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Carboxylic Acids Prep
🎓 Class 12
Chemistry
CBSE
Theory
Ch 8 – Aldehydes, Ketones and Carboxylic Acids
⏱ ~14 min
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Carboxylic Acids Prep
8.5 Carboxylic Acids — Introduction & Nomenclature L1
The vinegar in your kitchen, the lactic acid that aches your muscles after a sprint, the citric acid in lemons, the fatty acids in soap — all share the carboxyl group –COOH, a fusion of a carbonyl (C=O) and a hydroxyl (–OH) on the same carbon. The carboxyl carbon is again sp² hybridised; the C–O single bond shortens and the C=O lengthens because of resonance, so the two C–O bond lengths in the carboxylate anion become identical.
| Structure | Common name | IUPAC name |
|---|---|---|
| HCOOH | Formic acid | Methanoic acid |
| CH3COOH | Acetic acid | Ethanoic acid |
| CH3(CH2)2COOH | Butyric acid | Butanoic acid |
| (CH3)2CHCOOH | Isobutyric acid | 2-Methylpropanoic acid |
| HOOC-COOH | Oxalic acid | Ethanedioic acid |
| HOOC(CH2)2COOH | Succinic acid | Butanedioic acid |
| C6H5COOH | Benzoic acid | Benzenecarboxylic acid |
| C6H5CH2COOH | Phenylacetic acid | 2-Phenylethanoic acid |
| Phthalic acid (1,2-) | Phthalic acid | Benzene-1,2-dicarboxylic acid |
8.6 Methods of Preparation of Carboxylic Acids L3
8.6.1 From Primary Alcohols & Aldehydes
Strong oxidants (KMnO4/H+, CrO3/H2SO4, K2Cr2O7/H2SO4) oxidise 1° alcohols and aldehydes to the acid.
R-CH2OH alkaline KMnO4, then H3O⁺→ R-COOH
R-CHO Tollens, Fehling or [O]→ R-COOH
8.6.2 From Alkylbenzenes
Aromatic carboxylic acids form when alkylbenzenes are oxidised with alkaline KMnO4 or acidified Na2Cr2O7. The whole side chain (any length, provided it has a benzylic C-H) is chopped down to –COOH.
C6H5R KMnO4/KOH then H3O⁺→ C6H5COOH
tert-Butylbenzene is not oxidised because there is no benzylic H.
8.6.3 From Nitriles & Amides
Hydrolysis of a nitrile (or an amide) with mineral acid or alkali gives the carboxylic acid:
R-C≡N H3O⁺ / Δ→ R-CONH2 H2O/H+→ R-COOH + NH4⁺
If alkali is used (OH⁻/Δ) the immediate product is the carboxylate, R-COO⁻, which is acidified to give the free acid.
8.6.4 From Grignard Reagents
Grignard reagents add to CO2 (dry ice) to form the magnesium salt of a carboxylic acid; acid hydrolysis releases the carboxylic acid. The carbon count of the alkyl halide is increased by one — useful synthetic step.
R-MgX + CO2 dry ether→ R-COO-MgX H3O⁺→ R-COOH
8.6.5 From Acyl Halides & Anhydrides
Acyl chlorides are hydrolysed easily; anhydrides need warm water. Esters require more forcing conditions (acid or alkali hydrolysis — the latter is called saponification).
RCOCl + H2O → RCOOH + HCl
(RCO)2O + H2O → 2 RCOOH
RCOOR' + NaOH → RCOONa + R'OH → RCOOH (on acidification)
(RCO)2O + H2O → 2 RCOOH
RCOOR' + NaOH → RCOONa + R'OH → RCOOH (on acidification)
8.7 Physical Properties L2
Carboxylic acids are typical of organic compounds that show very strong intermolecular hydrogen bonding — two O–H···O=C bonds knit two acid molecules into a cyclic dimer. As a result, acids boil even higher than alcohols of comparable mass.
| Acid | M (g mol⁻¹) | B.p. (K) | Solubility in H2O |
|---|---|---|---|
| HCOOH | 46 | 374 | Miscible |
| CH3COOH | 60 | 391 | Miscible |
| C3H7COOH | 88 | 437 | Miscible |
| C9H19COOH (decanoic) | 172 | 542 | Insoluble |
| C6H5COOH | 122 | 523 | Sparingly |
The first four acids (formic to butanoic) are miscible with water because the –COOH H-bonds with H2O. Beyond 4 carbons, the hydrophobic tail wins and solubility falls off rapidly.
8.8 Acidic Character of Carboxylic Acids L4
Carboxylic acids ionise in water:
R-COOH(aq) + H2O ⇌ R-COO⁻(aq) + H3O⁺(aq) Ka = [RCOO⁻][H3O⁺] / [RCOOH]
Strong acidity (compared with alcohols and phenols) arises because the resulting carboxylate anion is stabilised by resonance — the negative charge is equally shared by two equivalent oxygens.
8.8.1 Effect of Substituents on Acidity
Any group that stabilises the carboxylate anion increases acidity. Hence:
- Electron-withdrawing groups (EWG) — –NO2, –CN, halogens — increase Ka (decrease pKa).
