Aldehydes, Ketones & Carboxylic Acids

8.1 Nomenclature & Structure of Carbonyl Group

Key Definitions

Carbonyl Group: C=O (carbon-oxygen double bond) — one of the most important functional groups in organic chemistry.

Aldehyde (RCHO): Carbonyl group bonded to one carbon (or H) and one hydrogen. The –CHO group is always at the end of the chain.

Ketone (RCOR'): Carbonyl group bonded to two carbon atoms. The C=O is within the chain.

Carboxylic Acid (RCOOH): Carbonyl group with an –OH group attached to the same carbon → –COOH (carboxyl group).

8.1.1 IUPAC Nomenclature of Aldehydes

IUPAC Rules — Aldehydes

Replace the ending '–e' of alkane with '–al'. (e.g., methane → methanal, ethane → ethanal)
The –CHO carbon is always C-1; numbering starts from the aldehyde end.
For aldehydes attached to a ring: add suffix 'carbaldehyde' after the full ring name. (e.g., cyclohexanecarbaldehyde)
Simplest aromatic aldehyde: benzenecarbaldehyde (IUPAC) or benzaldehyde (accepted IUPAC common name)

8.1.1 IUPAC Nomenclature of Ketones

IUPAC Rules — Ketones

Replace the ending '–e' of alkane with '–one'. (e.g., propane → propanone)
Number from the end nearer to the carbonyl group to give it the lowest locant.
For cyclic ketones: carbonyl carbon is C-1.
Common names: name both alkyl groups + "ketone". (e.g., dimethyl ketone = acetone)

Master Nomenclature Table

Structure	Common Name	IUPAC Name
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
CH₃CH₂CH₂CH₂CHO	Valeraldehyde	Pentanal
CH₂=CHCHO	Acrolein	Prop-2-enal
C₆H₅CHO	Benzaldehyde	Benzenecarbaldehyde (Benzaldehyde)
CH₃COCH₃	Acetone (Dimethyl ketone)	Propanone
C₆H₅COCH₃	Acetophenone / Methyl phenyl ketone	1-Phenylethan-1-one
C₆H₅COC₆H₅	Benzophenone	Diphenylmethanone
CH₃COCH₂CH₂CH₃	Methyl n-propyl ketone	Pentan-2-one

8.1.2 Structure of the Carbonyl Group

KEY CONCEPT — Carbonyl Group Structure

The carbonyl carbon is sp² hybridised — forms three σ bonds. The fourth electron remains in a p-orbital and forms a π bond with the oxygen p-orbital.
All three atoms bonded to carbonyl carbon lie in the same plane (trigonal planar, ~120° angles).
The π-electron cloud lies above and below this plane.
Polarity: C=O is highly polar — O is more electronegative → C^δ+, O^δ−. Carbonyl carbon acts as Lewis acid (electrophile); carbonyl oxygen acts as Lewis base (nucleophile).
Resonance: >C=O ↔ >C⁺–O^–. The dipolar structure explains the high polarity.
Carbonyl compounds are more polar than ethers but form no H-bonds with each other (unlike alcohols).

⚛️

Orbital Diagram — Formation of Carbonyl Group (Fig. 8.1)

NCERT p.231 (Fig. 8.1)

Shows three stages: (1) sp² orbitals of carbon with one p-orbital perpendicular; (2) lateral overlap of p-orbital of C with p-orbital of O to form π bond; (3) final trigonal planar structure with 120° bond angles, showing π-cloud above and below the plane.

AI Image Prompt

Professional chemistry orbital diagram showing formation of carbonyl group. Fully white background, clean minimalistic. Three diagrams side by side: (1) sp² hybridised carbon with unhybridised p orbital shown as dumbbell perpendicular to the plane, bonded to oxygen showing lone pairs; (2) sideways overlap of p orbitals forming π bond shown as shaded lobes above and below; (3) final trigonal planar structure with 120° bond angles marked, two groups R attached to carbon, oxygen double-bonded. No background patterns, no text paragraphs.

8.2a Preparation of Aldehydes

General Methods (for both Aldehydes & Ketones)

1. Oxidation of Primary Alcohols → Aldehydes

RCH₂OH —^{PCC / CrO₃ (anhyd.)}→ RCHO (Aldehyde)
RCH₂OH —^{KMnO₄ or K₂Cr₂O₇/H⁺}→ RCOOH (over-oxidation to acid!)
PCC = Pyridinium Chlorochromate — stops at aldehyde stage (no over-oxidation)

2. Dehydrogenation of Alcohols (Industrial Method)

RCH₂OH —^{Ag or Cu catalyst, vapour, 573 K}→ RCHO + H₂
R–CHOH–R' —^{Cu, 573 K}→ R–CO–R' (Ketone) + H₂

3. From Hydrocarbons — Ozonolysis of Alkenes

Ozonolysis of alkenes with O₃, followed by Zn/H₂O (reductive workup), gives aldehydes, ketones or mixtures depending on substitution.

