Carboxylic Acids

Carboxylic Acids

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Rapid summary for last-minute revision before your exam.

Carboxylic Acids — Key Facts for JEE Advanced

Structure:

• Functional group: –COOH (carboxyl group)
• Carbon is sp² hybridized, bonded to: O (double), O (single, with H), and R group
• Resonance stabilization: –COO⁻ has delocalized negative charge over two oxygens
• Monocarboxylic acids: one –COOH (formic HCOOH, acetic CH₃COOH)
• Dicarboxylic acids: two –COOH (oxalic HOOC–COOH, malonic HOOC–CH₂–COOH)

Nomenclature Quick Rules:

• Suffix: –oic acid (or –dioic acid for dicarboxylic)
• C-1 is always the carboxyl carbon
• Acetic acid = ethanoic acid; oxalic acid = ethanedioic acid; succinic acid = butanedioic acid
• Aromatic: benzoic acid = C₆H₅COOH; phthalic acid = benzene-1,2-dicarboxylic acid

Key Distinguishing Test: ⚡ Acids vs Phenols: Add NaHCO₃ — carboxylic acids effervesce CO₂; phenols do not (phenol pKa ~10, too weak to react with NaHCO₃ pKa ~10.3, barely favors products). Carboxylic acids (pKa ~4-5) readily react with NaHCO₃.

⚡ Exam Tip: Solubility in NaOH vs NaHCO₃ — both dissolve carboxylic acids. But only acids dissolve in NaHCO₃ (with effervescence). Phenols dissolve in NaOH but NOT NaHCO₃. This is JEE’s most frequently tested distinction between phenols and carboxylic acids.

⚡ Exam Tip: Remember the pKa trend: Formic acid (pKa 3.75) < Acetic acid (pKa 4.76) < Propionic acid (pKa 4.87). More alkyl groups = slightly less acidic (electron donating +I effect).

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🟡 Standard — Regular Study (2d–2mo)

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Carboxylic Acids — Chemistry Study Guide

1. Structure & Bonding:

Carboxyl Group:

• C(sp²)–O σ bonds: one to hydroxyl O, one to carbonyl O
• C(sp²)=O π bond: delocalized over C=O and C–O
• The C=O bond is shorter (120 pm) and stronger than the C–O bond (130 pm)
• In the neutral acid, the resonance form R–C(–O⁻)=O⁺H (separating charge on the hydroxyl oxygen) is a minor, high-energy contributor, so the neutral acid is only weakly resonance stabilized
• The carboxylate anion R–COO⁻ is strongly resonance stabilized: both C–O bonds are equivalent (bond order 1.5)
• The anion has delocalized negative charge over TWO oxygens → explains high acidity

Dimerization:

• Carboxylic acids form strong dimers via two H-bonds: R–COOH ⋯ HO–OC–R
• Dimerization raises boiling point significantly
• Formic acid BP 100.5°C, acetic acid 118°C — much higher than analogous alcohols

Acidity and pKa Values:

Acid	pKa	Reason
HCOOH	3.75	No alkyl group
CH₃COOH	4.76	+I effect of CH₃
CH₃CH₂COOH	4.87	+I effect
(CH₃)₂CHCOOH	5.05	More alkyl groups
FCH₂COOH	2.59	–I effect of F (stabilizes anion)
ClCH₂COOH	2.87	–I effect
CCl₃COOH	0.70	Strong –I, anion highly stabilized
Benzoic acid	4.20	Resonance with phenyl ring

⚡ The effect of substituents on acidity: Electron-withdrawing groups (–I, –M) increase acidity by stabilizing the conjugate base. Electron-donating groups decrease acidity. This is a major JEE topic for both aliphatic and aromatic acids.

