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Magazine Chapter 20: Q66P (page 1038)
An unknown compound gives a mass spectrum with a weak molecular ion at m/z113 and a prominent ion at m/z68. Its NMR and IR spectra are shown here. Determine the structure, and show how it is consistent with the observed absorptions. Propose a favorable fragmentation to explain the prominent MS peak at m/z68.
Short Answer
Step by step solution
Mass spectrometry
A particular molecule鈥檚 chemical composition can be understood with the help of mass spectrometry. The isotopes of a specific substance can also be identified from mass spectrometry. Mass spectroscopy considers the mass by charge ratio of the various components of the molecule to give an insight into the molecule's structure.
Proposing a favorable fragmentation to explain the prominent MS peak at m/z 68.
In the IR spectrum, the sharp spike at 2250 cm-1 corresponds to C鈮 . The peak at 1750 cm-1 corresponds to C=O, and the peak at 1200 cm-1 corresponds to C-O. The C=O and C-O peaks indicate that the compound may be an ester.
In the NMR spectrum, the triplet and quartet indicate the presence of -CH3CH2 group. The quartet at 未 =4.3 corresponds to (CH3CH2O-) group. The 2H singlet at 未 =3.5 corresponds to an isolated CH2.
The fragments can be combined in two ways:
Structures generated by combining the fragments
The NMR proves the structure is A. If the structure is B, the CH2 between oxygen and the carbonyl will come farther downfield than the CH2 of the ethyl, which is only deshielded by the oxygen atom. This does not happen, so the correct structure is A.
The peak in the mass spectrum at m/z 68 is due to the alpha cleavage of the ester
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