The ¹H NMR spectrum of this simple aromatic amine has three peaks in the ratio 1:2:2, which must correspond to 3H:6H:6H. The 6.38 ppm signal clearly belongs to the protons round the benzene ring, but why are they at 6.38 and not at around 7.2 ppm? We must also distinguish the two methyl groups at 2.28 ppm from those at 2.89 ppm. The chart on p. 276 suggests that these should both be at about 2.4 ppm, close enough to 2.28 ppm but not to 2.89 ppm. The solution to both these puzzles is the distribution of electrons in the aromatic ring. Nitrogen feeds electrons into the π system, making it electron rich: the ring protons are more shielded and the nitrogen atom becomes positively charged and its methyl groups more deshielded. The peak at 2.89 ppm must belong to the NMe₂ group.

Other groups, such as simple alkyl groups, hardly perturb the aromatic system at all and it is quite common for all five protons in an alkyl benzene to appear as one signal instead of the three we might expect. Here is an example with some non-aromatic protons too: there is another on p. 275—the Cbz-protected amino acid.

The five protons on the aromatic ring all have the same chemical shift. Check that you can assign the rest. The OCH₃ group (green) is typical of a methyl ester (the chart on p. 276 suggests 3.9 ppm). One CH₂ group (yellow) is between two carbonyl groups (compare 3.35 ppm for the similar CH2 group on p. 277). The other (red) is next to an ester and a benzene ring: we calculate 1.3 + 1.5 + 3.0 = 5.8 ppm for that—reasonably close to the observed 5.19 ppm. Notice how the Ph and the O together act to shift the Hs attached to this sp³ C down field into what we usually expect to be the alkene region. Don’t interpret the regions on p. 272 too rigidly!

مواضيع ذات صلة

How electron donation and withdrawal change chemical shifts

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