MCAT Organic Chemistry · Lesson 8
Carboxylic Acids
2 min read2 sectionsUpdated
2 sections
8.1 Description and Properties of Carboxylic Acids
Carboxylic acids contain carbonyl and hydroxyl groups, are highly oxidized, and exhibit significant hydrogen bonding. Their acidity is influenced by resonance stabilization and substituents, while dicarboxylic acids and beta-dicarboxylic acids show unique acid-base properties.
- Nomenclature
- Add
oic acidto parent chain - Cycloalkanes: use
carboxylic acidsuffix - Salts: cation first,
oatereplacesoic acid - Examples:
- Methanoic acid = formic acid
- Ethanoic acid = acetic acid
- Propanoic acid = propionic acid
- Dicarboxylic acids (COOH at both ends):
- Ethandioic acid = oxalic acid
- Propanedioic acid = malonic acid
- Butandioic acid = succinic acid
- Add
- Physical Properties
- H Bonding: Polar, strong intermolecular attractions; forms dimers → ↑ BP & MP
- Acidity: Hydroxyl H is very acidic; resonance stabilizes conjugate base
- pKa ≈ 4.8 (strong for organics, not strong vs. strong acids)
- Substituents:
- E⁻ withdrawing (NO₂, halides) → ↑ acidity
- E⁻ donating (NH₂, OCH₃) → ↓ acidity
- Dicarboxylic acids influence each other: first proton easier to remove than second
- Beta-dicarboxylic acids: COOH separated by one carbon; alpha-H highly acidic; carbanion stabilized by E⁻ withdrawing effect
8.2 Reactions of Carboxylic Acids
Carboxylic acids undergo oxidation, nucleophilic acyl substitution, reduction, decarboxylation, and saponification. Their reactivity is often enhanced via formation of derivatives like esters, amides, anhydrides, or salts.
- Synthesis
- Oxidation of aldehydes and primary alcohols (CrO₃, KMnO₄, dichromate salts)
- Secondary/tertiary alcohols cannot form COOH
- Grignard reagents, hydrolysis of nitriles (less MCAT relevant)
- Nucleophilic Acyl Substitution
- Nucleophile attacks carbonyl → tetrahedral intermediate → carbonyl reforms, leaving group departs
- Applies to COOH, amides, esters, anhydrides, other acyl derivatives
- Weak bases (CB of strong acids) = good leaving groups
- Amides
- COOH → amide via nucleophile: ammonia or amine
- Dehydration reaction
- N-alkyl groups indicated by prefix
N- - Cyclic amides = lactams (beta-lactam, delta-lactam)
- Esters
- Hybrid of COOH + ether (ROR)
- Condensation reaction; protonation of carbonyl O → ↑ electrophilicity
- Naming like salts: ethyl acetate = ethyl ethanoate
- Cyclic esters = lactones
- Anhydrides
- Condensation of 2 COOH → anhydride
- Nucleophilic acyl substitution; dehydration reaction
- Reduction
- COOH → primary alcohol via LAH
- Aldehyde intermediates formed briefly
- NaBH₄ too weak
- Decarboxylation
- Loss of carbonyl as CO₂ → reduces carbon count
- Some dicarboxylic acids decarboxylate spontaneously on heating
- Saponification
- Long-chain COOH + NaOH/KOH → soap (salt formation)
- Soap forms micelles → dissolves nonpolar compounds in aqueous solution
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