Carboxylic Acids: Structure, Preparation, and Reactions

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Topic 7 — Key Facts for Kenyatta University (Kenya) Core concept: Carboxylic acids contain the –COOH functional group; the –OH part of the carboxyl group is significantly more acidic (pKa ~4.76) than alcohols (pKa ~16) due to resonance stabilisation of the carboxylate anion High-yield point: Carboxylic acids can be reduced to primary alcohols (LiAlH₄ only), decarboxylated (lost as CO₂), and converted to acid chlorides (SOCl₂), anhydrides, esters, and amides — know the reactions and reagents for each ⚡ Exam tip: The pKa of formic acid (~3.75) is lower than acetic acid (~4.76); benzoic acid (~4.20) has a pKa between formic and acetic; these values reflect the electron-withdrawing effects of substituents and are frequently compared in exam questions

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Carboxylic Acids: The Most Acidic Organic Functional Group

Carboxylic acids are organic compounds characterised by the carboxyl functional group (–COOH). The name derives from the fact that these compounds were historically obtained by oxidising organic substances (Latin: carbo = “carbon,” pyre = “fire”).

The carboxyl group combines a carbonyl (C=O) and a hydroxyl (–OH) group attached to the same carbon. This creates a unique environment where the –OH hydrogen is highly acidic due to resonance stabilisation of the resulting carboxylate anion.

Nomenclature

IUPAC naming uses the suffix -oic acid:

• Methanoic acid (HCOOH): Formic acid — named from ants (formica in Latin)
• Ethanoic acid (CH₃COOH): Acetic acid — named from vinegar (acetum in Latin)
• Propanoic acid (C₂H₅COOH)
• Butanoic acid (C₃H₇COOH)

Aromatic carboxylic acid: Benzoic acid (C₆H₅COOH)

Dicarboxylic acids:

• Oxalic acid (HOOC–COOH): Ethanedioic acid
• Malonic acid (HOOC–CH₂–COOH): Propanedioic acid
• Succinic acid (HOOC–CH₂–CH₂–COOH): Butanedioic acid
• Adipic acid: Hexanedioic acid

Physical Properties of Carboxylic Acids

Hydrogen Bonding: The –COOH group can act as both a hydrogen bond donor (the –OH) and a hydrogen bond acceptor (the C=O). This dual hydrogen-bonding capability gives carboxylic acids unusually high boiling points compared to similar molecular weight compounds.

Boiling Points:

Acid	Formula	MW	BP (°C)
Formic acid	HCOOH	46	100.8
Acetic acid	CH₃COOH	60	118
Propanoic acid	C₂H₅COOH	74	141
Butanoic acid	C₃H₇COOH	88	163.5

Formic acid has the highest boiling point among monocarboxylic acids due to its ability to form strong hydrogen-bonded dimers.

Water Solubility:

• Formic acid, acetic acid, and propanoic acid are completely miscible with water
• Solubility decreases as the alkyl chain grows
• Butanoic acid is moderately soluble (5.6 g/100mL)

Odour:

• Formic acid: Pungent, penetrating
• Acetic acid: Pungent, vinegar-like
• Butanoic acid: Unpleasant, rancid butter smell
• Pentanoic acid: Similar unpleasant odour

⚡ Exam Tip: Butanoic acid is responsible for the smell of rancid butter. The “body odour” smell of human sweat is partly due to short-chain fatty acids produced by bacteria on skin.

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Acidity of Carboxylic Acids

Why Carboxylic Acids Are Acidic

When a carboxylic acid donates a proton (H⁺), it forms a carboxylate anion (R–COO⁻). This anion is highly stabilised by resonance:

Resonance Structures of Acetate Anion (CH₃COO⁻):

O⁻
‖
CH₃–C
/
O

… is equivalent to …

   O
   ‖
CH₃–C–O⁻

Both oxygen atoms are equivalent — the negative charge is delocalised over two oxygen atoms. This resonance stabilisation makes the loss of H⁺ energetically favourable.

