Carboxylic Acids

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Carboxylic Acids — Key Facts for SLMC Medical (Sri Lanka)

• Carboxylic acids contain the –COOH functional group (or –CO₂H)
• General formula: CₙH₂ₙ₊₁COOH (or R–COOH)
• They are the most oxidized organic functional group (containing 2 oxygen atoms)
• pKa values: Formic acid ~3.75; Acetic acid ~4.76; Propionic acid ~4.87
• ⚡ Exam tip: Esterification, decarboxylation, and the HVZ reaction are high-yield for SLMC

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Carboxylic Acids — SLMC Medical (Sri Lanka) Study Guide

What Are Carboxylic Acids?

Carboxylic acids contain the carboxyl functional group (–COOH or –CO₂H) attached to an alkyl group or hydrogen. The carbon of the –COOH group is the carbonyl carbon (C=O) bonded to an –OH group.

General formula: R–COOH (where R = H for formic acid, alkyl for higher acids)

Nomenclature

Common and IUPAC names:

• Methanoic acid = Formic acid (HCOOH) — from ants (formica)
• Ethanoic acid = Acetic acid (CH₃COOH) — from vinegar
• Propanoic acid = Propionic acid (C₂H₅COOH)
• Butanoic acid = Butyric acid (butyric = butter) — responsible for rancid butter odor
• Pentanoic acid = Valeric acid
• Hexanoic acid = Caproic acid

Dicarboxylic acids:

• Ethanedioic acid = Oxalic acid (HOOC–COOH) — found in rhubarb leaves; toxic
• Propanedioic acid = Malonic acid
• Butanedioic acid = Succinic acid
• Hexanedioic acid = Adipic acid — used in nylon synthesis

Physical Properties

Property	Trend
Boiling point	High (due to dimeric hydrogen bonding via two –COOH groups); increases with chain length
Melting point	Even-numbered dicarboxylic acids have higher MP than odd-numbered (due to packing efficiency)
Solubility	C₁–C₄: miscible with water; C₅–C₉: sparingly soluble; C₁₀+: insoluble
Odor	Formic acid: pungent; Acetic acid: vinegar; Butyric acid: rancid

Hydrogen bonding in carboxylic acids: Each –COOH group can form hydrogen bonds with TWO other molecules — this gives carboxylic acids unusually high boiling points (acetic acid BP 118°C vs ethanol BP 78°C despite similar MW).

In the solid and liquid states, carboxylic acids exist as dimers via intermolecular H-bonding:

HOOC–R······O=C–R
      ↑
   hydrogen bond

Chemical Properties

1. Acidity

Carboxylic acids are weak acids (pKa ~4–5):

• They dissociate: R–COOH ⇌ R–COO⁻ + H⁺
• The carboxylate anion (R–COO⁻) is stabilized by resonance — two equivalent resonance structures delocalize the negative charge across two oxygen atoms

Acid strength increases with:

• Electron-withdrawing substituents (e.g., –Cl, –NO₂) via inductive effect
• Conjugation with π-systems (e.g., benzoic acid: pKa 4.20)
• Formic acid is stronger than acetic acid because the R group is H (no inductive effect from alkyl group)

Neutralization reactions:

• R–COOH + NaOH → R–COONa + H₂O
• R–COOH + Na₂CO₃ → R–COONa + NaHCO₃
• R–COOH + NaHCO₃ → R–COONa + H₂O + CO₂↑ (effervescence — diagnostic test!)

2. Esterification (Fischer Esterification)

Acid-catalyzed reaction of carboxylic acid with alcohols:

• R–COOH + R’–OH ⇌ R–COOR’ + H₂O (conc. H₂SO₄ catalyst, heat)
• This is a reversible reaction — excess alcohol or removal of water drives the equilibrium toward ester
• Esters are named as alkyl alkanoates (e.g., ethyl acetate = ethyl ethanoate)

Mechanism: Nucleophilic acyl substitution — the nucleophile (alcohol) attacks the carbonyl carbon; the –OH group leaves as water.

3. Reduction

• R–COOH + LiAlH₄ → R–CH₂OH (primary alcohol — reduces entire acid to alcohol)
• R–COOH + R’/MgBr → NO direct reaction (Grignard does NOT directly add to carboxylic acids — the acid protonates the Grignard reagent first!)

To make tertiary alcohols from carboxylic acids:

1. Reduce acid to alcohol with LiAlH₄
2. Oxidize alcohol to ketone with PCC
3. Add 2 equivalents of Grignard

4. Decarboxylation

Heating calcium salts of carboxylic acids produces ketones (dry distillation):

• 2CH₃COONa + Ca(OH)₂ → (CH₃)₂C=O + Na₂CO₃ + H₂O (acetone from calcium acetate)

Sodium salts of carboxylic acids with NaOH + CaO at high temperature produce alkanes:

• CH₃COONa + NaOH + CaO → CH₄ + Na₂CO₃ (methane from sodium acetate) This is the Kolbe’s electrolysis principle for alkane preparation.

5. Hell-Volhard-Zelinsky (HVZ) Reaction

Alpha halogenation of carboxylic acids:

• R–CH₂–COOH + Br₂/PBr₃ → R–CH(Br)–COOH (α-bromo acid)
• Reaction occurs at the alpha carbon (carbon adjacent to carboxyl group)
• Used to make amino acids (via azide substitution) — biologically very important

6. Conversion to Acid Chlorrides

R–COOH + SOCl₂ → R–COCl + SO₂ + HCl or: R–COOH + PCl₅ → R–COCl + POCl₃ + HCl

Acid chlorides are highly reactive intermediates for organic synthesis.

Important Carboxylic Acids in Medicine

• Acetylsalicylic acid (Aspirin): acetyl ester of salicylic acid; NSAID; antipyretic; anticoagulant properties
• Salicylic acid: keratolytic agent used in wart treatments; precursor to aspirin
• Lactic acid (2-hydroxypropanoic acid): produced during muscle exertion; involved in the Cori cycle
• Citric acid: component of Krebs cycle; used as acidulant in pharmaceutical syrups
• Oxalic acid: toxic; found in rhubarb; chelates calcium — causes kidney stones
• Benzoic acid: preservative (sodium benzoate); antifungal
• EDTA: ethylenediaminetetraacetic acid — chelating agent used in heavy metal poisoning treatment

Clinical and Medical Relevance

• Acetic acid (vinegar): used as a solvent and for urinary alkalinization; 1% solution for ear infections (Swimmer’s ear)
• Citric acid in the Krebs cycle: each turn consumes 1 acetyl-CoA, produces 3 NADH, 1 FADH₂, 1 GTP
• Lactic acidosis: buildup of lactic acid in blood; seen in shock, severe exercise, metformin toxicity
• Sodium benzoate as a preservative in foods and medications — concerns about hyperactivity in children
• Aspirin mechanism: irreversibly inhibits COX-1 and COX-2 enzymes → decreased prostaglandin synthesis → anti-inflammatory, analgesic, antipyretic effects

Common SLMC Exam Traps

• Carboxylic acids do NOT react with Grignard reagents directly — the acidic H protonates the Grignard reagent first, destroying it
• To convert carboxylic acid to tertiary alcohol, you must FIRST reduce to alcohol (LiAlH₄), THEN react with Grignard
• Esterification occurs at the carbonyl carbon (acyl carbon), not the alpha carbon
• Sodium bicarbonate test (effervescence with NaHCO₃) distinguishes carboxylic acids from phenols — phenols are weak acids but do NOT effervesce with NaHCO₃

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