We can get an idea of the effect of electron distribution by looking at a series of benzene rings with the same substituent in the 1 and 4 positions. This pattern makes all four hydrogens on the ring identical. Here are a few compounds listed in order of chemical shift: largest shift (lowest field; most deshielded) first. Conjugation is shown by the usual curly arrows, and inductive effects by a straight arrow by the side of the group. Only one hydrogen atom and one set of arrows are shown.

The largest shifts come from groups that withdraw electrons by conjugation. Nitro is the most powerful—this should not surprise you as we saw the same in non-aromatic compounds in both ¹³C and ¹H NMR spectra. Then come the carbonyl and nitrile group followed by groups showing simple inductive withdrawal. CF₃ is an important example of this kind of group— three fluorine atoms combine to exert a powerful effect.

In the middle of our sequence, around the position of benzene itself at 7.27 ppm, come the halogens, whose inductive electron withdrawal and lone pair donation are nearly balanced.

Alkyl groups are weak inductive donators, but the groups which give the most shielding— perhaps surprisingly—are those containing the electronegative atoms O and N. Despite being inductively electron withdrawing (the C–O and C–N σ bonds are polarized with δ + C), on balance conjugation of their lone pairs with the ring (as you saw on p. 278) makes them net electron donors. They increase the shielding at the ring hydrogens. Amino groups are the best. Note that one nitrogen-based functional group (NO₂) is the best electron withdrawer while another (NH₂) is the best electron donor.

As far as the donors with lone pairs are concerned (the halogens plus O and N), two factors are important—the size of the lone pairs and the electronegativity of the element. If we look at the four halides at the top of this page the lone pairs are in 2p (F), 3p (Cl), 4p (Br), and 5p (I) orbitals. In all cases the orbitals on the benzene ring are 2p so the fluorine orbital is of the right size to interact well and the others too large. Even though fluorine is the most electro negative, it is still the best donor. The others don’t pull so much electron density away, but they can’t give so much back either. If we compare the first row of the p block elements—F, OH, and NH₂—all have lone pairs in 2p orbitals so now electronegativity is the only variable. As you would expect, the most electronegative element, F, is now the weakest donor.

مواضيع ذات صلة

Uneven electron distribution in aromatic rings

The benzene ring current causes large shifts for aromatic protons

Summary chart of proton NMR shifts

Chemical shifts of CH groups

CH and CH2 groups have higher chemical shift than CH3 groups

Methyl groups give us information about the structure of molecules

Proton chemical shifts tell us about chemistry

Protons on saturated carbon atoms Chemical shifts are related to the electronegativity of substituents

Regions of the proton NMR spectrum

Integration tells us the number of hydrogen atoms in each peak

The differences between carbon and proton NMR

Kinetic versus thermodynamic products