Bond Polarity: The oxygen atom is significantly more electronegative than the carbon atom, creating a permanent dipole where the carbon is electron-deficient () and the oxygen is electron-rich ().
Intermolecular Forces: Carbonyls experience permanent dipole-dipole interactions but cannot form hydrogen bonds with themselves because they lack a hydrogen atom bonded directly to an electronegative atom.
Solubility: Small aldehydes and ketones are water-soluble because the lone pairs on the oxygen atom can form hydrogen bonds with the hydrogen atoms in water molecules. Solubility decreases as the non-polar hydrocarbon chain length increases.
Oxidation: Aldehydes are easily oxidized to carboxylic acids using acidified potassium dichromate(VI) (), changing from orange to green. Ketones resist oxidation under standard conditions because they lack a C-H bond at the carbonyl carbon.
Reduction: Both can be reduced back to alcohols using lithium tetrahydridoaluminate () in dry ether. Aldehydes yield primary alcohols, while ketones yield secondary alcohols.
Nucleophilic Addition: The electron-deficient carbonyl carbon is attacked by nucleophiles like the cyanide ion (). This reaction with (often generated in situ from and ) produces hydroxynitriles and increases the carbon chain length.
| Test Reagent | Aldehyde Observation | Ketone Observation | Purpose |
|---|---|---|---|
| Tollens' Reagent | Silver mirror forms | No change (remains colorless) | Distinguish aldehydes from ketones |
| Fehling's Solution | Blue solution to brick-red ppt | No change (remains blue) | Distinguish aldehydes from ketones |
| 2,4-DNPH | Orange/Yellow precipitate | Orange/Yellow precipitate | Identify presence of group |
| Iodoform (I2/NaOH) | Yellow ppt (only for Ethanal) | Yellow ppt (only for Methyl Ketones) | Identify group |
Mechanism Precision: When drawing nucleophilic addition, always start the curly arrow from the lone pair of the nucleophile to the carbon, and show the -bond electrons moving to the oxygen.
Reagent Conditions: Remember that must be used in dry ether because it reacts violently with water. For oxidation, always specify acidified conditions for dichromate.
Chain Length: Note that the reaction with is one of the few ways to increase the length of a carbon chain in a controlled manner, making it a favorite in synthesis questions.
Melting Point Analysis: To identify a specific carbonyl after a 2,4-DNPH test, the resulting precipitate must be purified by recrystallization and its melting point compared to known data values.