2. Preparation Methods:

1. Oxidation of primary alcohols and aldehydes:

   • R–CH₂OH + PCC/PDC → R–CHO (aldehyde) + further oxidation → R–COOH
   • KMnO₄ hot, acidic: oxidizes primary alcohol → carboxylic acid (no stopping at aldehyde)
   • R–CHO + AgNO₃/NH₃ → R–COO⁻ (Tollens oxidation; benzoins do this too)

2. Hydrolysis of nitriles (CN):

   • R–CN + H₂O/H⁺ → R–COOH (acidic hydrolysis)
   • R–CN + OH⁻ → R–COO⁻ + NH₃ (basic hydrolysis)
   • This is how we make acetic acid from acetylene: C₂H₂ → CH₃CN → CH₃COOH

3. Hydrolysis of esters:

   • R–COOR′ + H₂O/H⁺ → R–COOH + R′OH (acidic hydrolysis, reversible)
   • R–COOR′ + OH⁻ → R–COO⁻ + R′OH (saponification, irreversible)

4. Hydrolysis of amides:

   • R–CONH₂ + H₂O/H⁺ → R–COOH + NH₄⁺ (slow hydrolysis)
   • R–CONH₂ + NaOH → R–COONa + NH₃ (faster)

5. Oxidation of alkylbenzenes:

   • Ar–CH₃ + KMnO₄/hot → Ar–COOH (side chain oxidation — all alkyl chain carbons eventually become COOH)
   • Ar–CH₂–CH₃ + KMnO₄ → benzoic acid (both carbons of ethyl group oxidize to COOH)
   • Ar–CH(CH₃)₂ + KMnO₄ → benzoic acid (tertiary butyl → one COOH)

6. Grignard + CO₂:

   • R–MgX + CO₂ → R–COOMgX → H₃O⁺ → R–COOH (how to increase carbon chain by one)
   • This is a very important JEE method for making carboxylic acids

7. Decarboxylation of calcium salts:

   • (R–COO)₂Ca + heat → R–R (ketone if two different R groups, decarboxylation gives dimer)
   • For sodium salt of carboxylic acid + NaOH + CaO → alkane (for one carbon loss)
   • Kolbe’s electrolysis: 2R–COONa → R–R + 2CO₂ + H₂ (anodic decarboxylation)

3. Reactions:

Acid Reactions (Characteristic):

1. Esterification:

   R–COOH + R′–OH ⇌ R–COOR′ + H₂O
   Mechanism: Nucleophilic attack by alcohol on carbonyl C → tetrahedral → loss of H₂O
   Acid catalyst: protonates carbonyl O making C more electrophilic

   ⚡ Esterification with 2° and 3° alcohols is slow and may involve rearrangements. Primary alcohols work best. Tertiary alcohols undergo elimination rather than esterification under acidic conditions.

2. Reaction with PCl₅/PCl₃/SOCl₂:

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

   ⚡ SOCl₂ is preferred in lab because byproducts are gases (SO₂, HCl) — easy to separate from product.

3. Amide formation:

   R–COOH + NH₃ → R–COONH₄ → heat → R–CONH₂ → R–CN (dehydration at higher T)

4. Reduction:

   • LiAlH₄: R–COOH → R–CH₂OH (complete reduction, very strong)
   • Borane (BH₃): R–COOH → R–CH₂OH (selective, reduces carboxylic acid but not esters)
   • NaBH₄ does NOT reduce carboxylic acids (too weak) ⚡ Borane reduction is selective: reduces COOH and aldehydes/ketones but not esters. Useful in multi-step synthesis.

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

   R–CH₂–COOH + Br₂/P (or PBr₃) → R–CH(Br)–COOH
   α-bromination of carboxylic acids
   Mechanism: PBr₃ converts acid to acyl bromide → enolization → bromination → hydrolysis

   ⚡ Only works for α-hydrogen acids (needs α-CH₂). Acetic acid → bromoacetic acid. Propionic → 2-bromopropanoic acid. Trimethylacetic acid (no α-H) does NOT react.

