1. AROMATIC RINGS

2. BENZENE RING HYDROGENS
Ring current causes protons attached to the
ring to appear in the range of 7 to 8 ppm.
An anisotropy diagram (next slide) shows
the origin of the effect.
Protons in a methyl or methylene group
attached to the ring appear in the range
of 2 to 2.5 ppm.

4. MONOSUBSTITUTED RINGS

5. ALKYL-SUBSTITUTED RINGS
In monosubstituted rings with an alkyl substitutent
all ring hydrogens come at the same place in the NMR
spectrum.
R = alkyl (only)
Apparently the ring current
equalizes the electron density
at all the carbons of the ring
and, therefore, at all of the
hydrogen atoms.

6. NMR Spectrum of Toluene5 3

7. SUBSTITUENTS WITH UNSHARED PAIRS
Electronegative elements with unshared pairs shield
the o- and p- ring positions, separating the hydrogens
into two groups. ..
unshared pair
Electron-donating groups
shield the o-, p- positions due to
resonance (see below). .. .. ..
X = OH, OR, + + + .. : : : : .. .. - -
NH2, NR2, : : ..
-O(CO)CH3 .. .. -
ester

8. Anisole (400 MHz) 2 3 shielded Compare: The ring protons in
toluene come at
about 7.2 ppm at
the red line.
Compare: 2 3
shielded

9. THE EFFECT OF CARBONYL SUBSTITUENTS
When a carbonyl group is attached to the ring the
o- protons are deshielded by the anisotropic
field of C=O
Only the o- protons are in range for this effect.
The same effect is sometimes seen with C=C bonds.

10. Acetophenone (90 MHz) 3 2 3 deshielded Compare: The ring protons in
toluene come at
about 7.2 ppm at
the red line.
Compare: 2 3
deshielded

11. para -DISUBSTITUTED RINGS

12. para-Disubstitution 1,4-Disubstituted benzene rings will show
a pair of doublets, when the two groups
on the ring are very different
an example:
1-iodo-4-methoxybenzene

13. NMR Spectrum of 1-iodo-4-methoxybenzene3
CHCl3 impurity 2 2

14. NMR Spectrum of 1-bromo-4-ethoxybenzene3 4 2

15. THE p-DISUBSTITUTED PATTERN CHANGES AS THE
TWO GROUPS BECOME MORE AND MORE SIMILAR
All peaks move closer.
Outer peaks get smaller …………………..… and finally disappear.
Inner peaks get taller…………………………. and finally merge.
all H
equivalent
X = X
X = Y
X ~ X’
same groups

16. NMR Spectrum of 1-amino-4-ethoxybenzene3 4 2 2

17. NMR Spectrum of p-Xylene (1,4-dimethylbenzene)6 4

21. HYDROXYL AND AMINO
PROTONS

22. Hydroxyl and Amino Protons
Hydroxyl and amino protons can appear
almost anywhere in the spectrum (H-bonding).
These absorptions are usually broader than
other proton peaks and can often be identified
because of this fact.
Carboxylic acid protons generally appear far
downfield near 11 to 12 ppm.

23. NMR Spectrum of Ethanol3 2 1

24. C O H H SPIN-SPIN DECOUPLING BY EXCHANGE
In alcohols coupling between the O-H hydrogen and
those on adjacent carbon atoms is usually not seen.
This is due to rapid exchange of
OH hydrogens between the various
alcohol molecules in the solution. C O H H
In ultrapure alcohols, however,
coupling will sometimes be seen.
R-O-Ha + R’-O-Hb R-O-Hb R’-O-Ha
The exchange happens so quickly that the C-H group
sees many different hydrogens on the O-H during the
time the spectrum is being determined (average spin = 0)