Oxygen bond polarisation

Alkyl halides and magnesium react to form Grignard reagents, which are good nucleophiles. Ethylene oxide Is an electrophile, with the C-0 bond polarised by the electronegative oxygen there Is substantial ring strain present. 

Rate enhancements of 104 - 106 are typically associated with the formation of chelated complexes in which the carbonyl oxygen atom is also co-ordinated to the metal (3.2). This results in a considerably greater polarisation of the C-0 bond. 

Peracids tend to adopt an intramolecularly hydrogen-bonded conformation in solution, and the high degree of polarisation results in an electrophilic oxygen atom that is able to add to alkenes. 

Dipole-dipole interactions can occur between polarised bonds other than N-H or O-H bonds. The most likely functional groups that can interact in this way are those containing a carbonyl group (C = O), the electrons in the carbonyl bond are polarised towards the more electronegative oxygen such that the oxygen acquires a... 

The carbonyl group is polarised in such a way that the oxygen is slightly negative and the carbon is slightly positive. Both the polarity of the carbonyl group and the presence of the weak n bond explain much of the chemistry and the physical properties of aldehydes and ketones. The polarity of the bond also means that the carbonyl group has a resultant dipole moment. 

The polarisation of the carbonyl group is away from carbon towards oxygen in the bonding orbital, and the opposite in the antibonding orbital, as usual. The wire-mesh pictures in Fig. 1.53 show more realistically an outer contour of these two orbitals in formaldehyde itself. Note that in Figs 1.52 and 1.53, it appears from the shape of the outer contour that the electron population in the bonding orbital is very similar on oxygen to that on carbon. 

---