Aldehydes and Ketones - Complete Summary

Introduction

Carbonyl compounds are of two types, aldehydes and ketones. Both have a carbon-oxygen double bond often called as carbonyl group.

General Formula: C_nH_2nO

Structure of Carbonyl Group

Carbonyl carbon atom is joined to three atoms by sigma bonds. Since these bonds utilise sp²-orbitals, they lie in the same plane and are 120° apart. The carbon-oxygen double bond is different than carbon-carbon double bond. Since, oxygen is more electronegative, the electrons of the bond are attracted towards oxygen. Consequently, oxygen attains a partial negative charge and carbon a partial positive charge making the bond polar.

Dipole moment = 2.3 – 2.8 D

O

||

C

↔

O⁻

||

⁺C

120° — σ-bond — 120°

C connected to 3 atoms with π-bond to O

Preparation of Carbonyl Compounds

(1) From Alcohols

By Oxidation

R–CH₂–OH →[Mild oxidising agents] R–CHO

R–CH(OH)–R' →[Mild oxidising agents] R–CO–R'

Mild oxidising agents: X₂, Fenton reagent (FeSO₄ + H₂O₂), K₂Cr₂O₇/H⁺, Jones reagent, Sarret reagent, MnO₂, Aluminium tertiary butoxide

Oppenauer oxidation: Secondary alcohols → ketones using aluminium tert-butoxide

PCC (Pyridinium chlorochromate): Converts allylic alcohols to aldehydes without affecting double bond

Dehydrogenation

R–CH₂–OH →[Cu/300°C] R–CHO + H₂

R–CH(OH)–R' →[Cu/300°C] R–CO–R' + H₂

(2) From Carboxylic Acids

Distillation of Ca/Ba/Sr/Th salts

(RCOO)₂Ca →[Δ] R–CO–R' + CaCO₃

Cyclic ketones from dibasic acids (1,4 and higher)

Decarboxylation by MnO/300°C

Case I: Both HCOOH → HCHO

Case II: One HCOOH + RCOOH → RCHO

Case III: Two RCOOH → R–CO–R'

(3) From Gem Dihalides

R–CHX₂ →[H₂O/HO⁻] R–CHO

R–CX₂–R' →[H₂O/HO⁻] R–CO–R'

(4) From Alkenes

Ozonolysis

RCH=CHR →[O₃ → Zn/H₂O] RCHO + RCHO

R₂C=CR₂ →[O₃ → Zn/H₂O] R₂CO + R₂CO

Oxo Process

R–CH=CH₂ + CO + H₂ →[Co, 150°C, 300 atm] R–CH₂–CH₂–CHO

Wacker Process

CH₂=CH₂ →[PdCl₂/HOH, CuCl₂/air] CH₃CHO

(5) From Alkynes

R–C≡CH →[H₂O/HgSO₄/H₂SO₄] R–CO–CH₃

(6) From Grignard Reagents

With HCOOC₂H₅

R–MgX + HCOOC₂H₅ → R–CHO

With R'COOC₂H₅

R–MgX + R'COOC₂H₅ → R–CO–R'

(7) From Acid Chlorides

With Dialkyl Cadmium

R'–COCl + R₂Cd → R'–CO–R

Rosenmund Reduction

R–COCl →[H₂, Pd/BaSO₄, Xylene] R–CHO

(8) From Cyanides

R–C≡N →[SnCl₂/HCl → H₂O] R–CHO (Stephen Synthesis)

(9) From Vic Diols

R–CH(OH)–CH(OH)–R →[HIO₄] R–CHO + R–CHO

(10) From Alkyl/Benzyl Halides

R–CH₂Cl →[DMSO] R–CHO

(11) From Nitro Alkanes (Nef Synthesis)

R–CH₂–NO₂ →[NaOH → 70% H₂SO₄] R–CHO

(12) From Excess Alkyl Lithium

R–COOH + 2R'Li → R–CO–R' (after hydrolysis)

Preparation of Aromatic Carbonyl Compounds

Etard Reaction

Ar–CH₃ →[CrO₂Cl₂ → HOH] Ar–CHO

Gattermann-Koch

ArH →[CO/HCl, AlCl₃/Cu₂Cl₂] Ar–CHO

Gattermann

ArH →[HCN/HCl, AlCl₃ → H₂O] Ar–CHO

Reimer-Tiemann

Phenol →[CHCl₃/Alc.KOH → H⁺] Salicylaldehyde (major)

Physical Properties

Physical State

Methanal: gas; Ethanal: volatile liquid (b.p. 294 K); Higher members: liquids/solids

Smell

Lower aldehydes: unpleasant; Higher: fragrant (used in perfumes)

Solubility

Upto C₄: miscible with water (H-bonding); Higher: insoluble in water, soluble in organic solvents

Boiling Points

Higher than hydrocarbons, lower than alcohols; Ketones > Aldehydes (isomeric)

Acetaldehyde: b.p. = 294 K, μ = 2.52 D
Acetone: b.p. = 329 K, μ = 2.88 D

Chemical Properties

(1) Nucleophilic Addition Reactions

Aldehydes > Ketones (electronic + steric reasons)

Addition of HCN

R–CHO + HCN → R–CH(OH)CN

Optically active (except HCHO); Used for α-hydroxy acids

NaHSO₃

R–CHO + NaHSO₃ → Crystalline adduct

Only aliphatic methyl ketones react

Alcohols

Base catalyzed → Hemiacetal/Hemiketal (reversible)

Reactivity Order

HCHO > RCHO > RCOR > ArCHO > ArCOR > (Ph)₂CO

JEE Main Weightage

Typically 2-3 questions from Carbonyl Compounds. Focus on preparation methods, nucleophilic addition, and named reactions.

Weightage High (2-3 Qs)