Spectroscopy
are recorded and analyzed by computer to give an intensity versus frequency plot of the
sample. This information can then be used to determine the structure of the compound.
Aromatics in H-NMR Electron Donating Groups vs. Electron Withdrawing
Groups
On monosubstituted rings, electron donating groups resonate at high chemical shifts. Elec-
tron donating groups increase the electron density by releasing electrons into a reaction
center, thus stabilizing the carbocation. An example of an electron donating group is
methyl (-CH3).
Accordingly, electron withdrawing groups are represented at low chemical shifts. Electron
withdrawinggroupspullelectronsawayfromareactingcenter. Thiscanstabilizeanelectron
rich carbanion. Some examples of electron withdrawing groups are halogens (-Cl, -F) and
carboxylic acid (-COOH).
Looking at the H NMR spectrum of ethyl benzene, we see that the methyl group is the
most electron withdrawing, so it appears at the lowest chemical shift. The aromatic phenyl
group is the most electron donating, so it has the highest chemical shift.
Figure 187