Physical Properties
Aldehydes and ketones have physical properties that sit between alkanes/ethers and alcohols. They do not have O-H bonds, so they cannot donate hydrogen bonds - but they have strong dipoles and can accept H-bonds from protic solvents. The result is boiling points higher than alkanes and ethers but lower than alcohols of similar molecular weight.
Boiling Points
Compare molecules of similar MW (≈ 72-74):
| Compound | Structure | MW | Boiling point |
|---|---|---|---|
| Pentane | CH₃(CH₂)₃CH₃ | 72 | 36°C |
| Diethyl ether | CH₃CH₂OCH₂CH₃ | 74 | 35°C |
| Butanal | CH₃CH₂CH₂CHO | 72 | 75°C |
| Butan-2-one | CH₃COCH₂CH₃ | 72 | 80°C |
| 1-butanol | CH₃(CH₂)₃OH | 74 | 118°C |
The carbonyl dipole raises the boiling point by about 40°C above the alkane/ether. Alcohols (H-bond donors) add another 40°C on top of that.
Water Solubility
Short-chain aldehydes and ketones are water-soluble because the carbonyl oxygen’s lone pairs can accept H-bonds from water:
- Acetone (propan-2-one): miscible with water in all proportions. Commonly used as a polar aprotic-ish solvent (technically it can accept H-bonds but not donate).
- Formaldehyde (in 37% water = formalin): very soluble.
- Butanone (MEK): fully miscible.
- Larger ketones (e.g., 2-octanone): sparingly soluble - the hydrophobic tail overpowers the polar carbonyl.
The same rule of thumb as alcohols: one polar group can solvate ~3-5 carbons of hydrophobic tail. Beyond that, water solubility drops sharply.
The Carbonyl as a Hydrogen-Bond Acceptor
The carbonyl oxygen has two lone pairs available to accept H-bonds. This lets aldehydes and ketones dissolve in water and other protic solvents, and it makes acetone a versatile co-solvent for many polar reactions.
Note: aldehydes and ketones cannot donate H-bonds (they have no O-H or N-H bond). So in PURE aldehyde or ketone liquid, there are no H-bonds at all - just dipole-dipole interactions, London forces, and the weak C-H…O interaction.
Acidity of Alpha-Hydrogens
A specific physical/chemical property preview: the C-H bonds on carbons directly next to a carbonyl (the alpha-carbons) are much more acidic than regular C-H bonds. Alpha-H pKa is about 17-20 for aldehydes and ketones, versus ~50 for ordinary alkane C-H bonds. This is because the conjugate base (the enolate) is resonance-stabilized onto the carbonyl oxygen.
Alpha-acidity is the gateway to Chapter 7’s enolate chemistry: aldol condensations, Claisen condensations, Michael additions, and alpha-alkylations. Preview for now; deep dive next chapter.