Aldehydes and Ketones: Structure, Properties, and Reactions
Expert reviewed • 04 March 2025 • 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