Aldehydes and Ketones

Overview

• Aldehydes and ketones are organic compounds with the carbonyl group (C=O) as their functional group.
• General molecular formula: CnH2nO.
• Often referred to as carbonyl compounds due to their similar chemical properties.
• Differences lie in the groups attached to the carbonyl carbon.

Structure

Aldehydes

• General formula: R-CHO.
• One valence of the carbonyl carbon is satisfied by a hydrogen atom.
• Common aldehydes:
  • Formaldehyde (R=H)
  • Acetaldehyde (R=Methyl group)
  • Benzaldehyde (R=Phenyl group)

Ketones

• General formula: R-CO-R'.
• Both valences of the carbonyl carbon are satisfied by alkyl or aryl groups.
• Common ketones:
  • Simple ketones: Acetone (R=R'=CH3)
  • Mixed ketones: Acetophenone (R=Methyl group, R'=Phenyl group)

Chemical Properties

• Aldehydes easily oxidize compared to ketones.
• Aldehydes and ketones participate in nucleophilic addition reactions.

Significance

• Fragrance and flavor:
  • Acetaldehyde (apple scent)
  • Vanillin (vanilla), Salicylaldehyde (Meadowsweet), Cinnamaldehyde (cinnamon)
• Biochemical significance:
  • Glucose (polyhydroxy aldehyde) in photosynthesis and respiration
• Industrial significance:
  • Aldehydes and ketones as solvents and intermediates in synthesis (e.g., bakelite, urea-formaldehyde glues).

Nomenclature

Common System

• Aldehydes named after the corresponding carboxylic acids by replacing '-ic acid' with '-aldehyde'.
• Positions indicated by Greek letters (α, β, γ, δ).

IUPAC System

• Aldehydes named by replacing ‘-e’ of the alkane with ‘-al’.
• Example: Methanal (Formaldehyde), Ethanal (Acetaldehyde).

Ketones

• Named by replacing ‘-e’ of the alkane with ‘-one’.
• Example: Propanone (Acetone).
• Substituents indicated by numerals and prefixes.

Methods of Preparation

From Alcohols

• Oxidation of alcohols:
  • Primary alcohols -> Aldehydes (using PCC)
  • Secondary alcohols -> Ketones
• Dehydrogenation of alcohols:
  • Volatile alcohols passed over metal catalysts.

From Alkenes and Alkynes

• Ozonolysis of alkenes -> Aldehydes/Ketones
• Hydration of alkynes -> Aldehydes/Ketones

Reductions

• Acid chlorides reduced to aldehydes (Rosenmund reduction).
• Dialkyl cadmium for ketones.

Other Methods

• Reduction of nitriles and hydrocarbons.
• Various reactions for aromatic aldehydes and ketones.

Physical Properties

• Solubility in water and higher boiling points compared to hydrocarbons due to dipole interactions.
• Physical state: Formaldehyde (gas), Acetaldehyde (liquid).

Reactivity

• Reactivity sequence: Formaldehyde > Other aldehydes > Ketones
• Electronic and steric factors influencing reactivity.
• Nucleophilic addition reactions are typical.

Overall Importance

Aldehydes and ketones play crucial roles in industrial applications, biochemistry, and organic synthesis, demonstrating significant versatility due to their dynamic reactivity and numerous preparation methods. Understanding their properties enhances the ability to manipulate and utilize these compounds effectively.