Carboxylic Acids, Acid Derivatives, and Claisen Condensation

🟢 Lite — Quick Review (1h–1d)

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• Carboxylic acids (RCOOH) are the most acidic among organic compounds (pKa ~ 4-5); conjugate base is resonance-stabilized carboxylate anion
• Acid strength order: Perfluoroacetic > Trichloroacetic > Dichloroacetic > Chloroacetic > Acetic > Propionic
• Fischer esterification: Carboxylic acid + alcohol + H⁺ → ester + H₂O; reversible; rate depends on steric bulk of alcohol and acid
• Acid derivatives reactivity: Acid chloride > Acid anhydride > Ester > Amide (highest to lowest reactivity)
• Claisen condensation: Ester + base + ester → β-keto ester (for esters with α-hydrogens); similar to aldol but between two ester molecules
• ⚡ Acidity of carboxylic acids is due to resonance stabilization of the carboxylate ion — the conjugate base has two equivalent resonance structures

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Carboxylic Acids and Their Derivatives

Carboxylic acids are among the most important functional groups in organic and pharmaceutical chemistry. They serve as building blocks for a vast array of drug molecules, natural products, and biological molecules (amino acids, fatty acids, hormones). Understanding their reactivity patterns is essential for pharmacy students.

Carboxylic Acids — Structure and Acidity

Structure

Carboxylic acids contain the –COOH (or –CO₂H) functional group:

• The carbonyl carbon is attached to an –OH group
• The –OH is bonded to the same carbonyl carbon — not like an alcohol on a separate carbon
• Both oxygens are equivalent by resonance, though the true structure is an average

Why Carboxylic Acids are Acidic

The acidity of RCOOH comes from the resonance stabilization of its conjugate base, RCOO⁻ (carboxylate anion):

RCOOH → RCOO⁻ + H⁺

Resonance in Carboxylate: The negative charge is delocalized equally over two oxygen atoms:

    O⁻           O
    ‖          ‖
R—C          R—C
    ‖          ‖
    O          O⁻

This resonance stabilization makes the conjugate base much more stable than if the charge were localized — hence the hydrogen is relatively acidic.

Comparison of Acidity

pKa Values (Lower = More Acidic):

Compound	pKa	Structure
Formic acid	3.75	HCOOH
Acetic acid	4.76	CH₃COOH
Propionic acid	4.87	CH₃CH₂COOH
Trichloroacetic acid	0.7	Cl₃CCOOH
Picric acid	0.3	2,4,6-trinitrophenol

Why does electron-withdrawing groups increase acidity?

• –Cl, –NO₂, –CN pull electron density away from the carboxyl group
• This further stabilizes the carboxylate anion (even lower energy)
• More stable conjugate base → stronger acid

Why does inductive effect matter more than resonance here?

• In carboxylate, the resonance structures are already equivalent and fully delocalized
• Additional stabilization from nearby electron-withdrawing groups operates through the σ-bond framework (inductive effect)

Nomenclature of Carboxylic Acids

IUPAC Rules:

• Identify the longest chain containing the –COOH
• Drop the -e from the alkane name and add -oic acid
• Number the chain starting from the carboxyl carbon as C1
• If the acid has a double bond, change -oic to -enoic

Common Names:

• Formic acid (HCOOH) — from ants (formica)
• Acetic acid (CH₃COOH) — from vinegar (acetum)
• Propionic acid (C₂H₅COOH) — first fat acid
• Butyric acid (C₃H₇COOH) — from butter (butyrum)
• Benzoic acid (C₆H₅COOH) — aromatic carboxylic acid

Important Reactions of Carboxylic Acids

1. Salt Formation

RCOOH + NaOH → RCOONa + H₂O

• Carboxylic acids react with bases to form carboxylate salts
• Soap is the sodium salt of a long-chain fatty acid (stearic acid + NaOH → sodium stearate)

2. Fischer Esterification

Mechanism (Acid-Catalyzed):

1. Protonation of carbonyl oxygen → activates carbonyl for nucleophilic attack
2. Alcohol attacks carbonyl → tetrahedral intermediate
3. Proton transfer → another intermediate
4. Loss of water → ester + regenerated acid catalyst

Key Characteristics:

• Reversible: Equilibrium reached; yield can be improved by removing water or using excess alcohol
• Steric Effect: More hindered acids and alcohols react more slowly
• Primary > Secondary > Tertiary alcohols: Tertiary alcohols give poor yields due to competing elimination

3. Reduction

LiAlH₄ reduces carboxylic acids to primary alcohols: RCOOH → RCH₂OH (via aldehyde intermediate, but aldehyde is too reactive to isolate)

Boron-based reagents (e.g., borane, BH₃·THF) selectively reduce carboxylic acids without affecting other functional groups.

Note: NaBH₄ does NOT reduce carboxylic acids.

4. Decarboxylation

Heating calcium salts of carboxylic acids yields ketones (R–CO–R) through decarboxylative coupling: 2 RCOOCa + heat → R–CO–R + CaCO₃

5. Halogenation (Hell-Volhard-Zelinsky Reaction)

α-Halogenation of carboxylic acids using Br₂/PBr₃: CH₃COOH + Br₂/PBr₃ → CH₂BrCOOH (bromoacetic acid)

Acid Derivatives — Comparative Overview

Acid derivatives differ in the group replacing –OH:

Derivative	Structure	Naming
Acid chloride	R–COCl	-oyl chloride
Acid anhydride	R–CO–O–CO–R	-oic anhydride
Ester	R–COOR’	alkyl -oate
Amide	R–CONH₂	-amide

Reactivity Order (Highest → Lowest)

Acid chloride > Acid anhydride > Ester > Amide

Why this order?

