Set 5 – Exchange Effects

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Page ID: 79419

Thomas Wenzel
Bates College

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Considering the spectrum of a compound such as methanol (CH₃OH). Using the rules for coupling we have established, predict the number of resonances and identify their multiplet structure.

Groups are usually able to determine this, although they may ask a question about whether having oxygen as one of the atoms makes a difference. If they ask that, I indicate that they should only count the number of bonds between the hydrogen atoms.

However, in other situations the spectrum of methanol appears as two singlets, one of area three and the other of area one. Propose a reason why singlets are observed instead of coupled multiplets.

Some groups may need a prompt about the ability of methanol to form hydrogen bonds. It can be useful to ask them to draw pictures of two hydrogen bonded methanol molecules and include the lone pairs as a way of getting them to realize that the hydrogen atoms can exchange between different methanol molecules.

Another time-dependent aspect to consider about NMR spectra can be illustrated by considering the ¹H NMR spectrum of N,N-dimethylacetamide (DMA) shown below.

What does the NMR spectrum of DMA look like?

They are able to determine that rapid rotation causes the two methyl groups to be equivalent and there is a singlet of area 6 for the methyl groups and a singlet of area 1 for the aldehyde hydrogen.

DMA has the following contributing resonance form. What would the NMR spectrum of DMA look like in this resonance form?

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Groups are able to determine that in this form the methyl groups are inequivalent and two singlets would be observed of area 3 for these two different methyl groups.