Exhaustive Guide: Carboxylic Acids | Class 12 Chemistry | ChemCA

Carboxylic Acids: Preparation & Properties

Module 4 | CBSE Class 12 Chemistry | Organic Chemistry

1. Structure of Carboxyl Group

Carboxylic acids are compounds containing the carboxyl functional group (-COOH). The carboxyl group consists of a carbonyl group attached to a hydroxyl group.

In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about 120° (sp² hybridization).

Crucial Difference from Aldehydes/Ketones:
The carboxylic carbon is less electrophilic than carbonyl carbon in aldehydes and ketones. This is due to resonance. The lone pair of electrons on the oxygen atom of the -OH group delocalizes towards the carbonyl carbon, severely diminishing its partial positive charge (δ+). Hence, carboxylic acids do not typically give the characteristic nucleophilic addition reactions of aldehydes and ketones.

2. Methods of Preparation

2.1 From Primary Alcohols & Alkylbenzenes

A. From Primary Alcohols & Aldehydes: Primary alcohols are readily oxidized to carboxylic acids using strong oxidizing agents such as neutral, acidic or alkaline potassium permanganate (KMnO₄) or potassium dichromate (K₂Cr₂O₇) and chromium trioxide (CrO₃) in acidic media (Jones reagent).

R-CH₂OH →(Alk. KMnO₄ / H₃O⁺) R-COOH

B. From Alkylbenzenes (Highly Tested): Aromatic carboxylic acids can be prepared by vigorous oxidation of alkylbenzenes with chromic acid or alkaline KMnO₄.

The Side-Chain Oxidation Rule: The entire alkyl side chain (regardless of its length) is oxidized completely to a carboxyl group (-COOH), provided the benzylic carbon has at least one hydrogen atom.
- Toluene → Benzoic acid
- Propylbenzene → Benzoic acid
- Tert-butylbenzene → No reaction (Because it lacks a benzylic hydrogen).

2.2 From Nitriles, Amides & Grignard Reagents

A. From Nitriles and Amides: Nitriles are hydrolyzed to amides and then to carboxylic acids in the presence of H⁺ or OH^- as catalysts. Mild reaction conditions stop the reaction at the amide stage.

R-CN →(H⁺ or OH^-, H₂O) R-CONH₂ (Amide) →(H⁺ or OH^-, Δ) R-COOH + NH₃

B. From Grignard Reagents: Grignard reagents react with crushed dry ice (solid carbon dioxide) to form salts of carboxylic acids, which on acidification yield corresponding carboxylic acids. This method increases the carbon chain by one carbon atom.

R-Mg-X + O=C=O →(Dry ether) R-COOMgX →(H₃O⁺) R-COOH

2.3 From Acyl Halides, Anhydrides, and Esters

Acyl Halides: Acid chlorides (R-COCl) when hydrolyzed with water give carboxylic acids. With aqueous base, they give carboxylate ions which on acidification yield the acid.
Anhydrides: Hydrolyze readily to form corresponding acids. (e.g., Acetic anhydride + H₂O → 2 Acetic acid).
Esters: Acidic hydrolysis of esters directly yields carboxylic acids and alcohols. Basic hydrolysis (Saponification) gives carboxylate salts which must be acidified.

3. Physical Properties

Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids with unpleasant odours. Higher acids are wax-like solids and practically odourless due to low volatility.

Boiling Points & Dimer Formation:
Carboxylic acids have boiling points higher than alcohols, ketones, and aldehydes of comparable molecular masses. This is due to the presence of more extensive intermolecular hydrogen bonding.
The hydrogen bonds are so strong that they do not break completely even in the vapour phase. In fact, most carboxylic acids exist as dimers (pairs of molecules held together by two H-bonds) in the vapour phase or in aprotic solvents.

Solubility: Simple aliphatic carboxylic acids having up to four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. Solubility decreases as the alkyl chain (hydrophobic part) increases.

4. Chemical Reactions: Acidity of Carboxylic Acids

Carboxylic acids turn blue litmus red. They react with electropositive metals to form salts and evolve hydrogen gas. They react with strong bases (NaOH) and weak bases (NaHCO₃) to form salts and water/CO₂.

4.1 Carboxylic Acids vs. Phenols (The Bicarbonate Test)

Both phenols and carboxylic acids are acidic, but carboxylic acids are stronger acids than phenols.

