Electronegativity hybridization effect

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded Normally the carbon of C=0 is the least shielded one m... 

The introduction of heteroatoms into cyclic systems produces significant variations in the molecular geometry that reflect the changes in covalent radii, relative electronegativity and effective hybridization. Consequently, there are changes in the bonding and the physico-chemical characteristics of these heterocyclic systems—particularly in small ring systems. 

Figure 3.20 clearly shows that there are some types of protons whose chemical shifts are not easily explained by simple considerations of the electronegativity of the attached groups. For instance, consider the protons of benzene and other aromatic systems. Aryl protons generally have a chemical shift as large as that of the proton of chloroform Alkenes, alkynes, and aldehydes also have protons whose resonance values are not in line with the expected magnitudes of any electron-withdrawing or hybridization effects. In each of these cases, the anomalous shift is due to the presence of an unsaturated system (one with r electrons) in the vicinity of the proton in question. 

It is also interesting to note that the hybrid lone pair at N is a weaker donor than the 100% p-lone pair at the carbonyl oxygen (9.2 vs. 18.7kcal) towards the same o-acceptor (the C1-C4 bond). This difference illustrates how hybridization effects can override differences in electronegativity (Figure 4.4). ... 

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