Decarboxylation:

• Sodium salts of carboxylic acids + NaOH + CaO → alkane (loss of CO₂)
• Simple heating of calcium salts: (CH₃COO)₂Ca + heat → CH₃–CO–CH₃ (acetone) + CaCO₃
• For dibasic acids: oxalic acid → formic acid → CO₂ + H₂O (successive decarboxylation on heating)
• Malonic acid decarboxylates when heated: HOOC–CH₂–COOH → CH₃–COOH + CO₂

Kolbe’s Electrolysis:

2CH₃COONa → CH₃–CH₃ + 2CO₂ (anode) [coupling of methyl radicals]
Mechanism: Anode: 2CH₃COO⁻ → 2CH₃• + 2CO₂ → CH₃–CH₃ (Wurtz-type coupling)
Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Carboxylic Acids — Comprehensive Chemistry Notes

1. Detailed Mechanisms:

Esterification Mechanism (Fischer Esterification):

Step 1: Protonation of carbonyl oxygen → C=O becomes more electrophilic
Step 2: ROH attacks carbonyl C → tetrahedral intermediate (C now has 4 bonds: O, O, R', OR)
Step 3: Proton transfer within the tetrahedral intermediate
Step 4: Loss of H₂O from protonated tetrahedral intermediate
Step 5: Deprotonation → ester + H⁺
Rate = k[acyl carbon][H⁺] (acid-catalyzed, first order in each)

⚡ This is an equilibrium reaction. Le Chatelier’s principle applies: use excess alcohol or remove water to drive toward ester.

⚡ Stereochemistry at the α-carbon is retained in esterification (no carbocation formation at α-carbon).

Ester Hydrolysis:

Acidic: R–COOR' + H₂O ⇌ R–COOH + R'OH
Basic: R–COOR' + OH⁻ → R–COO⁻ + R'OH (irreversible, saponification)

⚡ Basic hydrolysis is irreversible because carboxylate is resonance-stabilized and a poor electrophile. Acidic hydrolysis is reversible. This is why saponification (base-catalyzed) goes to completion.

Schmidt Reaction (Carboxylic Acid + Hydrazoic Acid):

R–COOH + HN₃ → R–NH₂ + CO₂ + N₂
Loss of N₂ drives the reaction
This is a rearrangement reaction — useful for making amines

Lossen, Curtius, Hofmann Rearrangements (all generate isocyanates):

Lossen: R–CO–NH–OH → R–N=C=O (isocyanate) → R–NH₂ + CO₂
Curtius: R–CO–N₃ → R–N=C=O → R–NH₂
Hofmann: R–CO–NHBr → R–N=C=O → R–NH₂ (bromamide variant)

⚡ All three degrade a carboxylic acid derivative to an amine with loss of one carbon. Important in synthesis for making amines from acids.

α-Halogenation (HVZ) Mechanism:

Step 1: PBr₃ converts –OH to –Br: R–CH₂–COOH + PBr₃ → R–CH₂–COBr
Step 2: The acyl bromide enolizes: R–CH₂–COBr ⇌ R–CH=C(OH)Br (enol form)
Step 3: Br₂ attacks the enol: bromination at α-carbon
Step 4: Hydrolysis of acyl bromide: R–CH(Br)–COBr + H₂O → R–CH(Br)–COOH + HBr
Net: α-H replaced by Br

2. Dicarboxylic Acids — Special Properties:

Dicarboxylic Acid	Formula	pKa1	pKa2	Notable Reaction
Oxalic	HOOC–COOH	1.27	4.27	Forms complex with Fe³⁺
Malonic	HOOC–CH₂–COOH	2.83	5.69	Decarboxylates on heating
Succinic	HOOC–CH₂–CH₂–COOH	4.21	5.64	Forms anhydride
Glutaric	HOOC–CH₂–CH₂–CH₂–COOH	4.34	5.42	—
Adipic	HOOC–(CH₂)₄–COOH	4.41	5.41	Nylon-6,6 precursor

⚡ Oxalic acid is a reducing agent (used in rust removers) and forms soluble complexes with calcium (why it’s used to remove calcium deposits). The Ca²⁺ complex is soluble.