Why are Carboxylic acids stronger than Phenols?
The conjugate base of a carboxylic acid (carboxylate ion, R-COO^-) is stabilized by two equivalent resonance structures where the negative charge is delocalized over two highly electronegative oxygen atoms.
In phenol, the conjugate base (phenoxide ion) has resonance structures where the negative charge sits on the less electronegative carbon atoms of the benzene ring. Delocalization over two oxygen atoms is far more stabilizing than over carbon atoms.

Distinguishing Test: Sodium Bicarbonate Test
Carboxylic acids react with weak bases like NaHCO₃ to produce brisk effervescence of CO₂ gas.
R-COOH + NaHCO₃ → R-COONa + H₂O + CO₂↑
Phenols are not strong enough acids to react with NaHCO₃ and do NOT give effervescence.

4.2 Effect of Substituents on Acid Strength (Highly Tested)

Substituents heavily influence the stability of the conjugate base (carboxylate ion), thus altering acidity.

Electron Withdrawing Groups (EWG): Groups with a -I effect (like -Cl, -F, -NO₂, -CN) withdraw electron density, disperse the negative charge on the carboxylate ion, and stabilize it. EWGs increase acidity.
Electron Donating Groups (EDG): Groups with a +I effect (like alkyl groups, -CH₃) push electron density towards the carboxylate ion, intensifying the negative charge and destabilizing it. EDGs decrease acidity.

Important Trends to Memorize:

Electronegativity of Halogen: FCH₂COOH > ClCH₂COOH > BrCH₂COOH > ICH₂COOH. (Fluorine has the strongest -I effect).
Number of Halogens: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH. (More -I groups = stronger acid).
Distance of Halogen: CH₃CH₂CH(Cl)COOH > CH₃CH(Cl)CH₂COOH > CH₂(Cl)CH₂CH₂COOH. (Inductive effect weakens heavily with distance).

5. Other Chemical Reactions

5.1 Reactions Involving Cleavage of C-OH Bond

Formation of Anhydride: Heating with H₂SO₄ or P₂O₅ removes water from two molecules to form anhydrides.
Esterification: Reaction with alcohols in the presence of conc. H₂SO₄ yields esters.
Reaction with PCl₅, PCl₃, and SOCl₂: Yields acid chlorides (R-COCl). SOCl₂ (Thionyl chloride) is preferred because the byproducts (SO₂ and HCl) are gases.
Reaction with Ammonia: Forms ammonium salts, which on heating at high temperatures lose water to form Amides.
CH₃COOH + NH₃ ⇔ CH₃COO^-NH₄⁺ →(Δ) CH₃CONH₂ (Acetamide) + H₂O

5.2 Reactions Involving -COOH Group

Reduction: Carboxylic acids are reduced to primary alcohols by Lithium aluminium hydride (LiAlH₄) or Diborane (B₂H₆).
Note: Sodium borohydride (NaBH₄) is a mild reducing agent and does not reduce carboxylic acids.
Decarboxylation: Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (a mixture of NaOH and CaO in the ratio of 3:1).
R-COONa + NaOH →(CaO, Δ) R-H (Alkane) + Na₂CO₃

5.3 Reactions Involving Hydrocarbon Part

A. Hell-Volhard-Zelinsky (HVZ) Reaction

Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of a small amount of red phosphorus to give α-halocarboxylic acids.

R-CH₂-COOH + X₂ / Red P →(H₂O) R-CH(X)-COOH
(where X = Cl, Br)

B. Ring Substitution in Aromatic Acids

Aromatic carboxylic acids undergo electrophilic substitution reactions. The carboxyl group acts as a deactivating and meta-directing group.

Why does Benzoic acid NOT undergo Friedel-Crafts reaction?
Benzoic acid fails to undergo Friedel-Crafts alkylation or acylation because the carboxyl group (-COOH) is strongly deactivating, and the catalyst employed (anhydrous aluminium chloride, AlCl₃, which is a Lewis acid) bonds permanently to the electron-rich oxygen of the carboxyl group instead of generating an electrophile.