Comparison with Alcohols: Alcohols (R–CH₂OH) form alkoxide anions (R–CH₂O⁻) when deprotonated. The negative charge on oxygen in alkoxides is not resonance-stabilised, making the conjugate base much less stable. Therefore alcohols are far less acidic (pKa ~16) than carboxylic acids (pKa ~4–5).

Factors Affecting Acidity

1. Electron-withdrawing substituents increase acidity: Formic acid (pKa 3.75) is more acidic than acetic acid (pKa 4.76) because the formyl hydrogen does not donate electron density.

Substituted acetic acids:

Acid	Substituent	pKa
Trichloroacetic acid	Cl₃C–	0.70
Dichloroacetic acid	Cl₂CH–	1.48
Chloroacetic acid	ClCH₂–	2.87
Fluoroacetic acid	FCH₂–	2.59
Acetic acid	CH₃–	4.76

The electron-withdrawing halogen atoms stabilise the carboxylate anion by inductive effect.

2. Resonance effects in aromatic acids: Benzoic acid (pKa 4.20) is more acidic than acetic acid because the benzoate anion is resonance-stabilised. The negative charge can delocalise into the aromatic ring.

3. Hybridisation of the acidic carbon: The more s-character in the orbital holding the acidic hydrogen, the more acidic the compound. Formic acid (sp² carbon bonded to H) > acetic acid (sp³ carbon attached to carbonyl).

Salt Formation

Carboxylic acids react with bases to form carboxylate salts:

CH₃COOH + NaOH → CH₃COO⁻Na⁺ + H₂O
CH₃COOH + NaHCO₃ → CH₃COO⁻Na⁺ + CO₂↑ + H₂O

Sodium bicarbonate test: Carboxylic acids (but not phenols) react with NaHCO₃ to evolve CO₂ gas — a distinction between carboxylic acids and phenols.

Naming salts: Sodium acetate (CH₃COONa), potassium benzoate (C₆H₅COOK), calcium propionate (Ca(C₂H₅COO)₂)

⚡ Preservation Note: Calcium propionate and sodium benzoate are used as food preservatives because they inhibit the growth of bacteria and fungi in bread and other food products.

Reactions of Carboxylic Acids

1. Formation of Derivatives

Carboxylic acids can be converted to various derivatives:

Acid Chlorides (R–COCl):

• Reagents: SOCl₂ (thionyl chloride), oxalyl chloride, PCl₅

R–COOH + SOCl₂ → R–COCl + SO₂↑ + HCl↑

Most reactive carboxylic acid derivative; used to make esters, amides, anhydrides.

Acid Anhydrides (R–CO–O–OC–R):

• From acid chlorides: R–COCl + R–COO⁻Na⁺ → R–CO–O–OC–R + NaCl
• Examples: Acetic anhydride (CH₃CO–O–OCCH₃)
• Mixed anhydrides: R–CO–O–OC–R’ (from two different acids)

Esters (R–COOR’):

• Fischer esterification: R–COOH + R’–OH ⇌ R–COOR’ + H₂O (acid-catalysed, reversible)
• From acid chloride + alcohol: R–COCl + R’OH → R–COOR’ + HCl

Amides (R–CONH₂):

• From acid chloride + NH₃: R–COCl + 2NH₃ → R–CONH₂ + NH₄Cl
• Amides are the least reactive carboxylic acid derivatives

⚡ Reactivity Order: Acid chloride > Acid anhydride > Ester > Amide (Acid chloride is most reactive because the chlorine is an excellent leaving group and the carbonyl carbon is most electrophilic)

2. Reduction Reactions

LiAlH₄ Reduction: Carboxylic acids can be reduced to primary alcohols — the only reagent that does this (NaBH₄ does not reduce acids):

R–COOH + 4[H] (LiAlH₄) → R–CH₂OH + 2H₂O + Al³⁺ species

CH₃COOH + [H] → CH₃CH₂OH   (acetic acid → ethanol)

Reduction of esters to alcohols also uses LiAlH₄:

R–COOR' + 4[H] → R–CH₂OH + R'–CH₂OH

This is the basis for converting fats (triglycerides) to fatty alcohols in industrial chemistry.

