Chemical Reactions of Carboxylic Acids

Full Notes

Carboxylic acids undergo a wide range of chemical reactions due to the versatile nature of their –COOH group.

Acidity of Carboxylic Acids

Carboxylic acids exhibit acidic behaviour due to the ease with which they lose a proton (H⁺) from the –COOH group.

Redox Reaction with Reactive Metals

Metal displaces hydrogen from the –COOH group, producing hydrogen gas and a carboxylate salt.

Example 2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂↑

Neutralisation with Alkalis

OH⁻ accepts the H⁺ from the –COOH group, forming a salt and water.

Example CH₃COOH + NaOH → CH₃COONa + H₂O

Reaction with Weaker Bases (Carbonates)

Example 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂↑ + H₂O

Typical acid–carbonate reaction producing carbon dioxide, water, and a salt.

Ionisation in Water & Resonance Stabilisation

Carboxylic acids dissociate in water and the carboxylate ion (R–COO⁻) is resonance-stabilised with two equivalent structures, making the loss of H⁺ easier (stronger acid).

The resonance involves delocalisation of negative charge over two electronegative oxygen atoms.

pK_a and Acid Strength

For the dissociation of a carboxylic acid in water, the equilibrium position can be described using the equilibrium constant, K_a.

• K_a (acid dissociation constant) and pK_a = −log K_a.
• Lower pK_a = Stronger acid.

• HCl: −7.0 (very strong)
• Trifluoroacetic acid: 0.23 (strongest carboxylic acid listed)
• Benzoic acid: 4.19
• Acetic acid: 4.76

Carboxylic acids are stronger acids than alcohols and phenols (e.g., phenol pK_a ≈ 10, ethanol ≈ 16).

Why Carboxylic Acids Are More Acidic Than Phenols

• In carboxylate ions, negative charge is equally delocalised over two electronegative oxygen atoms.
• In phenoxide ions, delocalisation involves carbon (less electronegative), and resonance structures are not equivalent.
• Thus, carboxylate ions are more stable, making carboxylic acids more acidic.

Relative Acidities of Carboxylic Acids, Phenols, and Alcohols

• Carboxylic acids are the strongest acids of the three.
• Phenols are weaker than carboxylic acids but stronger than alcohols.
• Alcohols are the weakest acids.

The carboxylate ion (RCOO⁻) formed from carboxylic acids is resonance-stabilised, making it easier to lose H⁺ (see above).

The negative charge is equally delocalised over two electronegative oxygen atoms. In phenoxide ions, delocalisation involves carbon (less electronegative), and resonance structures are not equivalent.

Alkoxide ions from alcohols are unable to be stabilised, making them harder to form and exist.

Effect of Substituents on Acidity

Electron-withdrawing groups like chlorine atoms increase acidity. Electron donating groups decrease acidity.

For Example:
Trichloroethanoic acid (CCl₃COOH) is much more acidic than ethanoic acid (CH₃COOH).

Chlorine atoms pull electron density away from the carboxylate ion via the inductive effect.

This weakens the O–H bond and stabilises the negative charge on the ion through increased delocalisation, making it easier to lose the H⁺ from the acid group.

Electron Donating Groups (EDG), destabilise carboxylate ion by increasing electron density. This decreases acidity.

For Example: –CH₃, –OCH₃.

Order of Acidity Based on Substituents

Increasing acidity (decreasing pK_a):

CF₃COOH < CCl₃COOH < CHCl₂COOH < NO₂CH₂COOH < CNCH₂COOH

FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > C₆H₅COOH > CH₃COOH

The more electronegative the substituent and closer it is to the –COOH group, the stronger the acid.

Effect of Phenyl/Vinyl Group Direct Attachment

• Directly bonded sp² hybridised carbons to –COOH increase acidity.
• Greater electronegativity of sp² carbon draws electron density away, aiding H⁺ release.

Examples

Reactions Involving Cleavage of C–OH Bond

These reactions involve the removal or replacement of the –OH group in the carboxyl group.

Formation of Anhydrides

When heated with a dilute acid (such as H₂SO₄), two carboxylic acid molecules lose water to form an acid anhydride.

Common with ethanoic acid (acetic acid) to form ethanoic anhydride (acetic anhydride).

Esterification

Carboxylic acids react with alcohols in the presence of an acid catalyst (H₂SO₄) to form esters in condensation reactions (water is released).

Mechansim:

Nucleophilic attack of alcohol oxygen on carbonyl carbon, followed by elimination of water.

Reaction with PCl₅, PCl₃ and SOCl₂

These reagents replace the –OH of the carboxylic acid with Cl, forming acid chlorides.

The use of SOCl₂ is preferred industrially as it gives gaseous byproducts only

Reaction with Ammonia

Carboxylic acids react with ammonia to initially form ammonium carboxylate salts. Upon heating, these salts undergo dehydration to yield amides.

Reactions Involving the –COOH Group

Reduction

Carboxylic acids can be reduced to primary alcohols using strong reducing agents.

Reagents: LiAlH₄ (powerful), BH₃ (selective)

Decarboxylation

Removal of CO₂ from the carboxyl group, especially when in β-position to another carbonyl.

Example CH₃COOH → CH₄ (methane)

This is commonly used for simplifying carbon chains.

Substitution Reactions

Aromatic carboxylic acids like benzoic acid undergo electrophilic substitutions at the meta position due to the electron-withdrawing –COOH group.

Halogenation (Hell–Volhard–Zelinsky Reaction)

Carboxylic acids that possess an α-hydrogen (a hydrogen on the carbon next to the –COOH group) undergo halogenation at this position when treated with chlorine or bromine. This reaction requires the presence of a small amount of red phosphorus.

The halogen replaces one α-hydrogen, forming an α-halo carboxylic acid. This transformation is known as the Hell–Volhard–Zelinsky (HVZ) reaction.

• Reagents used:
  • Step (i): Cl₂ or Br₂ with red phosphorus
  • Step (ii): Hydrolysis with water

Ring Substitution (in Aromatic Carboxylic Acids)

Aromatic carboxylic acids undergo electrophilic substitution reactions on the benzene ring. The carboxyl group (−COOH) is electron-withdrawing and therefore:

• Deactivates the aromatic ring toward electrophilic substitution.
• Directs incoming substituents to the 3rd (meta) position.

Examples Nitration and bromination of benzoic acid

Nitration with concentrated HNO₃ and H₂SO₄ yields 3-nitrobenzoic acid.

Bromination with Br₂/FeBr₃ gives 3-bromobenzoic acid.

Important exception: Carboxylic acids do not undergo Friedel–Crafts reactions because the –COOH group deactivates the ring. The catalyst (AlCl₃) forms a complex with the –COOH group, preventing the reaction.

Summary

• Carboxylic acids are weak acids due to resonance-stabilised carboxylate ions.
• Acidity trends depend on substituents, distance from –COOH, and hybridisation.
• Key reactions: neutralisation, metal displacement, carbonate reaction, esterification, anhydride and acyl chloride formation, amide formation, reduction to alcohols, and decarboxylation.
• Aromatic acids undergo meta-directing electrophilic substitution; HVZ halogenates at the α-position of carboxylic acids with α-hydrogen.