Section 14.15 Protons Bonded to Oxygen and Nitrogen 559In a sample of pure alcohol, the rate of proton exchange is very slow. This causes
the spectrum to look identical to one that would be obtained if proton exchange did not
occur. Acids and bases catalyze proton exchange, so if the alcohol is contaminated
with just a trace of acid or base, proton exchange becomes rapid. When proton ex-
change is rapid, the spectrum records only an average of all possible environments.
Therefore a rapidly exchanging proton is recorded as a singlet. The effect of a rapidly
exchanging proton on adjacent protons is also averaged. Thus, not only is its signal not
split by adjacent protons, the rapidly exchanging proton does not cause splitting.
The signal for an OH proton is often easy to spot in an NMR spectrum because it is
frequently somewhat broader than other signals (see the signal at 4.9 in Figure 14.30(b)).
The broadening occurs because the rate of proton exchange is not slow enough to result in
a cleanly split signal, as in Figure 14.28(a), or fast enough for a cleanly averaged signal, as
in Figure 14.28(b). NH protons also show broad signals, not because of chemical
exchange, which is generally quite slow for NH, but because of quadrupolar relaxation.
d1 HRO HH+ HOH RO HH+ HOH
+ROHH+ HOH
+ +mechanism for acid-catalyzed proton exchange
8
a.b.7654 3210CH 3 CH 2 OH87654 3210
CH 3 CH 2 OHδ (ppm)
frequencyδ (ppm)
freq en y
Figure 14.28
(a) NMR spectrum of pure ethanol. (b) NMR spectrum of ethanol containing a trace
amount of acid.
1 H 1 H