R–CH=CH–R' —^{(i) O₃ (ii) Zn/H₂O}→ RCHO + R'CHO (two aldehydes)
R–CH=CR'R'' → RCHO + R'–CO–R'' (aldehyde + ketone)

4. Hydration of Alkynes

HC≡CH + H₂O —^{H₂SO₄, HgSO₄}→ CH₃CHO (Acetaldehyde / Ethanal)
RC≡CH + H₂O —^{H₂SO₄, HgSO₄}→ RCOCH₃ (Ketone)
Only ethyne gives aldehyde; all other alkynes give ketones (Markovnikov addition)

8.2.2 Special Methods — Preparation of Aldehydes Only

★ 5. Rosenmund Reduction (Acyl Chloride → Aldehyde)

Acyl chloride (RCOCl) is hydrogenated with H₂ over palladium-on-barium sulphate (Pd/BaSO₄) catalyst. BaSO₄ poisons the Pd catalyst to prevent over-reduction to primary alcohol.

R–CO–Cl + H₂ —^Pd/BaSO₄→ R–CHO + HCl
Example: C₆H₅COCl + H₂ —^Pd/BaSO₄→ C₆H₅CHO (Benzaldehyde) + HCl

★ 6. Stephen Reaction (Nitrile → Aldehyde)

Nitrile (RCN) is reduced with SnCl₂ in anhydrous ether + dry HCl → stannous chloride imine complex → hydrolysis with water → aldehyde. Adds one carbon to the chain.

RCN —^SnCl₂/HCl→ R–CH=NH (imine intermediate) —^H₃O⁺→ RCHO

7. DIBAL-H Reduction (Nitrile or Ester → Aldehyde)

Diisobutylaluminium hydride (DIBAL-H) selectively reduces nitriles and esters to aldehydes at low temperature (–78°C). Does not over-reduce.

RCN —^{(i) DIBAL-H, –78°C (ii) H₂O}→ RCHO
RCOOC₂H₅ —^{(i) DIBAL-H (ii) H₂O}→ RCHO + C₂H₅OH

8.2.2 Aromatic Aldehyde Preparation

★ Etard Reaction (Toluene → Benzaldehyde)

Toluene + CrO₂Cl₂ (chromyl chloride) in CS₂ → chromium complex → hydrolysis with H₃O⁺ → benzaldehyde.

C₆H₅CH₃ + CrO₂Cl₂ —^CS₂→ [Complex] —^H₃O⁺→ C₆H₅CHO

★ Gattermann–Koch Reaction (Benzene → Benzaldehyde)

Benzene + CO + HCl with anhydrous AlCl₃ or CuCl (Lewis acid catalyst) → benzaldehyde. Direct formylation of benzene ring.

C₆H₆ + CO + HCl —^{anhyd. AlCl₃/CuCl}→ C₆H₅CHO

Side-Chain Chlorination → Benzaldehyde (Commercial Method)

C₆H₅CH₃ —^{Cl₂/hν (UV light)}→ C₆H₅CHCl₂ (Benzal chloride) —^{H₂O, 373 K}→ C₆H₅CHO

8.2b Preparation of Ketones

1. Oxidation of Secondary Alcohols

R–CHOH–R' —^{K₂Cr₂O₇/H⁺ or KMnO₄ or CrO₃}→ R–CO–R' (Ketone)

★ 2. From Acyl Chlorides — Dialkylcadmium Reagent

Acyl chlorides react with dialkylcadmium (R₂Cd) — prepared from Grignard reagent + CdCl₂ — to give ketones. Cadmium reagent is less reactive than Grignard and doesn't over-react with the ketone product.

2RMgX + CdCl₂ → R₂Cd + 2Mg(X)Cl
2R'–CO–Cl + R₂Cd → 2R'–CO–R + CdCl₂

★ 3. Friedel-Crafts Acylation (Aromatic Ketones)

Benzene (or substituted benzene) + acyl chloride (RCOCl) or acid anhydride with anhyd. AlCl₃ (Lewis acid) → aryl ketone. This is the best method for aromatic ketones.

C₆H₆ + CH₃COCl —^{anhyd. AlCl₃}→ C₆H₅–CO–CH₃ (Acetophenone) + HCl
C₆H₆ + (CH₃CO)₂O —^{anhyd. AlCl₃}→ C₆H₅–CO–CH₃ + CH₃COOH

4. From Nitriles (using Grignard Reagent)

R–C≡N + R'MgX —^ether→ R–C(=NMgX)–R' —^H₃O⁺→ R–CO–R' (Ketone)

8.3 Physical Properties of Aldehydes & Ketones

Key Physical Properties

Physical state: Methanal (HCHO) is a gas at room temperature. Ethanal is a volatile liquid. All others are liquids or solids.
Boiling points — ordered (same MW, ~58–60):

n-Butane
273 KAlkane (van der Waals only)

Methoxyethane
281 KEther (dipole-dipole)

Propanal
322 KAldehyde (stronger dipole)

Acetone
329 KKetone

Propan-1-ol
370 KAlcohol (H-bonding)

Why higher bp than hydrocarbons/ethers? Strong dipole-dipole interactions in the polar C=O group.
Why lower bp than alcohols? No intermolecular hydrogen bonding (no O–H bond in aldehydes/ketones).
Solubility: Lower members (methanal, ethanal, propanone) are miscible with water in all proportions — form H-bonds with water via the carbonyl oxygen. Solubility decreases with increasing chain length.
Odour: Lower aldehydes have pungent odours; higher members are more fragrant. Many natural fragrances are aldehydes and ketones (vanillin, cinnamaldehyde).