- Electron-donating groups (EDG) — alkyl, –OCH3, –NH2 — decrease Ka (raise pKa).
| Acid | Ka | pKa | Comment |
|---|---|---|---|
| HCOOH | 1.77 × 10⁻⁴ | 3.75 | No alkyl R; strongest aliphatic monocarboxylic acid |
| CH3COOH | 1.74 × 10⁻⁵ | 4.76 | +I of CH3 weakens it |
| FCH2COOH | 2.6 × 10⁻³ | 2.59 | -I of F enhances acidity |
| ClCH2COOH | 1.36 × 10⁻³ | 2.87 | |
| BrCH2COOH | 1.25 × 10⁻³ | 2.90 | |
| ICH2COOH | 7.5 × 10⁻⁴ | 3.12 | -I order F > Cl > Br > I |
| Cl2CHCOOH | 5.5 × 10⁻² | 1.26 | Two Cl, stronger |
| Cl3CCOOH | 2.3 × 10⁻¹ | 0.65 | Three Cl, nearly mineral-acid strong |
| O2N-C6H4-COOH (4-NO2) | 3.9 × 10⁻⁴ | 3.41 | EWG enhances |
| CH3O-C6H4-COOH (4-OMe) | 3.6 × 10⁻⁵ | 4.46 | EDG weakens |
| C6H5COOH | 6.27 × 10⁻⁵ | 4.20 | Stronger than acetic acid |
Why benzoic acid is stronger than acetic acid: the sp²-hybridised C of the ring is less electron-donating than sp³ alkyl, and resonance pulls electron density into the ring, helping stabilise the carboxylate.
Visual pKa ladder — lower bar = stronger acid
Inductive effect falls off rapidly with distance. Compare: 4-Cl-butanoic acid (pKa 4.52, nearly like butanoic itself) vs 2-Cl-butanoic acid (pKa 2.86). The –I effect of Cl 3 bonds away barely matters.
Interactive: pKa Comparator
Pick any two acids and the simulator tells you which is stronger and why.
Pick two acids.
Activity 8.3 — Rank the Acids
Setup: You have these four acids on the bench: acetic acid, fluoroacetic acid, trichloroacetic acid, and 4-methoxybenzoic acid.
Predict: Arrange them in order of increasing acid strength. Explain using inductive and resonance effects.
Order (weakest → strongest):
4-OMe-C6H4COOH (pKa 4.46) < CH3COOH (4.76)? — recall lower pKa = stronger acid.
So in INCREASING strength: CH3COOH (4.76) < 4-OMe-C6H4COOH (4.46) < FCH2COOH (2.59) < CCl3COOH (0.65).
Acetic is weakest: only an EDG alkyl. 4-OMe-benzoic is weaker than benzoic but stronger than acetic (resonance donation from OMe partly offset by sp²-C ring). FCH2COOH and CCl3COOH have strong –I substituents stabilising the carboxylate; three Cl together > one F.
Worked Example 8.5 — Why HCOOH is stronger than CH3COOH L4
Explain on the basis of inductive effect why formic acid (pKa 3.75) is stronger than acetic acid (pKa 4.76).
Formic acid carries only an H on the carboxyl carbon, while acetic acid carries a methyl group. The methyl is electron-donating (+I), pushing electron density toward –COO⁻ and destabilising the anion. With less stabilisation, equilibrium of dissociation lies further to the left for acetic — hence smaller Ka (larger pKa).
Worked Example 8.6 — Predict the stronger acid L5
Which is the stronger acid: 2-chlorobutanoic acid or 4-chlorobutanoic acid? Justify.
The –I effect of Cl falls rapidly with the number of σ-bonds between it and the carboxyl carbon. In 2-chlorobutanoic acid, Cl is 2 bonds away and exerts strong stabilisation on the carboxylate. In 4-chlorobutanoic acid Cl is 4 bonds away — its influence is negligible. 2-Chlorobutanoic acid is the stronger acid (pKa 2.86 vs 4.52).
Intext Practice L3
Intext 8.3 — Synthesis problems
Show how each conversion can be carried out: (i) Butan-1-ol → butanoic acid, (ii) Benzyl alcohol → phenylethanoic acid, (iii) 3-Nitrobromobenzene → 3-nitrobenzoic acid, (iv) 4-Methylacetophenone → benzene-1,4-dicarboxylic acid, (v) Cyclohexene → hexane-1,6-dioic acid, (vi) Butanal → butanoic acid.
(i) CH3CH2CH2CH2OH → (CrO3/H2SO4) → CH3CH2CH2COOH.
(ii) C6H5CH2OH → (HBr) → C6H5CH2Br → (KCN) → C6H5CH2CN → (H3O⁺/Δ) → C6H5CH2COOH.
(iii) 3-NO2-C6H4-Br → (Mg/ether) → 3-NO2-C6H4-MgBr → (CO2, then H3O⁺) → 3-NO2-C6H4-COOH.
(iv) 4-CH3-C6H4-COCH3 → (KMnO4/KOH then H3O⁺) → terephthalic acid (1,4-C6H4(COOH)2).
