CBSE Class 12 Chemistry Aldehyde Ketones and Carboxylic

This Class 12 Chemistry summary covers Unit 12 Aldehydes, Ketones and Carboxylic Acids. It gives the structure of the carbonyl and carboxyl groups, nomenclature tables, the methods of preparation of aldehydes, ketones and carboxylic acids, and their chemical reactions: nucleophilic addition, reduction, oxidation, aldol condensation, Cannizzaro reaction, acidity, esterification, decarboxylation, Hell-Volhard-Zelinsky reaction and ring substitution.

SUMMARY OF Unit 12: Aldehyde, Ketones and Carboxylic - PART 1

Introduction:

Organic compounds containing carbon-oxygen double bond (>C=O) called carbonyl group.

[Figure: General structures of carbonyl compounds, each with a C=O group: Aldehyde R-CHO (R and H on carbon); Ketone R-CO-R'; Carboxylic acid R-COOH; Acyl halide R-CO-X, X = (Halogen); Acid anhydride R-CO-O-CO-R'; Ester R-COOR'; Amide R-CONH₂]

Nomenclature of Aldehyde and Ketone

Nomenclature of Carboxylic acids

Structure of functional groups:

Structure Of carbonyl functional group:

[Figure: Orbital picture of the carbonyl group - a planar σ-bond framework of the carbon with its three substituents and a π-bond formed by the sideways overlap of p orbitals above and below the plane of the C and O atoms. Below it, the resonance structures: (A) C=O, with a double bond, and (B) C⁺-O^-, joined by a double-headed arrow]

Structure Of -COOH functional group:

[Figure: Three resonance structures of the -COOH group joined by double-headed arrows: the first with C=O and C-O-H (with curved arrows showing movement of electrons), the second with C⁺, O^- and :O-H, and the third with C-O^- and a positively charged O⁺-H]

Preparation of aldehydes and Ketones:

1. By oxidation of alcohols:
RCH₂OH → (Oxidation) R-CHO (Aldehyde) → R-COOH (Carboxylic acid)
R-CH(OH)-R' (Sec-alcohol) → (CrO₃) R-CO-R' (Ketone)

2. By dehydrogenation of Alcohols:
RCH₂OH → (Cu, 573 K) RCHO
R-CH(OH)-R' → (Cu, 573 K) R-CO-R'
CH₃-C(CH₃)(OH)-CH₃ → (Cu, 573 K) CH₃-C(CH₃)=CH₂

3. By ozonolysis of Alkenes:
>C=C< + O₃ / Zn + H₂O → >C=O + O=C<

4. By hydration of alkynes:

Ethyne + H₂O → (Hg²⁺, H₂SO₄) CH₃-CHO (Ethanal)
Propyne + H₂O → (Hg²⁺, H₂SO₄) CH₃-CO-CH₃ (Propanone)
(Ethyne gives an aldehyde; other alkynes give ketones.)

Preparation of Ketones:

1. From acyl chlorides:
2 R-Mg-X + CdCl₂ → R₂Cd + 2Mg(X)Cl
2 R'-CO-Cl + R₂Cd → 2 R'-CO-R + CdCl₂

2. From nitriles:
CH₃-CH₂-C≡N + C₆H₅MgBr → (ether) CH₃CH₂-C(=NMgBr)-C₆H₅ → (H₃O⁺) C₆H₅-CO-C₂H₅ (Propiophenone, 1-Phenylpropanone)

3. Friedel-Crafts acylation reaction:
Benzene + Ar/R-CO-Cl → (Anhyd. AlCl₃) Ar/R-CO-C₆H₅ (a ketone in which the benzene ring is joined to the C=O)

Preparation of Aldehydes

1. From acyl chloride - Rosenmund reduction:
Benzoyl chloride (C₆H₅COCl) → (H₂, Pd-BaSO₄) Benzaldehyde (C₆H₅CHO)

2. From nitriles (Stephen Reaction):
RCN + SnCl₂ + HCl → RCH=NH → (H₃O⁺) RCHO
This reaction is called Stephen reaction.

3. From hydrocarbons:
i. By oxidation of methylbenzene -
a. use of chromyl chloride (CrO₂Cl₂):
Toluene + CrO₂Cl₂ → (CS₂) Chromium complex, C₆H₅CH(OCrOHCl₂)₂ → (H₃O⁺) Benzaldehyde
This reaction is called Etard reaction.
b. use of chromic oxide (CrO₃):
Toluene + CrO₃ + (CH₃CO)₂O → (273-283 K) C₆H₅CH(OCOCH₃)₂ → (H₃O⁺, Δ) Benzaldehyde
ii. By side chain chlorination followed by hydrolysis:
Toluene → (Cl₂/hν) Benzal chloride (C₆H₅CHCl₂) → (H₂O, 373 K) Benzaldehyde
iii. By Gatterman-Koch reaction:
Benzene → (CO, HCl; Anhyd. AlCl₃/CuCl) Benzaldehyde