8.4a Chemical Reactions — Nucleophilic Addition

Why Nucleophilic Addition? (Mechanism)

Mechanism of Nucleophilic Addition

Carbonyl carbon is sp² (planar). A nucleophile attacks the electrophilic δ+ carbonyl carbon perpendicularly to the plane of the sp² orbitals (from above or below). Carbon rehybridises from sp² → sp³ → tetrahedral alkoxide intermediate forms → proton from medium → neutral product.

Nu^– + >C=O → [>C(–O^–)(–Nu)] (tetrahedral intermediate) —^H⁺→ >C(–OH)(–Nu)

Aldehydes are more reactive than ketones in nucleophilic addition. Two reasons:

Steric: Ketones have two bulky alkyl groups hindering approach of nucleophile; aldehydes have only one alkyl group.
Electronic: Alkyl groups (electron-donating, +I effect) in ketones reduce the δ+ charge on carbonyl carbon → less electrophilic → less reactive. Aldehydes have only one +I group.

Reactivity order: HCHO > RCHO > RCOR'

Benzaldehyde is less reactive than propanal because the C₆H₅ group donates electrons to carbonyl carbon through resonance (π-π conjugation) → reduces electrophilicity of carbonyl C.

Important Nucleophilic Addition Reactions

(a) Addition of HCN — Cyanohydrin Formation

>C=O + HCN —^{base catalyst (CN⁻ generated)}→ >C(–OH)(–CN) (Cyanohydrin)
Example: CH₃CHO + HCN → CH₃–CH(OH)–CN (Acetaldehyde cyanohydrin = Lactonitrile)

Mechanism: Pure HCN reacts slowly (weak nucleophile). Base catalyst (NaOH/KCN) generates CN⁻ ion (strong nucleophile). CN⁻ attacks carbonyl C → alkoxide → picks up H⁺ from HCN → cyanohydrin + CN⁻ (regenerated).

Importance: Cyanohydrins are useful synthetic intermediates — can be hydrolysed to α-hydroxy acids or reduced to amino alcohols.

Limitation (steric): Sterically hindered ketones (e.g., 2,2,6-trimethylcyclohexanone) give very low yield due to steric crowding around carbonyl carbon.

(b) Addition of NaHSO₃ — Bisulphite Addition

>C=O + NaHSO₃ ⇌ >C(–OH)(–OSO₂Na) (Bisulphite addition compound, crystalline)
To regenerate: treat with dil. HCl or NaOH

Key features:

Equilibrium favours product for aldehydes and methyl ketones; unfavourable for most ketones (steric reasons).
Addition compound is water-soluble and crystalline → useful for separation and purification of aldehydes from non-reacting impurities.
Reversible — original carbonyl compound easily regenerated with dilute acid or alkali.

R–CHO + R'OH —^{dry HCl}→ R–CH(OH)(OR') [Hemiacetal] —^{R'OH, H⁺}→ R–CH(OR')(OR') + H₂O (Acetal)

Ketone + ethylene glycol:
R–CO–R' + HOCH₂CH₂OH —^{dry HCl}→ Cyclic ethylene glycol ketal + H₂O

Acetals and ketals are hydrolysed back to aldehydes/ketones with aqueous mineral acid. They are used as protecting groups for carbonyl compounds in synthesis.

(e) Addition of Ammonia and its Derivatives — Nucleophilic Addition-Elimination

Nucleophiles of the type H₂N–Z add to the carbonyl group. The addition is followed by loss of water (elimination) → C=N–Z double bond (imine-type products). Reaction is acid-catalysed and reversible.

N-Substituted Derivatives — Complete Table (Table 8.2)

Reagent (H₂N–Z)	Z group	Product	Product Name
Ammonia (NH₃)	–H	>C=NH	Imine
Primary amine (R–NH₂)	–R	>C=N–R	Substituted imine / Schiff's base
Hydroxylamine (NH₂OH)	–OH	>C=N–OH	Oxime
Hydrazine (NH₂NH₂)	–NH₂	>C=N–NH₂	Hydrazone
Phenylhydrazine (C₆H₅NHNH₂)	–NHC₆H₅	>C=N–NH–C₆H₅	Phenylhydrazone
2,4-Dinitrophenylhydrazine (DNPH)	–NHC₆H₃(NO₂)₂	>C=N–NHC₆H₃(NO₂)₂	2,4-DNP derivative (yellow/orange/red solid)
Semicarbazide (NH₂NHCONH₂)	–NHCONH₂	>C=N–NHCONH₂	Semicarbazone

★ 2,4-DNP derivatives are yellow, orange or red crystalline solids → used for characterisation and identification of aldehydes and ketones (sharp melting point).

