Compound A
The first things to note are that the hydrogens labelled Hb in the diagram below are diastereotopic (hence the red and blue shading). Similarly, the hydrogen atoms (Hd) of the two methyl groups are diastereotopic. Diastereotopic hydrogens will give rise to different signals in the NMR spectrum, and will exhibit different couplings to adjacent hydrogens.
Thus, the signals which will be observed for compound A are:
1. Ha multiplicity dd due to coupling to Hb
and Hb.
2. Hb multiplicity ddd due
to coupling to Ha, Hb, and Hc
3. Hb multiplicity ddd due
to coupling to Ha, Hb, and Hc
4. Hc multiplicity ddqq due to coupling to Hb,
Hb, Hd,
and Hd
5. H3d muliplicity d due to
coupling to Hc
6. H3d muliplicity d due to
coupling to Hc
Hence, there will be six signals in total.
The first things to note are that the hydrogens labelled Hb in the diagram below are diastereotopic (hence the red and blue shading), the hydrogens labelled Hc are also diastereotopic, and so are the hydrogens labelled Hd. Diastereotopic hydrogens will give rise to different signals in the NMR spectrum, and will exhibit different couplings to adjacent hydrogens.
Thus, the signals which will be observed for compound B are:
1. Ha multiplicity dd due to coupling to Hb
and Hb.
2. Hb multiplicity dddd due
to coupling to Ha, Hb,
Hc, and Hc
3. Hb multiplicity dddd due
to coupling to Ha, Hb,
Hc, and Hc
4. Hc multiplicity ddddd due
to coupling to Hb, Hb,
Hc, Hd,
and Hd
5. Hc multiplicity ddddd due
to coupling to Hb, Hb,
Hc, Hd,
and Hd
6. Hd muliplicity ddd due to
coupling to Hc, Hc,
and Hd
7. Hd muliplicity ddd due to
coupling to Hc, Hc,
and Hd
Hence, there will be seven signals in total.
The only two hydrogens which remain after deuterium exchange are the
hydrogens of the CH2 group. These are enantiotopic, and so cannot
be distinguished by NMR. Thus, the spectrum will consist of a single signal
which will be a singlet.
The difference between compounds C and D, is that the hydrogens of the
CH2 groups in compound D are diastereotopic due to the presence
of a stereocentre in the molecule. Thus, the two Ha's are now
diastereotopic, as are the two Hb's, as shown below.
The signals which will be observed in the spectrum are:
1. Ha multiplicity d due to
coupling to Ha
2. Ha multiplicity d due to
coupling to Ha
3. Hb multiplicity d due to
coupling to Hb
4. Hb multiplicity d due to
coupling to Hb
5. Hc multiplicity q due to coupling to Hd
6. Hd multiplicity d due to coupling to Hc
Hence, there will be six signals in total.
Back to Chapter 7 Back to answers to problems Back to STEREOCHEMISTRY home page