Para-nitroanilinium ion

Electrostatic potential map for para-nitroanilinium ion shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

The para-nitroanilinium ion is 3.60 p a units more acidic than the anilinium ion (pX a = 0.98 versus 4.58), but para-mtxoh azoic acid is only 0.76 p a unit more acidic than benzoic acid (p fa = 3.44 versus 4.20). Explain why the nitro substituent causes a large change in p a iu one case and a small change in the other. 

When a proton is lost from the para-nitroanilinium ion, the electrons it leaves behind are shared by five atoms. (Draw resonance contributors if you want to see which atoms share the electrons.) In contrast, when a proton is lost from para-nitrobenzoic acid, the electrons it leaves behind are shared by two atoms. In other words, loss of a proton leads to greater electron delocalization in one base than in the other. Electron delocalization stabilizes a compound, so the difference in electron delocalization explains why a nitro substituent has a greater effect on the acidity of an anilinium ion than on the acidity of benzoic acid. 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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