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Structure of Carbonyl Group

Both aldehydes and ketones have carbonyl group as the functional group. The carbonyl carbon is sp2 hybridized and, thus, it has three sp2 hybrid orbitals and one unhybridized p-orbital. It uses sp2 hybrid orbitals to form three sigma bonds, one with oxygen atom and remaining two with two other atoms or groups (R or H). All these three sigma bonds lie in same plane at the angle of 120°.

The unhybridized p-orbital of carbonyl carbon forms 1t bond with oxygen atom by sidewise overlapping with half filled p-orbital of oxygen atom as shown in Fig. 45.1.

Since carbon and oxygen have different values of celectronegativity, the bond between carbon and oxygen is polar. In fact, electron density around the oxygen atom is increased which causes the development of partial positive charge (o+) on carbon and a partial negative charge co-) on oxygen atom. It has been shown in Fig. 45.2. Thus, the carbonyl carbon is an electrophillic and carbonyl oxygen is nucleophillic centre.

The polar nature of carbonyl group is confirmed by the fact that dipole moments of aldehydes and ketones (2.3 to 3.02 D) are quite high. The dipole moment of ethanal, propanone and diethyl ether are 2.72, 2.88 and 1.18 D respectively.