8.4b Reduction Reactions of Aldehydes & Ketones

★ (i) Reduction to Alcohols

RCHO —^{NaBH₄ or LiAlH₄ or H₂/Pt}→ RCH₂OH (1° alcohol)
RCOR' —^{NaBH₄ or LiAlH₄}→ R–CHOH–R' (2° alcohol)

★ (ii) Reduction to Hydrocarbons (C=O → CH₂)

Clemmensen Reduction: Zn-Hg amalgam + conc. HCl. For acid-stable substrates.

>C=O —^{Zn-Hg/conc. HCl}→ >CH₂ + H₂O

Wolff-Kishner Reduction: NH₂NH₂ (hydrazine) → hydrazone → heat with KOH in ethylene glycol. For acid-sensitive substrates.

>C=O —^NH₂NH₂→ >C=NNH₂ —^KOH/Δ→ >CH₂ + N₂

🧠

Clemmensen vs Wolff-Kishner Clemmensen: Acidic conditions (Zn-Hg/HCl) → for acid-stable molecules.
Wolff-Kishner: Alkaline conditions (KOH/hydrazine) → for acid-sensitive molecules (e.g., acetals, base-stable groups).
Both convert C=O → CH₂ completely (remove the oxygen entirely).

8.4c Oxidation Reactions — Distinguishing Aldehydes from Ketones

KEY CONCEPT — Oxidation Difference

Aldehydes are easily oxidised to carboxylic acids by mild or strong oxidising agents. They have a C–H bond on the carbonyl carbon which is easily broken.

R–CHO —^[O]→ R–COOH

Ketones are generally resistant to mild oxidising agents. Under vigorous conditions (strong oxidising agents + high T), C–C bond cleavage occurs → mixture of carboxylic acids with fewer carbons.

→ This difference is exploited in two classic tests to distinguish aldehydes from ketones:

★ Tollens' Test (Silver Mirror Test)

Reagent: Freshly prepared ammoniacal silver nitrate solution [Ag(NH₃)₂]⁺OH^–.

Positive test: Bright silver mirror deposited on the inner wall of the test tube (Ag metal formed). Aldehyde is oxidised to carboxylate anion (in alkaline medium).

R–CHO + 2[Ag(NH₃)₂]⁺ + 3OH^– → RCOO^– + 2Ag↓ (silver mirror) + 4NH₃ + 2H₂O

Ketones: No reaction. Aromatic aldehydes (benzaldehyde): Positive test.

★ Fehling's Test

Reagent: Fehling Solution A (aq. CuSO₄) + Fehling Solution B (alkaline sodium potassium tartarate). Mixed in equal amounts before use.

Positive test: Reddish-brown precipitate of Cu₂O (cuprous oxide) forms on heating. Cu²⁺ is reduced to Cu⁺.

R–CHO + 2Cu²⁺ + 5OH^– → RCOO^– + Cu₂O↓ (red-brown) + 3H₂O

Important: Aromatic aldehydes (benzaldehyde) do NOT respond to Fehling's test. Aliphatic aldehydes give positive test. Ketones: No reaction.

★ (iii) Iodoform Reaction (Haloform Reaction) — Oxidation by NaOX

Compounds having CH₃CO– group (methyl ketones) or CH₃CHOH– group (which oxidises to CH₃CO–) react with iodine/NaOH (sodium hypoiodite) to give iodoform (CHI₃) — a yellow precipitate with characteristic smell.

R–CO–CH₃ + 3I₂ + 3NaOH → R–CO–CI₃ → RCOONa + CHI₃↓ (Iodoform, yellow)
R–CO–CH₃ —^{NaOCl (general)}→ RCOONa + CHX₃ (haloform, X = Cl, Br, I)

Iodoform test gives yellow precipitate with:

CH₃CO– group: acetaldehyde (CH₃CHO), acetone, methyl ketones (CH₃COR)
CH₃CHOH– group: ethanol (CH₃CH₂OH), propan-2-ol (oxidised to CH₃CO– then reacts)

Important: This is also used as a detection test for the CH₃CO– or CH₃CH(OH)– group.

8.4d Reactions Due to α-Hydrogen

Acidity of α-Hydrogen

The hydrogen atoms on the carbon adjacent to the C=O group (α-carbon) are acidic. Reasons:

Strong electron-withdrawing effect of the C=O group weakens the C–H bond.
The conjugate base (enolate ion) is resonance-stabilised: –C^––C=O ↔ –C=C–O^– (enolate)

★ Aldol Condensation — KEY NAMED REACTION

Aldehydes and ketones with at least one α-hydrogen undergo aldol reaction in the presence of dilute alkali (NaOH, Ba(OH)₂) to form β-hydroxy carbonyl compounds (aldol or ketol).