• The leaving group ability of the substituent on the carbonyl carbon determines reactivity
• Better leaving groups → faster substitution
• Cl⁻ leaves more easily than –OOCR (anhydride) which leaves more easily than RO⁻ (ester) which leaves more easily than NH₂⁻ (amide)
• NH₂⁻ is the poorest leaving group — amides are the least reactive

1. Acid Chlorides (RCOCl)

• Most reactive acid derivative
• Prepared from RCOOH + SOCl₂ (thionyl chloride) or oxalyl chloride
• Used in synthesis where you need to “activate” the acid for nucleophilic attack
• Reacts violently with water (hydrolysis) — must be protected from moisture

2. Acid Anhydrides (RCOOCOR)

• Second most reactive
• Symmetric (e.g., acetic anhydride) or mixed
• Formed from acid chloride + carboxylate salt: RCOCl + R’COONa → (RCO)₂O + NaCl
• In pharmacy: Acetylsalicylic acid (aspirin) is an ester, not an anhydride

3. Esters (RCOOR’)

• Fruity smells — many natural aromas are esters
• Formed by Fischer esterification or from acid chloride + alcohol
• Key reactions: Hydrolysis (acid or base), Aminolysis (with amines → amides), Alcoholysis (with alcohols → exchange)

4. Amides (RCONH₂)

• Least reactive acid derivative
• Found in nature: Peptide bonds in proteins are amide bonds
• Prepared from acid chloride + amine (or ester + amine — aminolysis)
• Basic hydrolysis (with strong base like NaOH) converts amide → carboxylate + amine
• Do not undergo Fischer esterification (reverse is impossible — amines don’t react with acids to give esters)

Claisen Condensation

The Claisen condensation is the ester equivalent of the aldol reaction:

Two equivalents of ester with α-hydrogens react under basic conditions to give a β-keto ester:

2 CH₃COOC₂H₅ (ethyl acetate) + 2 NaOEt → CH₃COCH₂COOC₂H₅ (ethyl acetoacetate) + 2 EtOH + NaEt

Mechanism:

1. Enolate of ester attacks carbonyl of another ester molecule
2. Alkoxide leaves — BUT only if there is a full equivalent of base to deprotonate the β-keto product (this drives reaction to completion)

Mixed Claisen: If one ester has no α-hydrogen, it can act as the electrophile only, giving mixed products

Significance: Ethyl acetoacetate (acetoacetic ester) is a key intermediate in organic synthesis; it can undergo alkylation at the α-carbon → hydrolysis → decarboxylation → ketones

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Hydrolysis of Esters

Acid-Catalyzed Hydrolysis:

• Reverse of Fischer esterification
• Ester + H₂O/H⁺ → Carboxylic acid + alcohol
• Same mechanism, just in reverse

Base-Catalyzed Hydrolysis (Saponification):

• Ester + NaOH → Carboxylate salt + alcohol
• Irreversible — the carboxylate is too stable to reform the ester
• This is how soap is made (triglyceride + NaOH → soap + glycerol)

Pharmaceutical Examples

Aspirin (Acetylsalicylic Acid):

• Salicylic acid (2-hydroxybenzoic acid) + acetic anhydride → aspirin + acetic acid
• This is an esterification reaction
• Mechanism: –OH on salicylic acid attacks the carbonyl of acetic anhydride

Parabens (Methylparaben, Propylparaben):

• Used as preservatives in pharmaceuticals and cosmetics
• Are esters of p-hydroxybenzoic acid
• Subject to hydrolysis in aqueous formulations

Beta-Lactam Antibiotics (Penicillins, Cephalosporins):

• Contain a four-membered cyclic amide (lactam) ring
• The amide bond in the β-lactam ring is unusually strained and reactive
• Amidase enzymes (in bacteria) hydrolyze this bond → antibacterial activity lost

Fluoroquinolone Antibiotics (Ciprofloxacin, Levofloxacin):

• Contain a carboxylic acid group (piperazinyl ring attached to a quinolone core)
• The carboxylic acid is essential for antibacterial activity
• Ciprofloxacin synthesis involves multiple steps including carboxylic acid chemistry

Lipids and Carboxylic Acids

Fatty Acids: Long-chain carboxylic acids found in triglycerides and phospholipids

• Saturated: Stearic acid (C18), Palmitic acid (C16)
• Unsaturated: Oleic acid (C18, one double bond), Linoleic acid (C18, two double bonds)

Triglycerides: Triesters of glycerol (propane-1,2,3-triol) with three fatty acid molecules

• Hydrolysis of triglycerides under basic conditions = saponification (soap making)
• Hydrogenation of unsaturated triglycerides = conversion to saturated (vegetable ghee → vanaspati)

Phospholipids: Glycerol backbone + 2 fatty acids + 1 phosphate group

• Form the lipid bilayer of cell membranes
• Amphipathic nature (hydrophilic head, hydrophobic tails)

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