Views: 7
ALKANALS AND ALKANONES
Aldehydes and ketones incorporate a carbonyl functional group
-C=O
These are organic compounds with structures -CHO and RC(=O)R’, where R and R’ represent carbon-containing substituents respectively.
In aldehydes, the carbonyl group has one hydrogen atom attached to it together with either a 2nd hydrogen atom or a hydrogen group which may be an alkyl group or one containing a benzene ring.
Example:
One can notice that all these have the exact same end to the molecule. The only difference is the complexity of the other attached group.
In ketones, the carbonyl group has 2 hydrocarbon groups attached to it. These can be either the ones containing benzene rings or alkyl groups. Ketone does not have a hydrogen atom attached to the carbonyl group.
Example:
Propane is generally written as CH3COCH3. In pentanone, the carbonyl group could be in the middle of the chain or next to the end – giving either pentan-3-one or pentan-2-one.
Properties of Aldehydes
- The structure of aldehydes shows a sp2 hybridized central carbon which is doubly connected to oxygen and has a single bond with hydrogen.
- Small aldehydes are quite soluble in water.
- Formaldehyde and acetaldehyde are great examples of this. These two aldehydes are quite important industrially.
- Aldehydes generally exhibit a tendency to undergo oligomerization or polymerization.
- The carbonyl centre of the aldehyde has an electron-withdrawing nature. Therefore, the aldehyde group can be considered somewhat polar.
Nomenclature of Aldehydes
- Acyclic and Aliphatic Aldehydes are named after their longest carbon chain along with the “-al” suffix. For example, CH3CH3CH2CHO is called butanal since it has four carbons in the chain.
- When the aldehyde functional group is added to a ring, the “-carbaldehyde” suffix must be used. For example, C6H11CHO is called cyclohexanecarbaldehyde.
- In the case of natural compounds or carboxylic acids, “-oxo” is used as a prefix to highlight the carbon which is a part of the aldehyde functional group. For example, (CHO)-CH2COOH is called 3-oxopropanoic acid.
Some common and IUPAC names for some aldehydes are tabulated below.
Formula |
Common name |
IUPAC name |
HCHO |
Formaldehyde |
Methanal |
CH3CHO |
Acetaldehyde |
Ethanal |
CH3-CH(CH3)-CHO |
Isobutyraldehyde |
2-Methylpropanal |
CH3-CH=CH-CHO |
Crotonaldehyde |
2-Butenal |
Properties of Ketones
- Ketones are polar in nature due to the presence of a polar carbonyl group. Therefore they have higher boiling points than non-polar compounds.
- It cannot form any intermolecular hydrogen bond-like alcohols because there is no hydrogen attached to an oxygen atom.
- Ketones have large dipole moments compared to alcohols or ethers due to the shifting of pi electrons.
- Ketones react with hydrogen cyanide to form cyanohydrins. The reaction is normally carried out in the presence of a base, which acts as a catalyst in the absence of a base the reaction proceeds slowly.
- Most of the ketones form bisulphite addition products when it is added to sodium bisulphite.
Nomenclature of Ketones
- Ketones are named after their parent alkanes with the suffix “-anone”. The carbonyl group’s position in the ketone is denoted by a number while naming the ketone. For example, CH3(CO)CH3 is called 2 propanone. However, this compound is generally referred to as acetone.
- Commonly, ketones are named by writing the name of each individual alkyl group attached to the carbonyl carbon and then “ketone”as the third word of the name. For example, butanone can be written as methyl ethyl ketone.
Formula |
Common name |
IUPAC name |
CH3-C(O)-CH3 |
Acetone |
Propanone |
CH3-C(O)-CH2-CH3 |
Ethylmethylketone |
Butanone |
CH3-CH2-C(O)-CH2-CH3 |
Diethyl ketone |
Pentan-3-one |
CH3-CH(CH3)-C(O)-CH3 |
Isopropyl methyl ketone |
3-Methylbutan-2-one |