Question

(8) 4. The diagram below is a composite of seven 'H NMR spectra, showing the aromatic protons for benzene (CH), nitrobenzene (C.H.NO,), and aniline (CHNH). There are three spectra each for nitrobenzene and aniline, showing the protons that are in the ortho-, meta-, and para-positions with respect to the NO, and NH, functional groups of nitrobenzene and aniline, respectively. Briefly explain why the protons have different chemical shifts, and why do the protons in each of these positions appear to be in the opposite order for nitrobenzene and aniline. benzene nitrobenzene pm | m 2 0 aniline 0 8.5 8.0 6.5 7.5 7.0 8('H)/ppm NO, NO

Answer 1

Let us consider nitrobenzene first. It can be seen from the resonance structures that the nitro group generates the positive charge on ortho and para positions. The negative resonance and negative inductive effect appear strongest at ortho position. This results in decrease in the electron density at ortho and para position and therefore the protons at ortho and para position appears deshielded.  

In case of anilne , the positive resonance and inductive effect creates negative charge on the ortho and para position, which increases the electron density at these two position and thus the protons appear shield, exactly opposite the nitrobenzene chemical shift.