Resonance structures electronegative than other

Finally, the inductive and resonance effects in compounds having the general structure C6Hs-Y=Z (with Z more electronegative than Y) are both electron withdrawing in other words, the two effects reinforce each other. This is true for benzaldehyde (C HsCHO) and all other compounds that contain a carbonyl group directly bonded to the benzene ring. 

C-H bond on the a carbon is more acidic than many other sp hybridized C-H bonds, because the resulting enolate is resonance stabilized. Moreover, one of the resonance structures is especially stable because it places a negative charge on an electronegative oxygen atom. 

For molecules in which atoms A and B are identical, ft c n (see p e I4l for the H, molecule), and the contribution of the ionic structures is small. If B is more electronegative than A. then the energy of the contributing structure A B approaches more nearly that of ihe purely covalent structure A—B and resonance is enhanced. On the other hand, the energy of is so prohibitively high that this... 

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