ALDEHYDES & KETONES

Definition of Aldehydes and Ketones

Compounds containing a carbonyl group (C=O).

Aldehydes: RCHO.

Ketones: R1COR2.

Carbonyl Group Characteristics

Composed of a sigma (σ\sigma) and pi (π\pi) bond. Polar due to oxygen's electronegativity.

Naming Aldehydes

Replace "-e" of parent alkane with "-al" (e.g., methanal).

Naming Ketones

Replace "-e" of parent alkane with "-one" (e.g., propanone).

Physical Properties of Carbonyl Compounds (Aldehyde & Ketone)

Higher boiling points than alkanes, lower than alcohols.

Soluble in water up to 2–3 carbons due to hydrogen bonding.

Preparation of Carbonyl Compounds: Alcohol Oxidation

Primary alcohols → Aldehydes (mild oxidants like PCC).

Secondary alcohols → Ketones.

Preparation of Carbonyl Compounds: Ozonolysis of Alkenes

Cleavage forms aldehydes and ketones depending on substitution.

Preparation of Carbonyl Compounds: Hydration of Alkynes

Terminal alkynes react with water (Hg2+^{2+}, acid catalyst) to form ketones.

Nucleophilic Addition Reactions

Carbonyl carbon (δ+) attracts nucleophiles, breaking the π bond.

Hydration of Aldehydes and Ketones

Forms geminal diols (e.g., chloral hydrate, a hypnotic).

Cyanohydrin Formation

Reaction with HCN produces cyanohydrins, used in synthesizing α-hydroxy acids.

Sodium Hydrogen Sulfite Addition

Forms crystalline addition products for purification of lower aldehydes and ketones.

Reduction of Carbonyls//Addition of hydride

Aldehydes → Primary alcohols, Ketones → Secondary alcohols (reagents: NaBH4,LiAlH4).

Grignard Reagents

React with aldehydes and ketones to produce alcohols.

Polymerization of Formaldehyde

Forms paraformaldehyde (solid) and paraldehyde (liquid hypnotic).

Hemiacetal and Acetal Formation

Aldehydes react with alcohols to form hemiacetals; further reaction produces acetals.

Keto-Enol Tautomerism

Carbonyl compounds exist in equilibrium with their enol form.

Haloform Reaction

Methyl ketones react with halogens (e.g., iodine) to form haloforms (positive iodoform test).

Aldol Condensation

Enolate ions react with aldehydes to form β-hydroxy aldehydes, which dehydrate to α,β-unsaturated compounds.

Aldehyde Oxidation

Oxidized to carboxylic acids by mild oxidants (e.g., KMnO4).

Drug Metabolism of Aldehydes

Catalyzed by enzymes like aldehyde dehydrogenase.

Sugar Cyclization

Intramolecular hemiacetals form stable cyclic structures (e.g., glucose as pyranose).

Mutarotation of Sugars

Interconversion between anomers (α and β) in solution, changing optical rotation.

Addition of Amines

Primary amines react with aldehydes to form imines, intermediates in organic synthesis.

Oxime Formation

Aldehydes react with hydroxylamine to form oximes, useful for characterization.