LiAlH4 or more easily diborane

• Diborane has difficulty to reduce functional group like ester, halo, nitro

• NaBH4 does not reduce carboxyl group

Decarboxylation

Using soda lime

• Carboxylic acids form hydrocarbons when sodium salt is heated with soda lime (NaOH and CaO in ratio 3:1)

Decarboxylation

Kolbe’s Electrolysis

• Aqueous solutions of alkali metal salts of carboxylic acids undergo electrolysis to form hydrocarbons having twice the number of carbon atoms compare to alkyl group of acid

Halogenation

Hell-Volhard-Zelinsky Reaction

• Carboxylic acids with α-hydrogen react with chlorine and bromine substituted at α position in the presence of red phosphorus to give α-halocarboxylic acids

Ring Substitution

• Aromatic carboxylic acids undergo electrophilic substitution where carboxyl group is deactivating and meta-directing

• Do not undergo Friedel-Crafts because the carboxyl group is deactivating and the catalyst, aluminum chloride get attached to carboxyl group