Preparation of Carboxylic Acids:

1. From primary alcohols and aldehydes -
RCH₂OH → (Oxidation) R-CHO (Aldehyde) → R-COOH (Carboxylic acid)

2. From alkylbenzenes by oxidation:
C₆H₅-CH₂CH₂CH₃ → (KMnO₄-KOH, Δ) C₆H₅-COOK → (H₃O⁺) Benzoic acid (C₆H₅COOH)

3. From nitriles and amides:
R-CN → (H⁺ or OH^-, H₂O) R-CONH₂ → (H⁺ or OH^-, Δ) RCOOH

4. From Grignard reagents:
R-Mg-X + O=C=O → (Dry ether) R-C(=O)-OMgX → (H₃O⁺) RCOOH

5. From acyl halides and anhydrides:
RCOCl → (H₂O) RCOOH + Cl^-
RCOCl → (OH^-/H₂O) RCOO^- + Cl^- → (H₃O⁺) RCOOH

6. From esters:
Ethyl benzoate (C₆H₅COOC₂H₅) + H₂O ⇌ (H₃O⁺) Benzoic acid (C₆H₅COOH) + C₂H₅OH

SUMMARY OF Unit 12: Aldehyde, Ketones and Carboxylic - PART 2

Chemical Reactions of aldehydes and ketones

1. Nucleophilic addition reactions:

[Figure: Mechanism of nucleophilic addition. A planar carbonyl carbon with substituents a and b, C=O with partial charges δ+ and δ-, is attacked by Nu^- to give a Tetrahedral intermediate with O^-; with H⁺ (fast, step 2) this gives the Addition product containing C(Nu)(OH)]

(a) Addition of hydrogen cyanide (HCN): Formation of Cyanohydrin
HCN + OH^- ⇌ :CN^- + H₂O
>C=O + :CN^- ⇌ Tetrahedral intermediate (>C(O^-)(CN)) → (H⁺) Cyanohydrin (>C(OH)(CN))

(b) Sodium hydrogensulphite adds to aldehydes and ketones to form the addition products
>C=O + NaHSO₃ ⇌ >C(OH)(SO₃Na) (addition product, formed through proton transfer)

(c) Addition of alcohols:
R-CHO + R'OH (HCl gas) ⇌ Hemiacetal, R-CH(OH)(OR') → (R'OH, H⁺) Acetal, R-CH(OR')₂ + H₂O
Ketones react with ethylene glycol to form cyclic products known as ethylene glycol ketals.
>C=O + HOCH₂CH₂OH ⇌ (HCl gas, dil. HCl) Ethylene glycol ketal (a five-membered ring with two O atoms joined by CH₂-CH₂) + H₂O

(d) Addition of Grignard reagents:
HCHO + RMgX → RCH₂OMgX → (H₂O) RCH₂OH + Mg(OH)X
RCHO + R'MgX → R-CH(R')-OMgX → (H₂O) R-CH(R')-OH + Mg(OH)X
RCOR + R'MgX → R-C(R)(R')-OMgX → (H₂O) R-C(R)(R')-OH + Mg(OH)X

(d) Addition of ammonia and its derivatives:
>C=O + H₂N-Z ⇌ [>C(OH)(NHZ)] → >C=N-Z + H₂O
Z = Alkyl, aryl, OH, NH₂, C₆H₅NH, NHCONH₂

2,4-DNP derivatives are yellow, orange or red solids, useful for characterisation of aldehydes and ketones.

2. Reduction:
(i) Reduction to alcohols:
RCHO + H₂ → (Pd) RCH₂OH
RCOR' → (NaBH₄) R-CH(OH)-R'
(ii) Reduction to hydrocarbons:
a. Clemmensen reduction: >C=O → (Zn-Hg, HCl) >CH₂ + H₂O
b. Wolff-Kishner reduction: >C=O → (NH₂NH₂, -H₂O) >C=NNH₂ → (KOH/ethylene glycol, heat) >CH₂ + N₂

3. Oxidation: by (i) KMnO₄/Acidic (ii) K₂Cr₂O₇/Acidic
R-CHO → ([O]) R-COOH
R-CH₂-CO-CH₂-R' → ([O]) R-COOH + R'-CH₂COOH (By cleavage of C₁-C₂ bond) and R-CH₂COOH + R'-COOH (By cleavage of C₂-C₃ bond)
Oxidation by mild oxidizing agent - Tollens' test:
RCHO + 2[Ag(NH₃)₂]⁺ + 3 OH^- → RCOO^- + 2Ag + 2H₂O + 4NH₃
Oxidation by mild oxidizing agent - Fehling's test:
R-CHO + 2Cu²⁺ + 5OH^- → RCOO^- + Cu₂O + 3H₂O (Red-brown ppt)
(iii) Oxidation of methyl ketone by haloform reaction:
R-CO-CH₃ → (NaOX) R-COONa + CHX₃ (X = Cl, Br, I)
(CH₃)₂C=CH-CO-CH₃ → (NaOCl) (CH₃)₂C=CH-COONa + CHCl₃