⚡ Malonic acid decarboxylates at 150°C to give acetic acid + CO₂. This is important because malonic ester synthesis uses the decarboxylation step to make substituted acetic acids.

⚡ Succinic acid forms succinic anhydride when heated (intramolecular dehydration). Glutaric acid likewise forms its cyclic anhydride.

Acidity of Dicarboxylic Acids:

• First pKa is lower than expected because one –COOH is electron-withdrawing toward the other
• Second pKa is higher because the mono-anion (⁻OOC–CH₂–COOH) carries a negative charge that resists further deprotonation
• For oxalic acid: pKa1 = 1.27 because after first dissociation, the anion (⁻OOC–COOH) has the second –COOH strongly electron-withdrawing
• For malonic: pKa2 (5.69) is much higher than pKa1 (2.83) because after losing the first H⁺, the anion already carries a negative charge — the second dissociation is harder (electrostatic work required)

3. Functional Derivatives — Relative Reactivity:

Order of reactivity toward nucleophiles:

Acid chloride (R–COCl) > Acid anhydride (R–CO–O–CO–R) > Ester (R–COOR')
> Amide (R–CONH₂) > Carboxylate (R–COO⁻)

⚡ Reason: Leaving group ability. Better leaving groups (Cl⁻, –OCOR⁻) make better acylating agents. Amides have NH₂⁻ as leaving group — very poor (pKa of NH₃ ~38), so amides are the least reactive.

Conversion hierarchy:

Acid → Ester (Fischer esterification, reversible)
Ester → Amide (R–COOR' + NH₃ → R–CONH₂ + ROH; slow)
Acid → Acid chloride (PCl₅/SOCl₂; irreversible)
Acid chloride → Ester (RCOCl + R'OH → RCOOR'; fast)
Acid chloride → Amide (RCOCl + NH₃ → RCONH₂; very fast)

Transesterification:

R–COOR' + R''OH ⇌ R–COOR'' + R'OH
Acid or base catalyzed
Biochemistry: triglycerides → biodiesel (transesterification with methanol)

4. Synthetic Applications:

Malonic Ester Synthesis:

CH₂(COOR)₂ + base → ⁻CH(COOR)₂ (malonic ester anion)
⁻CH(COOR)₂ + R–X → R–CH(COOR)₂ (alkylation)
→ H₃O⁺/heat → R–CH₂–COOH (decarboxylation, loses one CO₂)

⚡ This is THE JEE classic for making substituted carboxylic acids. The decarboxylation step is critical: the diacid intermediate has a β-carboxyl arrangement that readily loses CO₂.

Example: Make butanoic acid from malonic ester:

CH₂(COOR)₂ + EtBr → CH₃CH₂–CH(COOR)₂
→ hydrolyze + decarboxylate → CH₃CH₂CH₂COOH (butanoic acid)

To target propanoic acid instead, alkylate with MeI:

CH₂(COOR)₂ + MeI → CH₃–CH(COOR)₂ → decarboxylate → CH₃CH₂COOH (propanoic acid)

Acetoacetic Ester Synthesis:

CH₃–CO–CH₂–COOR + base → CH₃–CO–CH(⁻)–COOR
→ alkylation → CH₃–CO–CH(R)–COOR
→ H₃O⁺, heat → CH₃–CO–CH₂–R (substituted acetone) [ketonic hydrolysis]

⚡ The acetoacetic ester synthesis yields ketones (ketonic hydrolysis) when the alkylated intermediate is heated with dilute acid. This is distinct from malonic ester synthesis, which gives carboxylic acids.

Gabriel Synthesis (making primary amines):

Phthalimide + KOH → K⁺ ⁻N(Phthaloyl) 
+ R–X → R–N(Phthaloyl)
→ H₂N–NH₂ (hydrazine) → R–NH₂ (primary amine)
This is a standard method for primary amine synthesis without over-alkylation

⚡ Gabriel synthesis makes primary amines cleanly. If you use NH₃ directly, you get a mixture of primary, secondary, and tertiary amines.