On heating, aldol and ketol readily lose water (dehydration) → α,β-unsaturated carbonyl compound (aldol condensation product).

Ethanal → Aldol → But-2-enal (Crotonaldehyde):
2 CH₃CHO —^{dil. NaOH}→ CH₃–CH(OH)–CH₂–CHO (3-Hydroxybutanal, aldol) —^{Δ, –H₂O}→ CH₃–CH=CH–CHO (But-2-enal)

Propanone → Ketol → 4-Methylpent-3-en-2-one (Mesityl oxide):
2 CH₃COCH₃ —^Ba(OH)₂→ (CH₃)₂C(OH)–CH₂COCH₃ (ketol) —^{Δ, –H₂O}→ (CH₃)₂C=CH–COCH₃

Mechanism of aldol: Base deprotonates α-H → enolate anion (nucleophile) → attacks carbonyl carbon of another molecule (electrophile) → β-hydroxy carbonyl compound.

Cross Aldol Condensation

Aldol condensation between two different aldehydes/ketones. If both have α-H, a mixture of 4 products forms (all combinations). Useful when one component has no α-H (acts only as electrophile).

Example: Benzaldehyde (no α-H) + Acetone (has α-H) → Benzalacetone + Dibenzalacetone (1,3-Diphenyl-2-propen-1-one / Benzalacetophenone) as major product.

C₆H₅CHO + C₆H₅COCH₃ —^{OH^–, 293 K}→ C₆H₅CH=CH–CO–C₆H₅ (Chalcone, Benzalacetophenone)

8.4e Other Reactions of Aldehydes & Ketones

★ Cannizzaro Reaction — Disproportionation

Aldehydes which do NOT have α-hydrogen undergo self oxidation-reduction (disproportionation) on treatment with concentrated NaOH: one molecule is oxidised to the carboxylate (acid salt) and another is reduced to the alcohol.

Formaldehyde:
2HCHO + conc. KOH —^Δ→ CH₃OH (methanol) + HCOOK (potassium formate)

Benzaldehyde:
2C₆H₅CHO + conc. NaOH —^Δ→ C₆H₅CH₂OH (benzyl alcohol) + C₆H₅COONa (sodium benzoate)

Examples without α-H: HCHO, C₆H₅CHO, (CH₃)₃CCHO, 2,2-dimethylpropanal. Note: Formaldehyde in this reaction is always the reductant (gives methanol) because it has no alkyl group stabilisation.

Electrophilic Substitution of Aromatic Aldehydes and Ketones

The –CHO and –COR groups on benzene ring are deactivating and meta-directing (due to –M and –I effects of C=O which withdraws electrons from the ring). EAS occurs at meta position.

C₆H₅CHO + HNO₃/H₂SO₄ —^{273–283 K}→ m-NO₂–C₆H₄–CHO (m-Nitrobenzaldehyde, major)

8.6 Nomenclature & Structure of Carboxyl Group

Carboxylic Acids

Compounds containing the carboxyl group (–COOH). The carboxyl group consists of a carbonyl (C=O) group attached to a hydroxyl (–OH) group.

Aliphatic: R–COOH (fatty acids C₁₂–C₁₈ occur naturally as glycerol esters)

Aromatic: Ar–COOH (e.g., benzoic acid)

IUPAC Rules for Carboxylic Acids

Replace the ending '–e' of alkane with '–oic acid'. (e.g., methane → methanoic acid)
The –COOH carbon is always C-1.
For dicarboxylic acids: retain 'e' of alkane + add 'dioic acid'. (e.g., ethanedioic acid)
For aromatic: benzenecarboxylic acid (IUPAC) or benzoic acid (accepted)

Common and IUPAC Names — Master Table

Structure	Common Name	IUPAC Name	Source/Origin
HCOOH	Formic acid	Methanoic acid	Red ants (Latin: formica)
CH₃COOH	Acetic acid	Ethanoic acid	Vinegar (Latin: acetum)
CH₃CH₂COOH	Propionic acid	Propanoic acid	—
CH₃CH₂CH₂COOH	Butyric acid	Butanoic acid	Rancid butter (Latin: butyrum)
(CH₃)₂CHCOOH	Isobutyric acid	2-Methylpropanoic acid	—
HOOC–COOH	Oxalic acid	Ethanedioic acid	—
HOOC–CH₂–COOH	Malonic acid	Propanedioic acid	—
HOOC–(CH₂)₂–COOH	Succinic acid	Butanedioic acid	—
HOOC–(CH₂)₄–COOH	Adipic acid	Hexanedioic acid	Nylon-6,6 manufacture
C₆H₅COOH	Benzoic acid	Benzenecarboxylic acid	Benzoin resin

8.6.2 Structure of the Carboxyl Group

Structure of –COOH

The bonds to the carboxyl carbon lie in one plane, separated by ~120°. Three resonance structures stabilise the carboxyl group:

–C(=O)(O–H) ↔ –C⁺(–O^–)(–OH) ↔ –C(–O)(=OH⁺)

Key consequence: Carboxyl carbon is less electrophilic than aldehyde/ketone carbonyl carbon because resonance involving the –OH lone pair reduces the δ+ charge on carbon. This is why carboxylic acids do NOT undergo typical nucleophilic addition reactions like aldehydes and ketones.

