Functional groups inductive electron-withdrawing

First, the carbon atom is much closer to the substituent dian the proton. In the compounds in Table 15.2, the methyl carbon atom is directly bonded to the substituent, while the protons are separated from it by the carbon atom of the methyl group. If the functional group is based on a large electron-withdrawing atom like sulfur, the protons will experience a simple inductive electron withdrawal and have a proportional downfield shift. The carbon atom is close enough to the sulfur atom to be shielded as well by the lone-pair electrons in the large 3sp3 orbitals. The proton shift... 

The carbonyl group whose carbon is most electrophilic, or more aptly electron-deficient, would be the site of the nucleophilic attack.The attack would have to occur at the carbonyl group para to the nitro substituent since only here will the nitro functionality exert its electron-withdrawing resonance effect as well as its inductive effect. This resonance effect can put positive charge on the carbon para to the nitro group. This positively charged carbon will make the adjacent carbonyl carbon more electrophilic. 

FIGURE 3.10 Maps of electrostatic potential at approximately the bond density surface for acetic acid and ethanol. The positive charge at the carbonyl carbon of acetic acid is evident in the blue color of the electrostatic potential map at that position, as compared to the hydroxyl carbon of ethanol. The inductive electron-withdrawing effect of the carbonyl group in carboxylic acids contributes to the acidity of this functional group. 

Alkyl groups are weak inductive donators and at the smallest shift we have the groups that, on balance, donate elections to the ring and increase the shielding at the carbon atoms. Amino is the best of thesje. So a nitrogen-based functional group (NO2) is the best electron withdrawer while another (NH2) is the best electron donor. 

---