4. Reactions due to α-hydrogen:
(i) Aldol Condensation:
2 CH₃-CHO (Ethanal) ⇌ (dil. NaOH) CH₃-CH(OH)-CH₂-CHO (3-Hydroxybutanal, Aldol) → (Δ, -H₂O) CH₃-CH=CH-CHO (But-2-enal, aldol condensation product)
2 CH₃-CO-CH₃ (Propanone) ⇌ (Ba(OH)₂) CH₃-C(OH)(CH₃)-CH₂-CO-CH₃ (4-Hydroxy-4-methylpentan-2-one, Ketol) → (Δ, -H₂O) (CH₃)₂C=CH-CO-CH₃ (4-Methylpent-3-en-2-one, aldol condensation product)
(ii) Cross aldol Condensation:
CH₃CHO + CH₃CH₂CHO → (1. NaOH, 2. Δ) CH₃-CH=CH-CHO (But-2-enal) + CH₃CH₂-CH=C(CH₃)-CHO (2-Methylpent-2-enal) - from two molecules of ethanal and from two molecules of propanal: simple or self aldol products
+ CH₃-CH=C(CH₃)-CHO (2-Methylbut-2-enal) + CH₃CH₂-CH=CH-CHO (Pent-2-enal) - from one molecule of ethanal and one molecule of propanal: cross aldol products

5. Other reactions:
(i) Cannizzaro reaction:
HCHO + HCHO + Conc. KOH → (Δ) CH₃OH (Methanol) + HCOOK (Potassium formate)
(ii) Electrophilic substitution reaction:
Benzaldehyde → (HNO₃/H₂SO₄, 273-283 K) m-Nitrobenzaldehyde

Chemical Reactions of carboxylic acid

1. Reaction involving cleavage of O-H bond, Acidity of Carboxylic acid:
Carboxylic acids dissociate to give resonance stabilised carboxylate anions and hydronium ion -
R-COOH + H₂O ⇌ H₃O⁺ + [R-COO^- ↔ R-COO^- (resonance forms)] = R-C(O^-)=O
Electron withdrawing group (EWG) stabilises the carboxylate anion and strengthens the acid. Electron donating group (EDG) destabilises the carboxylate anion and weakens the acid.
The effect of the following groups in increasing acidity order is: Ph < I < Br < Cl < F < CN < NO₂ < CF₃
Thus, the following acids are arranged in order of increasing acidity (based on pK_a values):
CF₃COOH > CCl₃COOH > CHCl₂COOH > NO₂CH₂COOH > NC-CH₂COOH > FCH₂COOH > ClCH₂COOH > BrCH₂COOH > HCOOH > ClCH₂CH₂COOH > C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH
The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.
4-Methoxybenzoic acid (pK_a = 4.46); Benzoic acid (pK_a = 4.19); 4-Nitrobenzoic acid (pK_a = 3.41)

2. Reaction involving cleavage of O-H bond:
i. Formation of anhydride:
Ethanoic acid + Ethanoic acid → (H⁺, Δ or P₂O₅, Δ) Ethanoic anhydride, CH₃-CO-O-CO-CH₃
ii. Esterification:
RCOOH + R'OH ⇌ (H⁺) RCOOR' + H₂O
iii. Reaction with PCl₃, PCl₅, SOCl₂:
RCOOH + PCl₅ → RCOCl + POCl₃ + HCl
3RCOOH + PCl₃ → 3RCOCl + H₃PO₃
RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
iv. Reaction with ammonia:
CH₃COOH + NH₃ ⇌ CH₃COO^-NH₄⁺ (Ammonium acetate) → (Δ, -H₂O) CH₃CONH₂ (Acetamide)

3. Reactions Involving -COOH Group:
i. Reduction:
R-COOH → (i) LiAlH₄/ether or B₂H₆, (ii) H₃O⁺) R-CH₂OH
ii. Decarboxylation:
R-COONa → (NaOH and CaO, Heat) R-H + Na₂CO₃

4. Substitution Reaction in the Hydrocarbon Part:
i. Halogenation, Hell-Volhard-Zelensky reaction:
R-CH₂-COOH → ((i) X₂/Red phosphorus, (ii) H₂O) R-CH(X)-COOH, X = Cl, Br (α-Halocarboxylic acid)
ii. Ring substitution:
Benzoic acid → (Br₂/FeBr₃) m-Bromobenzoic acid
Benzoic acid → (Conc. HNO₃ + Conc. H₂SO₄) m-Nitrobenzoic acid