Exhaustive Guide: Aldehydes & Ketones - Prep | Class 12 Chemistry | ChemCA

Aldehydes & Ketones: Prep & Basics

Module 1 | CBSE Class 12 Chemistry | Organic Chemistry

1. Introduction to Carbonyl Compounds

Organic compounds containing the carbon-oxygen double bond (>C=O) are called carbonyl compounds. They form the structural backbone of numerous vital organic and biological molecules.

Aldehydes: The carbonyl group is bonded to at least one hydrogen atom (R-CHO).
Ketones: The carbonyl group is bonded to two carbon atoms (R-CO-R').

Real-world Significance: Aldehydes and ketones are widespread in nature. They add fragrance and flavor to nature. Examples include Vanillin (from vanilla beans), Salicylaldehyde (from meadowsweet), and Cinnamaldehyde (from cinnamon).

2. Nomenclature (Common & IUPAC)

Aldehydes: IUPAC names replace the 'e' of the alkane with 'al' (e.g., Ethanal). Numbering starts with the carbonyl carbon as C-1. If the -CHO group is attached to a ring, the suffix carbaldehyde is used (e.g., Cyclohexanecarbaldehyde).

Ketones: IUPAC names replace the 'e' of the alkane with 'one' (e.g., Propanone). Numbering begins from the end nearer to the carbonyl group.

Structure	Common Name	IUPAC Name
HCHO	Formaldehyde	Methanal
CH₃CHO	Acetaldehyde	Ethanal
CH₃COCH₃	Acetone	Propanone
C₆H₅COCH₃	Acetophenone	1-Phenylethan-1-one

3. Structure of the Carbonyl Group

The carbonyl carbon atom is sp² hybridized and forms three sigma (σ) bonds. The fourth valence electron forms a pi (π) bond with oxygen by overlap with the p-orbital of oxygen.

Geometry: The carbon and the three atoms attached to it lie in the same plane, resulting in a trigonal planar geometry. The bond angles are approximately 120°.
Polarity: The carbon-oxygen double bond is highly polarized due to the higher electronegativity of oxygen relative to carbon. Hence, the carbonyl carbon behaves as an electrophile (Lewis acid), and carbonyl oxygen acts as a nucleophile (Lewis base).

Dipole Moment: Carbonyl compounds have substantial dipole moments. The high polarity of the carbonyl group is explained by resonance structures, resulting in a partial positive charge on carbon and a partial negative charge on oxygen.

4. General Methods of Preparation (Both Aldehydes & Ketones)

These methods yield both aldehydes and ketones depending upon the starting material used.

A. By Oxidation of Alcohols

Primary (1°) alcohols are oxidized to aldehydes using mild oxidizing agents like PCC (Pyridinium chlorochromate) to prevent further oxidation to carboxylic acids.
Secondary (2°) alcohols are oxidized to ketones using CrO₃ or KMnO₄/K₂Cr₂O₇.

B. By Dehydrogenation of Alcohols

Passing vapours of a primary or secondary alcohol over heated copper at 573 K yields an aldehyde or a ketone respectively.

R-CH₂OH →(Cu, 573 K) R-CHO + H₂
R-CH(OH)-R' →(Cu, 573 K) R-CO-R' + H₂

C. From Hydrocarbons (Ozonolysis & Hydration)

Ozonolysis of Alkenes: Reaction of alkenes with ozone followed by cleavage with Zinc dust and water yields aldehydes/ketones.
Hydration of Alkynes: Addition of water to ethyne in the presence of H₂SO₄ and HgSO₄ (Kucherov's reaction) yields acetaldehyde. All other alkynes give ketones.

5. Preparation of Aldehydes ONLY (Crucial Name Reactions)

5.1 From Acyl Chloride (Rosenmund Reduction)

Acyl chloride (acid chloride) is hydrogenated over a catalyst consisting of palladium on barium sulphate.

R-COCl + H₂ →(Pd / BaSO₄) R-CHO + HCl

NCERT Question: Why is BaSO₄ used?
Palladium (Pd) is a strong reducing catalyst. If used alone, it would reduce the acid chloride completely into an alcohol. Barium sulphate (often treated with a poison like quinoline or sulfur) acts as a catalytic poison, decreasing the activity of Pd and arresting the reduction specifically at the aldehyde stage.

5.2 From Nitriles and Esters (Stephen Reaction)

Nitriles are reduced to corresponding imine intermediates with stannous chloride (SnCl₂) in the presence of hydrochloric acid. This is followed by hydrolysis to yield the aldehyde.

R-CN + SnCl₂ + HCl → R-CH=NH (Imine)
R-CH=NH →(H₃O⁺) R-CHO

The Magic of DIBAL-H:
Nitriles and esters can also be selectively reduced to aldehydes using Diisobutylaluminium hydride (DIBAL-H). The reaction is typically carried out at a very low temperature (-78°C) to prevent further reduction to alcohols.
Reaction: R-CN →(1. DIBAL-H, 2. H₂O) R-CHO

5.3 From Hydrocarbons (Etard Reaction)

Strong oxidizing agents oxidize toluene directly to benzoic acid. To stop the oxidation at the aldehyde stage, specific reagents are used to form intermediates.

