Aldehydes and Ketones: Structure, Properties, and Reactions

Expert reviewed • 23 November 2024 • 5 minute read

The carbonyl group (C=O) is a key feature in organic chemistry, appearing in several important functional groups. This article explores two major carbonyl-containing compounds: aldehydes and ketones.

Structure and Bonding

Aldehydes and ketones both contain a carbonyl group (C=O), where the carbon atom is sp² hybridized. The main difference between these compounds lies in their molecular structure:

  • Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom
  • Ketones: The carbonyl carbon is bonded to two carbon groups

The C=O bond consists of:

  • One σ bond (strong and unreactive)
  • One π bond (weaker and responsible for reactivity)

Nomenclature

Aldehydes

  • Suffix: -al
  • Prefix: formyl-
  • Example: CH₃CH₂CHO (propanal)

Ketones

  • Suffix: -one
  • Prefix: oxo-
  • Example: CH₃COCH₃ (propanone)

When naming compounds with multiple functional groups, priority follows this order:

  • Aldehydes
  • Ketones
  • Alcohols
  • Alkenes
  • Alkynes
  • Alkanes

Physical Properties

Intermolecular Forces

Both aldehydes and ketones are polar molecules due to the electronegative oxygen atom. They can form:

  • Dipole-dipole interactions
  • Van der Waals forces (dispersion forces)
  • Hydrogen bonds with water (but not with themselves)

Boiling and Melting Points

  • Higher than equivalent hydrocarbons due to stronger intermolecular forces
  • Lower than equivalent alcohols (which can form hydrogen bonds)
  • Increase with molecular mass within each homologous series

Water Solubility

  • Small molecules are water-soluble due to hydrogen bonding with water
  • Solubility decreases as carbon chain length increases
  • More polar than hydrocarbons but less polar than alcohols

Oxidation Reactions

Aldehydes

  • Can be oxidized to carboxylic acids
  • Common oxidizing agents include:
    • Acidified potassium permanganate (H⁺/KMnO₄)
    • Tollens' reagent (Ag(NH₃)₂⁺)
    • Jones reagent (CrO₃/H⁺)

Ketones

  • Cannot be oxidized under normal conditions
  • The carbon adjacent to the carbonyl group has no hydrogen atoms

[Insert Diagram 1: Carbonyl group structure] [Insert Table 1: Boiling points comparison]

Laboratory Tests

Aldehydes and ketones can be distinguished using:

  • Tollens' test (silver mirror test) - positive for aldehydes only
  • Fehling's solution - positive for aldehydes only
  • 2,4-dinitrophenylhydrazine test - positive for both

Up Next

Carboxylic Acids: Structure, Properties, and Reactions

Return to Methods of Alcohol Production in Organic Chemistry