5. Special Reactions of Unsaturated Acids:

Perkin’s Reaction:

Ar–CHO + (CH₃CO)₂O (acetic anhydride) → Ar–CH=CH–COOH (cinnamic acid)
Base removes α-H from anhydride → carbanion → attacks aldehyde → elimination

Knoevenagel Condensation:

R–CHO + CH₂(COOR')₂ → R–CH=C(COOR')₂ + H₂O (catalyzed by base)
Active methylene compounds with aldehydes/ketones → α,β-unsaturated products

Erlenmeyer (Azlactone) Synthesis:

Ar–CHO + hippuric acid (PhCONH–CH₂–COOH) → azlactone intermediate
→ hydrolysis → α,β-unsaturated (and ultimately α-) amino acid
Used in amino acid synthesis

6. Analysis & Identification:

pKa and Strength:

• Carboxylic acid pKa ~4-5 (stronger than carbonic acid H₂CO₃, pKa ~6.4, which is why carbonates dissolve acids: CO₃²⁻ + H⁺ → HCO₃⁻)
• Compare: HCl pKa ~-7, H₂SO₄ pKa1 ~-3 (strong mineral acids are far stronger)
• Note: H₂CO₃ pKa ~6.35 but the apparent pKa of a CO₂ solution is ~7 (weakly acidic)

Qualitative Analysis:

Solubility in NaHCO₃ (effervescence) = carboxylic acid
Does not reduce Tollens/Fehling's = saturated acid (if it reduces, it's a reducing acid like formic)
Forms amide with NH₃ = carboxylic acid
Calcium salt test: Ca²⁺ + 2R–COO⁻ → (R–COO)₂Ca (precipitate for C₄+ acids)
Calcium salts of higher (C₄+) acids are sparingly soluble

⚡ JEE Quantitative Problem Pattern: “A monocarboxylic acid sample requires 40 mL of 0.5 M NaOH for complete neutralization. Determine its molar mass from the mass taken, then identify the acid.”

A monocarboxylic acid reacts 1:1 with NaOH:
   R–COOH + NaOH → R–COONa + H₂O
So moles of acid = moles of NaOH = V × M.

Worked example — take 1.20 g of the acid:
   Moles NaOH = 0.040 L × 0.5 M = 0.020 mol
   Moles acid = 0.020 mol (1:1)
   Molar mass = mass / moles = 1.20 / 0.020 = 60 g/mol

Identify: for a saturated monocarboxylic acid CₙH₂ₙO₂,
   12n + 2n + 32 = 60 → 14n = 28 → n = 2
   So the acid is C₂H₄O₂ = CH₃COOH (acetic acid).

⚡ Method to remember: for a monoprotic acid, moles of acid = (volume of base in L) × (molarity of base). Divide the mass taken by these moles to get the molar mass, then fit it to CₙH₂ₙO₂ (for a saturated acid) to identify the acid.

7. Biological Significance:

• Formic acid: ant venom (H–COOH, pKa 3.75, strongest of the simple aliphatic acids)
• Acetic acid: vinegar (CH₃COOH, 5-8% in dilute form)
• Citric acid: citrus fruits (a tribasic hydroxy acid, three –COOH groups)
• Lactic acid: CH₃–CH(OH)–COOH (muscle metabolism, sour milk, chiral)
• Tartaric acid: HOOC–CH(OH)–CH(OH)–COOH (wine, cream of tartar)
• Oxalic acid: found in rhubarb leaves (toxic, chelating agent for calcium)
• Adipic acid: industrially from cyclohexane → cyclohexanol → cyclohexanone → adipic acid (nylon precursor)
• Benzoic acid: preservative (sodium benzoate in soft drinks), occurs in gum benzoin

⚡ Food chemistry note: Sorbic acid (CH₃–CH=CH–CH=CH–COOH) is a widely used food preservative (acts against mold and bacteria). It is a polyunsaturated short-chain acid.

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