Physical properties of aldehydes and ketones
1. Physical State: Formaldehyde is a gas at room temperature and acetaldehyde is a volatile liquid. All
other aldehydes and ketones upto to C11 are colourless liquids while the higher ones are solids. 2. Boiling points
Aldehydes and ketones have relatively high boiling point as compared to hydrocarbons and ethers of comparable molecular mass. It is due to the weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.
C = O C = O C = O δ δ δ δδ δ
These dipole-dipole interactions are weaker than intermolecular H-bonding. The boiling points of aldehydes and ketones are much lower than those of corresponding alcohols and carboxylic acids which possess inter molecular hydrogen bonding.
Compound Molar mass
Boiling point (K)
Compound Molar mass
Boiling point (K)
CH3 CH2( ) 3 CH3
Pentane 72 309
CH3CH2COCH3
butan - 2- one 72 353
CH3 CH2( ) 2 CHO
butanal 72 349
CH3CH2COOH Propanoicacid
74 414
CH3 CH2( ) 3OH
butanol 74 391
XII U12-Carbonyl Compounds.indd 153 2/19/2020 5:14:27 PM
| C ompound | Molarmass | Boiling point (K) |
|---|---|---|
| CH ()CH CHPentan e3 2 3 | 72 | 309 |
| CH ()CH CHbutana l3 2 2 | 72 | 349 |
| CH ()CH OHbutanol3 2 3 | 74 | 391 |
| C ompound | Molarmass | Boiling point (K) |
|---|---|---|
| CH CH COCHbutan - 2- one3 2 3 | 72 | 353 |
| CH CH COOHPropanoicaci d3 2 | 74 | 414 |
154
3. Solubility Lower members of aldehydes and ketones like formaldehyde, acetaldehyde and acetone
are miscible with water in all proportions because they form hydrogen bond with water. Solubility of aldehydes and ketones decreases rapidly on increasing the length of alkyl
chain.
C =
O R
R H
H O
=
C
R
R
O
4. Dipolemoment: The carbonyl group of aldehydes and ketones contains a double bond between carbon and
oxygen. Oxygen is more electronegative than carbon and it attracts the shared pair of electron which makes the carbonyl group as polar and hence aldehydes and ketones have high dipole moments.
C O