8.7 Methods of Preparation of Carboxylic Acids

1. Oxidation of Primary Alcohols and Aldehydes

RCH₂OH —^{alk. KMnO₄ or Jones reagent (CrO₃–H₂SO₄)}→ RCOOH
RCHO —^{Tollens' / Fehling's / KMnO₄}→ RCOOH

2. Oxidation of Alkylbenzenes (Vigorous)

Entire alkyl side chain (regardless of length) is oxidised to –COOH. Primary and secondary alkyl groups react; tertiary groups are not affected.

C₆H₅CH₃ (Toluene) —^{KMnO₄/KOH, heat}→ C₆H₅COOK —^H₃O⁺→ C₆H₅COOH (Benzoic acid)
C₆H₅CH₂CH₂CH₃ —^{same conditions}→ C₆H₅COOH (same product, side chain length doesn't matter)

3. Hydrolysis of Nitriles and Amides

R–CN —^{H⁺ or OH⁻, H₂O}→ R–CONH₂ (Amide) —^{H⁺ or OH⁻, H₂O}→ RCOOH
Nitriles from alkyl halides → acids = ascending the series (1 carbon added)

4. From Grignard Reagents + CO₂ (Dry Ice)

RMgX + O=C=O (CO₂, dry ice) —^{dry ether}→ R–COOMgX —^H₃O⁺→ RCOOH
Also ascending the series — gives carboxylic acid with 1 more C than alkyl halide

5. Hydrolysis of Acyl Halides and Anhydrides

RCOCl + H₂O → RCOOH + HCl
(RCO)₂O + H₂O → 2RCOOH
(C₆H₅CO)₂O + H₂O → 2C₆H₅COOH

6. Hydrolysis of Esters

RCOOR' —^{H₃O⁺ or NaOH/H₂O}→ RCOOH + R'OH (acidic hydrolysis, direct)
RCOOR' —^NaOH→ RCOONa —^H₃O⁺→ RCOOH (saponification)

8.8 Physical Properties of Carboxylic Acids

Physical Properties

Physical state: C₁–C₉: colourless liquids with unpleasant odours. C₁₀+: wax-like solids, practically odourless (low volatility).
Boiling points: Higher than alcohols of same molecular mass — due to more extensive intermolecular hydrogen bonding (two H-bond donors + two acceptors per molecule). Carboxylic acids exist as dimers in vapour phase or aprotic solvents via double hydrogen bonding (cyclic dimer structure).
Solubility: C₁–C₄ are fully miscible with water (form H-bonds). Solubility decreases with carbon chain length. Higher acids are nearly insoluble in water but soluble in organic solvents (benzene, ether, ethanol, CHCl₃). Benzoic acid is nearly insoluble in cold water.

8.9 Chemical Reactions of Carboxylic Acids

8.9.1 Reactions Involving O–H Bond Cleavage (Acidity)

Acidity — Why Carboxylic Acids Are the Most Acidic

Acidity order: Carboxylic acids >> Phenols >> Alcohols >> Water (for organic compounds)

Why stronger than phenols? The carboxylate ion (RCOO⁻) is stabilised by two equivalent resonance structures with negative charge on both oxygens (both electronegative). The phenoxide ion has non-equivalent structures with negative charge on less electronegative ring carbons. Therefore, carboxylate is more stable → carboxylic acid is stronger.

RCOOH ⇌ RCOO⁻ + H⁺ [pKa values: formic = 3.75, acetic = 4.76, benzoic = 4.19, trifluoroacetic = 0.23]

Reactions with bases:

2R–COOH + 2Na → 2R–COONa + H₂↑
R–COOH + NaOH → R–COONa + H₂O
R–COOH + NaHCO₃ → R–COONa + H₂O + CO₂↑ ★ (distinguishes RCOOH from phenol — phenol does NOT react with NaHCO₃)

Effect of Substituents on Acidity of Carboxylic Acids

Electron-withdrawing groups (EWG): Stabilise the carboxylate anion → increase acidity (decrease pKa). Effect increases with number of EWG and decreases with distance from –COOH.
Electron-donating groups (EDG): Destabilise the carboxylate anion → decrease acidity (increase pKa).

