Aldehydes, ketones and carboxylic acids are organic compounds that contain a carbonyl group (carbon and oxygen with a double bond, >C=O).
In aldehydes, the carbonyl group is attached to one H atom, and one C atom of an alkyl or aryl group.
In ketones, the carbonyl group is attached to two C atoms belonging to two different alkyl or aryl groups.
In carboxylic acids, the carbonyl group is attached to one C atom of an alkyl or aryl group, and an O atom of the hydroxyl moiety (-OH). The word 'carboxyl' is a combination of carbonyl (>C=O) and hydroxyl (-OH).
2. Structure and Nature of Carbonyl Group (>C=O)
Let's look at the structural features and natureof the carbonyl group of aldehydes and ketones:
2.1 Structure of Carbonyl Group of Aldehdyes and Ketones
The carbonyl C atom is sp2 hybridised.
It is bonded to 3 other atoms through 3σ bonds.
The 4th valence electron of the carbonyl carbon remains in p-orbital. This unhybridised p-orbital overlaps with a p-orbital of O to form a π bond.
The bond angles of the carbonyl carbon and the 3 other atoms to which it is attached are approximately 120 .
The structure formed is trigonal coplanar structure.
Apart from this, the O atom has 2 non-bonding electron pairs.
The C atom of the carbonyl group, and the three atoms it is attached to lie in one plane, while the π electron cloud exists above and below the plane.
2.2. Nature of Carbonyl Group of Aldehdyes and Ketones
The O atom in >C=O is more electronegative than carbon. It pulls the shared electrons closer, and acquires partial negative charge, while the C atom acquires partial positive charge. Overall, the >C=O bond is polarised. As a result of this, the C atom of >C=O is electrophilic, while the carbonyl oxygen is nucleophilic. Because of the polar nature, the molecules of aldehydes and ketones have high dipole moments and are more polar than alkyl halides and ethers.
3. Structure and Nature of Carboxyl Group (-COOH)
3.1. Structure of Carboxyl Group of Carboxylic Acids
The carboxyl group of carboxylic acids comprises of a carbonyl group (>C=O) and a hydroxyl group (-OH). In the carbonyl group, both C and O are sp2 hybridised.The O atom of hydroxyl group is also sp2 hybridised.The carboxyl group has a trigonal planar structure.One of the lone pair of electrons of oxygen conjugates with the π system of the >C=O group.The bond angles of the carboxyl carbon are approximately 120. 3.2. Nature of Carboxyl Group of Carboxylic Acids
As stated earlier, the carboxyl group of carboxylic acids is made of a carbonyl group (>C=O) and a hydroxyl group (-OH). The carbonyl group of carboxylic acids behaves differently from that of aldehydes and ketones. The carbon in >C=O of carboxylic acids is less electrophilic than that of aldehydes and ketones. This is because of the presence of resonance in the molecule, which makes the C atom of the carbonyl group less positive, and hence, less electrophilic.
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