(v) Cyclohexene → (KMnO4/H+, heat) → HOOC(CH2)4COOH (adipic acid).
(vi) CH3CH2CH2CHO → (Tollens or [O]) → CH3CH2CH2COOH.
Competency-Based Questions
Q1. Why is the boiling point of propanoic acid (414 K) higher than 1-propanol (370 K)? L4
Carboxylic acids form a cyclic dimer through two O–H···O=C hydrogen bonds, effectively doubling the molar mass for evaporation. Alcohols form an open chain of weaker H-bonds and only one H per O.
Q2. Which is the strongest acid in the list? L3
(d) CCl3COOH (pKa 0.65). Three Cl provide maximum –I stabilisation of CCl3COO⁻.
Q3. Sodium ethanoate solution turns red litmus blue. Why? L2
CH3COO⁻ is the conjugate base of a weak acid; it hydrolyses to give some OH⁻ in solution, making the solution mildly basic.
Q4. Predict which is stronger: 4-methoxybenzoic acid or benzoic acid. L4
Benzoic acid (pKa 4.20) is stronger than 4-methoxybenzoic acid (pKa 4.46). The –OMe group donates electron density into the ring by resonance, destabilising the carboxylate and weakening the acid.
Q5. You isolated an unknown white solid M.p. 410 K, soluble in NaOH/Na2CO3, evolves CO2 with NaHCO3. Suggest the class of compound. L5
The compound is a carboxylic acid. NaHCO3 distinguishes acids (release CO2) from phenols (do not). Phenols dissolve in NaOH but not in NaHCO3.
Assertion–Reason Questions
Options: (A) Both A & R true; R correct explanation of A. (B) Both true; R not correct explanation. (C) A true, R false. (D) A false, R true.
A1. Carboxylic acids have higher boiling points than alcohols of comparable molar mass.
R1. Carboxylic acids form an additional intermolecular H-bond through C=O.
Answer: (A) — both true and R correctly explains A. Acids dimerise via two H-bonds, twice as many as in primary alcohols.
A2. Trichloroacetic acid is stronger than acetic acid.
R2. Three chlorines impart a strong –I effect that stabilises the carboxylate anion.
Answer: (A) — both true and R correctly explains A.
A3. Benzoic acid is a weaker acid than ethanoic acid.
R3. Sp²-hybridised ring carbon is more electron-donating than sp³ alkyl.
Answer: (D) — assertion false (benzoic is stronger, not weaker), reason also false (sp² is less, not more, electron-donating). Pure D — both false.
Frequently Asked Questions - Carboxylic Acids Prep
What is the main concept covered in Carboxylic Acids Prep?
In NCERT Class 12 Chemistry Chapter 8 (Aldehydes, Ketones and Carboxylic Acids), "Carboxylic Acids Prep" covers the core chemistry principles and reactions students need for board exam success. The MyAiSchool lesson explains the topic with definitions, structural diagrams, reaction mechanisms, worked examples, and interactive simulations. Key reactions, IUPAC names, and chemical reasoning are highlighted throughout aligned with CBSE 2025-26 syllabus.
How is Carboxylic Acids Prep useful in real-life or applied chemistry?
Real-life applications of "Carboxylic Acids Prep" from NCERT Class 12 Chemistry Chapter 8 include drug design, polymer industry, food chemistry, electrochemical cells, fuel cells, dyes/pigments, agrochemicals, and biochemistry. The MyAiSchool lesson links every concept to a tangible industrial or biological example so students see chemistry as a problem-solving framework for the molecular world.
What are the key reactions students should memorize for Carboxylic Acids Prep?
Key reactions in "Carboxylic Acids Prep" (NCERT Class 12 Chemistry Chapter 8 Aldehydes, Ketones and Carboxylic Acids) are tabulated in the MyAiSchool reaction map. Students should memorize each reaction with its reagent, conditions, mechanism class (SN1/SN2/E1/E2/electrophilic addition/etc), product, and stereochemistry. The Summary section provides a quick-reference reaction chart for last-minute revision.
How does this part connect to other parts of Chapter 8?
NCERT Class 12 Chemistry Chapter 8 (Aldehydes, Ketones and Carboxylic Acids) is structured so each part builds chemical understanding sequentially. "Carboxylic Acids Prep" connects to neighbouring parts via shared functional groups, reaction mechanisms, and structural concepts. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Carboxylic Acids Prep?
CBSE board questions from "Carboxylic Acids Prep" typically include: (1) 1-mark MCQs on definitions and IUPAC naming, (2) 2-mark short-answer reactions/products, (3) 3-mark mechanism questions, (4) 5-mark long-answer combining mechanism + product + stereochemistry + application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Carboxylic Acids Prep" lesson allows students to explore reaction outcomes, predict products, or compare reaction conditions, with live visual feedback. To use it effectively: (1) try every option/configuration, (2) compare with the analytical reasoning, (3) check IUPAC names and structural correctness, (4) test edge cases from worked examples. The simulation reinforces conceptual intuition that pure mechanism memorisation cannot provide.
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