Etard Reaction: Chromyl chloride (CrO₂Cl₂) oxidizes the methyl group of toluene to a chromium complex. This complex, upon hydrolysis, yields benzaldehyde.

C₆H₅CH₃ (Toluene) + CrO₂Cl₂ (in CS₂) → Chromium Complex
Chromium Complex →(H₃O⁺) C₆H₅CHO (Benzaldehyde)

Alternative Method: Toluene can also be oxidized using Chromium oxide (CrO₃) in acetic anhydride to form a benzylidene diacetate intermediate, which is then hydrolyzed to benzaldehyde.

5.4 Gattermann-Koch Reaction

When benzene or its derivative is treated with carbon monoxide (CO) and hydrogen chloride (HCl) in the presence of anhydrous aluminium chloride (AlCl₃) or cuprous chloride (CuCl), it gives benzaldehyde or substituted benzaldehyde.

Benzene + CO + HCl →(Anhyd. AlCl₃ / CuCl) Benzaldehyde

6. Preparation of Ketones ONLY

A. From Acyl Chlorides

Treatment of acyl chlorides with dialkylcadmium (prepared by the reaction of cadmium chloride with Grignard reagent) yields ketones.

2 R-Mg-X + CdCl₂ → R₂Cd + 2 Mg(X)Cl
2 R'-COCl + R₂Cd → 2 R'-CO-R (Ketone) + CdCl₂

B. From Nitriles

Treating a nitrile with a Grignard reagent followed by hydrolysis yields a ketone.

CH₃-CH₂-C≡N + C₆H₅MgBr →(ether) CH₃CH₂-C(C₆H₅)=NMgBr →(H₃O⁺) Propiophenone (Ketone)

C. From Benzene (Friedel-Crafts Acylation)

When benzene is treated with an acid chloride in the presence of anhydrous aluminium chloride, it forms an aromatic ketone.

C₆H₆ + R-COCl →(Anhyd. AlCl₃) C₆H₅-CO-R + HCl

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

NCERT Example 12.2: Write the structures of products of the following reactions:
(i) Benzene + C₂H₅COCl in presence of Anhyd. AlCl₃
(ii) (C₆H₅CH₂)₂Cd + 2CH₃COCl

Solution:
(i) This is a Friedel-Crafts Acylation reaction. The acyl group (C₂H₅CO-) substitutes a hydrogen atom on the benzene ring.
Product: C₆H₅-CO-C₂H₅ (Propiophenone or 1-Phenylpropan-1-one).

(ii) This reaction uses a dialkylcadmium reagent to synthesize a ketone from an acid chloride.
Product: 2 C₆H₅CH₂-CO-CH₃ (1-Phenylpropan-2-one) + CdCl₂.

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

Part A: Conceptual (1-2 Marks)

[CBSE 2018, 2021]

Q1. Name the reagent used in the Rosenmund reduction. What is the specific role of BaSO₄ in this reaction?

Answer: The reagent is Hydrogen gas (H₂) in the presence of a Palladium catalyst supported on Barium Sulphate (Pd/BaSO₄). The specific role of BaSO₄ is to act as a catalytic poison. It partially deactivates the Palladium catalyst, preventing the newly formed aldehyde from being further reduced into a primary alcohol.

[CBSE 2017, 2020]

Q2. What happens when toluene is treated with Chromyl chloride in CS₂ followed by hydrolysis? Name the reaction.

Answer: Toluene reacts with Chromyl chloride (CrO₂Cl₂) in CS₂ to form a brown chromium complex. Upon subsequent hydrolysis, this complex decomposes to yield Benzaldehyde. This specific conversion is known as the Etard Reaction.

Part B: Assertion-Reason Type (1 Mark)

[CBSE Sample Paper 2024]

Q3. Assertion (A): DIBAL-H is an excellent reagent to convert nitriles into aldehydes.
Reason (R): DIBAL-H reduces nitriles to imines which on hydrolysis yield aldehydes without further reduction to alcohols.

Answer: Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion. Diisobutylaluminium hydride (DIBAL-H), especially when used at low temperatures (-78°C), is highly selective. It stops the reduction at the imine stage, avoiding the formation of primary alcohols.

Part C: Synthesis and Conversions (3 Marks)

[CBSE 2016, 2022]

Q4. How will you bring about the following conversions?
(a) Ethanenitrile to Ethanal
(b) Benzene to Benzaldehyde
(c) Acid chloride to Ketone

Answer:
(a) Stephen Reaction: Treat ethanenitrile (CH₃CN) with SnCl₂ and HCl to form an imine intermediate, then hydrolyze (H₃O⁺) to get ethanal (CH₃CHO).

(b) Gattermann-Koch Reaction: Treat benzene with a mixture of Carbon Monoxide (CO) and Hydrogen Chloride (HCl) gas in the presence of anhydrous AlCl₃ and CuCl to yield benzaldehyde.

(c) Treat the acid chloride with a dialkylcadmium (R₂Cd) reagent. The R group from the cadmium reagent replaces the Cl to form a ketone (R'-CO-R).

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Exhaustive Guide: Aldehydes & Ketones - Prep | Class 12 Chemistry