Organic Chemistry

Section 14.14 Time Dependence of NMR Spectroscopy 557

the multiplicity of the signal for the adjacent methyl hydrogens indicates that the signal

should be a doublet of doublets. The signal, however, is a triplet. Using a splitting diagram,

explain why it is a triplet rather than a doublet of doublets.

SOLUTION The observation of a triplet means that the rule did not have to be

applied to the diastereotopic hydrogens separately but could have been applied to the two

protons as a set ( so ). This means that the coupling constant for split-

ting of the methyl signal by one of the methylene hydrogens is similar to the coupling con-

stant for splitting by the other methylene hydrogen.

PROBLEM 31

Draw a splitting diagram for where

a. and b. and

14.14 Time Dependence of NMR Spectroscopy

We have seen that the three methyl hydrogens of ethyl bromide give rise to one signal
in the NMR spectrum because they are chemically equivalent due to rotation about
the bond. At any one instant, however, the three hydrogens can be in quite
different environments: One can be anti to the bromine, one can be gauche to the
bromine, one can be eclipsed with the bromine.

An NMR spectrometer is very much like a camera with a slow shutter speed—it is too
slow to be able to detect these different environments, so what it sees is an average
environment. Because each of the three methyl hydrogens has the same average envi-
ronment, we see one signal for the methyl group in the NMR spectrum.
Similarly, the NMR spectrum of cyclohexane shows only one signal, even
though cyclohexane has both axial and equatorial protons. There is only one signal be-
cause the interconversion of the chair conformers of cyclohexane occurs too rapidly at

1 H

1 H

anti gauche eclipsed

H

H

H

H

Br

H

H

H

H

H

Br

H

H

HH H

HBr

C¬C

1 H

CCC

Ha Hb Hc

Jba= 12 Hz Jbc= 6 Hz Jba= 12 Hz Jbc= 12 Hz

Hb,

Jab

Ha

Jac

N=2, N+ 1 = 3

N+ 1

nonequivalent protons

CH 3 CHCH 2 CH 3

Br

*