Acidity order of haloacetic acids: CF₃COOH > CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH

Acidity order of chloroacetic acids by position: α-Cl > β-Cl > γ-Cl (inductive effect decreases rapidly with distance)

For aromatic acids: EWG at o/p → more acidic; EDG at o/p → less acidic. 4-Nitrobenzoic acid (pKa 3.41) > Benzoic acid (4.19) > 4-Methoxybenzoic acid (4.46)

EWG power: CF₃ > NO₂ > CN > F > Cl > Br > I > Ph

8.9.2 Reactions Involving C–OH Bond Cleavage

1. Formation of Acid Anhydride

2CH₃COOH —^{H₂SO₄ or P₂O₅, Δ}→ CH₃CO–O–COCH₃ (Ethanoic anhydride) + H₂O

★ 2. Esterification (Fischer Esterification)

Carboxylic acid + alcohol with conc. H₂SO₄ catalyst. Reversible — remove water or ester to drive equilibrium right.

RCOOH + R'OH ⇌^{H⁺, Δ} RCOOR' (Ester) + H₂O

Mechanism (nucleophilic acyl substitution): H⁺ protonates carbonyl O → electrophilicity of carbonyl C increases → alcohol (nucleophile) attacks → tetrahedral intermediate forms → proton transfer → –OH₂ (good leaving group) departs → protonated ester → deprotonation → ester product.

3. Reactions with PCl₅, PCl₃, SOCl₂ (→ Acyl Chloride)

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl
3RCOOH + PCl₃ → 3RCOCl + H₃PO₃
RCOOH + SOCl₂ → RCOCl + SO₂↑ + HCl↑ ★ (SOCl₂ preferred — both by-products are gaseous)

4. Reaction with Ammonia (→ Amide)

CH₃COOH + NH₃ ⇌ CH₃COONH₄ —^{Δ, –H₂O}→ CH₃CONH₂ (Acetamide)

8.9.3 Reactions Involving the –COOH Group

1. Reduction of –COOH to –CH₂OH

LiAlH₄ (best) or diborane (B₂H₆) reduces –COOH to primary alcohol. NaBH₄ cannot reduce carboxyl group.

R–COOH —^{(i) LiAlH₄ or B₂H₆ (ii) H₃O⁺}→ R–CH₂OH

Diborane advantage: does not reduce –NO₂, –Cl, –COOR groups selectively.

★ 2. Decarboxylation

Sodium salt of carboxylic acid + sodalime (NaOH + CaO, 3:1) → hydrocarbon (one carbon fewer). Called decarboxylation.

R–COONa —^{sodalime (NaOH/CaO), heat}→ R–H + Na₂CO₃
CH₃COONa → CH₄ + Na₂CO₃

8.9.4 Substitution Reactions in the Hydrocarbon Part

★ Hell-Volhard-Zelinsky (HVZ) Reaction — α-Halogenation

Carboxylic acids with α-hydrogen undergo halogenation at the α-position with Cl₂ or Br₂ in presence of small amount of red phosphorus (catalyst) → α-halocarboxylic acid.

R–CH₂–COOH —^{(i) Cl₂ or Br₂ / red P (ii) H₂O}→ R–CHX–COOH (α-Halocarboxylic acid, X = Cl or Br)

Example: CH₃CH₂COOH + Cl₂/red P → CH₃CHClCOOH (2-Chloropropanoic acid)

Importance: α-Halocarboxylic acids are useful synthetic intermediates (can be further modified at the α-position).

Ring Substitution of Aromatic Carboxylic Acids

The –COOH group is a deactivating and meta-directing group (–M effect withdraws electrons). Therefore, EAS in benzoic acid gives meta products.

C₆H₅COOH —^{HNO₃/H₂SO₄}→ m-O₂N–C₆H₄–COOH (m-Nitrobenzoic acid, major)
C₆H₅COOH —^{Br₂/FeBr₃}→ m-Br–C₆H₄–COOH (m-Bromobenzoic acid, major)

Important: Benzoic acid does NOT undergo Friedel-Crafts reaction because the –COOH group is deactivating AND the Lewis acid AlCl₃ gets bonded to the –COOH group (forms a complex with it).

✏️ Practice Questions

Arrange the following in increasing order of boiling points and explain the reason:
n-Butane, Ethoxyethane (diethyl ether), Butanal, Butan-1-ol

Explain why aldehydes are generally more reactive than ketones in nucleophilic addition reactions. Also explain why benzaldehyde is less reactive than propanal.

An organic compound (A) with molecular formula C₈H₈O forms an orange-red precipitate with 2,4-DNP reagent but neither reduces Tollens' nor Fehling's reagent. It gives yellow precipitate with I₂/NaOH (iodoform test). On drastic oxidation with chromic acid, it gives compound (B) with formula C₇H₆O₂. Identify A and B.