3. Decarboxylation

Heating calcium salts: Calcium salt of carboxylic acid + heat → ketone + calcium carbonate:

(CH₃COO)₂Ca + heat → CH₃–CO–CH₃ + CaCO₃↓   (calcium acetate → acetone)

Kolbe’s Electrolysis: Electrolytic decarboxylation of sodium carboxylate → alkane + CO₂ + carbonate:

2CH₃COONa + 2H₂O → 2CH₄ + 2CO₂ + 2NaOH + H₂   (at the cathode)

Hunsdiecker Reaction: Silver salt of carboxylic acid + Br₂ → alkyl bromide + CO₂ + AgBr:

CH₃COOAg + Br₂ → CH₃Br + CO₂ + AgBr

This reaction shortens the carbon chain by one carbon atom (decarboxylative halogenation).

4. Esterification Mechanism (Fischer Esterification)

Mechanism:

1. Protonation of the carbonyl oxygen of the carboxylic acid
2. The alcohol oxygen attacks the carbonyl carbon (nucleophilic addition)
3. Proton transfers within the tetrahedral intermediate
4. Elimination of water to form the ester

Key Feature: The reaction is reversible. In the presence of isotopically labelled oxygen (¹⁸O), both oxygens of the ester come from the alcohol, confirming that the C–O bond of the original acid is retained.

⚡ Exam Tip: This isotopic labelling experiment established the mechanism of Fischer esterification. In the reverse reaction (ester hydrolysis), the C–O bond of the ester is retained, and the OH in the carboxylic acid comes from water.

5. The Hell-Volhard-Zelinsky (HVZ) Reaction

Alpha-bromination of carboxylic acids using Br₂ and P (or PBr₃):

R–CH₂–COOH + Br₂/PBr₃ → R–CH(Br)–COOH + HBr

Purpose: Introduces bromine at the alpha position (α-bromo acid formation) Mechanism: Involves enolisation of the carboxylic acid derivative (acyl bromide intermediate)

Example:

CH₃CH₂COOH + Br₂/P → CH₃CH(Br)COOH + HBr   (2-bromopropanoic acid)

⚡ Exam Tip: The HVZ reaction only affects the α-carbon. Butanoic acid → 2-bromobutanoic acid. Acetic acid has no α-hydrogen and cannot undergo HVZ reaction.

6. Amide Formation

Carboxylic acid + amine (or ammonia) → amide (requires dehydrating agent):

Dicyclohexylcarbodiimide (DCC) coupling: Used in peptide bond formation (solid-phase peptide synthesis):

R–COOH + H₂N–R' + DCC → R–CO–NH–R' + DCU

Direct amide formation requires high temperatures and removal of water.

Natural Occurrence of Carboxylic Acids

Acid	Source	Notes
Formic acid	Ant venom	First carboxylic acid discovered
Acetic acid	Vinegar	Dilute (4–8%) solution of acetic acid
Butanoic acid	Rancid butter	Odour compound
Citric acid	Citrus fruits	Present in lemons, oranges; used as food acidulant
Lactic acid	Sour milk, muscles	Produced by fermentation; builds up during strenuous exercise
Tartaric acid	Grapes	Byproduct of winemaking; cream of tartar
Malic acid	Apples	Contributes to tart taste of green apples
Salicylic acid	Willow bark	Precursor to aspirin
Benzoic acid	Balsam, berries	Food preservative (sodium benzoate)

⚡ Health Note: Sodium benzoate (used as a preservative in carbonated drinks) can combine with vitamin C to form benzene (a carcinogen) under certain conditions. This has led to restrictions in some countries.

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