6. NCERT Solved Examples (Step-by-Step)

NCERT Example 12.6: Show how the following compounds can be converted to benzoic acid.
(i) Ethylbenzene
(ii) Acetophenone
(iii) Bromobenzene

Solution:
(i) Ethylbenzene to Benzoic acid: The side chain of alkylbenzenes is oxidized regardless of length if it has a benzylic hydrogen. Treat with Alkaline KMnO₄ and heat, followed by acidification.
C₆H₅CH₂CH₃ →(KMnO₄-KOH, Δ) C₆H₅COOK →(H₃O⁺) C₆H₅COOH.

(ii) Acetophenone to Benzoic acid: Acetophenone (a methyl ketone) can be oxidized using the Iodoform reaction. Treat with NaOI (NaOH + I₂).
C₆H₅COCH₃ →(NaOI) C₆H₅COONa + CHI₃↓. Acidify the salt to get Benzoic acid.

(iii) Bromobenzene to Benzoic acid: Convert to a Grignard reagent first.
C₆H₅Br + Mg →(Dry ether) C₆H₅MgBr.
Then react with Dry ice (CO₂) followed by hydrolysis.
C₆H₅MgBr + CO₂ → C₆H₅COOMgBr →(H₃O⁺) C₆H₅COOH.

7. Previous Year Questions (PYQs) & Exhaustive Question Bank

Part A: Conceptual & Tests (1-2 Marks)

[CBSE 2018, 2021]

Q1. Give a simple chemical test to distinguish between Phenol and Benzoic acid.

Answer: Sodium Bicarbonate (NaHCO₃) Test. Add aqueous NaHCO₃ solution to both compounds. Benzoic acid will react to give brisk effervescence due to the evolution of CO₂ gas. Phenol is a weaker acid and will not react with NaHCO₃, hence no effervescence is observed.

[CBSE 2017, 2020]

Q2. Why is chloroacetic acid more acidic than acetic acid?

Answer: Chlorine is an electronegative atom and exerts a strong electron-withdrawing -I effect. This effect withdraws electron density away from the O-H bond, facilitating the release of H⁺. Furthermore, it disperses the negative charge on the resulting chloroacetate ion, stabilizing it. Acetic acid has an electron-donating methyl group (+I effect), which destabilizes the acetate ion. Hence, chloroacetic acid is a stronger acid.

Part B: Assertion-Reason Type (1 Mark)

[CBSE Sample Paper 2023]

Q3. Assertion (A): Benzoic acid does not undergo Friedel-Crafts alkylation reaction.
Reason (R): The carboxyl group is activating and ortho-para directing.

Answer: Assertion is Correct but Reason is Incorrect.
Benzoic acid indeed fails to undergo Friedel-Crafts reactions. However, the reason is that the carboxyl group (-COOH) is a strongly deactivating and meta-directing group. Additionally, the Lewis acid catalyst AlCl₃ forms a complex with the carboxyl group itself, stopping the reaction.

Part C: Synthesis and Reactions (3 Marks)

[CBSE 2016, 2022]

Q4. Write the chemical equations for the following Name Reactions:
(a) Hell-Volhard-Zelinsky reaction
(b) Decarboxylation

Answer:
(a) HVZ Reaction: Carboxylic acids with α-hydrogens are halogenated at the α-position using Red Phosphorus and X₂ (Cl₂ or Br₂).
CH₃COOH + Cl₂ →(i. Red P, ii. H₂O) Cl-CH₂-COOH (Chloroacetic acid).

(b) Decarboxylation: Sodium salts of carboxylic acids lose CO₂ to form alkanes when heated with sodalime (NaOH + CaO).
CH₃COONa + NaOH →(CaO, Δ) CH₄ (Methane) + Na₂CO₃.

[CBSE 2019]

Q5. Arrange the following in increasing order of their acidic character:
4-Methoxybenzoic acid, Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid.

Answer:
Acidity is increased by Electron Withdrawing Groups (-NO₂) and decreased by Electron Donating Groups (-OCH₃).
- 4-Methoxybenzoic acid: Has an EDG, weakest acid.
- Benzoic acid: Reference point.
- 4-Nitrobenzoic acid: Has one strong EWG, stronger acid.
- 3,4-Dinitrobenzoic acid: Has two strong EWGs, strongest acid.
Order: 4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid.

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Exhaustive Guide: Carboxylic Acids | Class 12 Chemistry