Science & Fun | Home |  Introduction into the 1H NMR Spectroscopy



Spectrum (A) Spectrum (B)
option A


Based on the composition, we can derive the following isomers:

1 Isomer 1 2 Isomer 2
3 Isomer 3 4 Isomer 4
5 Isomer 5 6 Isomer 6
7 Isomer 7 8 Isomer 8

Spectrum (A)
The characteristic multiplicity of the signals in the spectral range d = 0.5 ... 2 ppm (A triplet and a quartet) can only be caused by a structure of the type

This motif occurs in the isomers 1, 3, 4, 6 and 8. But only for the isomers 6 and 8 would we expect three signals from the aliphatic side chains.
(E.g., in the case of the isomer 2 we would expect signals from five non-equivalent groups of protons in this spectral range)
To distinguish between the two remaining options, we will have to consider the single signal at d = 1.25 ppm. Since this signal is a singulet (Multiplicity M = 1), only isomer 6 could give rise to it.
In this molecule the methyls CH3 of the group

are equivalent and ouple with no other group, thus being represented by only a single signal in the spectrum.
Spectrum (B)
Matching this spectrum to isomer 7 is rather straight forward based on the number of aliphatic signals (2 groups of equivalent protons) and the lack of spin-spin coupling between the signals.

Now continue to the next problem!