Which compounds will undergo Aldol condensation, which will undergo Cannizzaro reaction, and which will undergo neither?
(i) Methanal (ii) 2-Methylpentanal (iii) Benzaldehyde (iv) Benzophenone (v) Cyclohexanone

Arrange in increasing order of acid strength and explain:
(i) CF₃COOH, CCl₃COOH, CH₂ClCOOH, CH₃COOH, CH₃CH₂COOH
(ii) Benzoic acid, p-Nitrobenzoic acid, p-Methoxybenzoic acid

Although phenoxide ion has MORE resonance structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why? (Exercise 8.20)

Give simple chemical tests to distinguish between:
(i) Propanal vs Propanone (ii) Benzaldehyde vs Acetophenone (iii) Pentan-2-one vs Pentan-3-one

How will you convert Ethanal into: (i) Butane-1,3-diol (ii) But-2-enal (iii) But-2-enoic acid

An organic compound contains 69.77% C, 11.63% H, and the rest is O. Molecular mass = 86. It does not reduce Tollens' reagent but forms an addition compound with NaHSO₃ and gives positive iodoform test. On vigorous oxidation, it gives ethanoic acid and propanoic acid. Identify the compound.

Q10

How will you prepare Benzoic acid from: (i) Ethylbenzene (ii) Acetophenone (iii) Bromobenzene?

Q11

An organic compound (A) with molecular formula C₉H₁₀O forms 2,4-DNP derivative, reduces Tollens' reagent and undergoes Cannizzaro reaction. On vigorous oxidation it gives 1,2-benzenedicarboxylic acid (phthalic acid). Identify compound (A).

🎯 Important Exam Points — Quick Reference

CONCEPTCarbonyl C is sp² hybridised, trigonal planar (~120°). C=O is polar (C is δ+, electrophile; O is δ−, nucleophile). Aldehydes more reactive than ketones in nucleophilic addition (steric + electronic).

CONCEPTReactivity in nucleophilic addition: HCHO > RCHO > RCOR'. Aldehydes have one alkyl group; ketones have two bulky groups hindering approach of nucleophile + more +I effect reducing δ+.

CONCEPTIodoform test: CH₃CO– or CH₃CH(OH)– group gives yellow CHI₃ (iodoform) precipitate with I₂/NaOH. Gives positive test with CH₃CHO, CH₃COCH₃, CH₃COR, C₂H₅OH, CH₃CHOHР.

CONCEPTAldol condensation: requires α-H, dilute NaOH. Cannizzaro reaction: no α-H, conc. NaOH. Self oxidation-reduction (disproportionation). Examples without α-H: HCHO, C₆H₅CHO, (CH₃)₃CCHO.

CONCEPTCarboxylic acids: more acidic than phenols because carboxylate ion has charge on two equivalent electronegative O atoms (equivalent resonance). Phenoxide: charge on one O + ring C atoms (less electronegative, non-equivalent).

CONCEPTEWG increases acidity of RCOOH (stabilises RCOO⁻). EDG decreases acidity. EWG effect decreases with distance from COOH. CF₃COOH > CCl₃COOH > CH₂ClCOOH > CH₃COOH > CH₃CH₂COOH.

REACTIONTollens' test (silver mirror): RCHO + [Ag(NH₃)₂]⁺OH⁻ → Ag (silver mirror) + RCOO⁻. Aromatic aldehydes: positive. Ketones: negative. Aliphatic aldehydes: positive.

REACTIONFehling's test: RCHO + Cu²⁺(alkaline) → Cu₂O↓ (red-brown) + RCOO⁻. Aromatic aldehydes (benzaldehyde): NEGATIVE! Only aliphatic aldehydes give positive test. Ketones: negative.

REACTIONClemmensen: C=O → CH₂ using Zn-Hg/conc. HCl (acid conditions). Wolff-Kishner: C=O → CH₂ using NH₂NH₂ then KOH/ethylene glycol/heat (basic conditions). Both reduce carbonyl to methylene completely.

REACTIONRosenmund: RCOCl + H₂ →(Pd/BaSO₄)→ RCHO. Stephen: RCN + SnCl₂/HCl → RCHO. DIBAL-H: RCN or RCOOR' → RCHO. Gattermann-Koch: C₆H₆ + CO/HCl →(AlCl₃/CuCl)→ C₆H₅CHO.

REACTIONHVZ (Hell-Volhard-Zelinsky): RCHCOOH + Br₂/red P → RCHBrCOOH (α-halogenation). Decarboxylation: RCOONa + sodalime → RH + Na₂CO₃. Esterification: RCOOH + R'OH ⇌(H⁺) RCOOR' + H₂O.

MCQNaHCO₃ test: RCOOH + NaHCO₃ → effervescence (CO₂). Phenol does NOT react with NaHCO₃. This is the key test to distinguish carboxylic acids from phenols. Both react with NaOH.

MCQNaBH₄ cannot reduce –COOH or –COOR. LiAlH₄ can reduce all carbonyl groups. Diborane reduces –COOH selectively without reducing –NO₂, –Cl, etc. PCC oxidises 1° alcohol → aldehyde only (not over-oxidised to acid).

MCQAromatic carboxylic acids (–COOH) are deactivating + meta-directing. Aromatic aldehydes and ketones (–CHO, –COR) are also deactivating + meta-directing. Neither undergoes Friedel-Crafts reaction (benzoic acid — AlCl₃ forms complex with COOH).

Aldehydes, Ketones & Carboxylic Acids –
Complete Detailed Notes

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