Chapter 38: Problem 1

You are trying to identify a compound \(M_{\mathrm{r}}=126\). Infrared, \({ }^{1} \mathrm{H}-\) and \({ }^{13} \mathrm{C}\)-NMR spectroscopy have indicated thatit is either \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{COOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\), but have not provided enough detail for a satisfactory conclusion. Predict the mass spectra of the two compounds from the possible \(\alpha\) - and \(\beta\)-cleavages for each molecule so that a distinction can be made.

Short Answer

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The mass spectra predictions of the two compounds - \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{COOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\) - from the alpha- and beta-cleavages vary: For the first compound, the major fragments will be with mass-to-charge (m/z) ratios of 59, 67 (alpha-cleavage) and 45, 81 (beta-cleavage), whereas the second compound will have major fragments with m/z ratios of 73, 53 (alpha-cleavage) and 59, 67 (beta-cleavage).

Step by step solution

Identify the Compounds

The two compounds given are \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{COOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\). The only difference between these two compounds is the location of the carbonyl group. The first compound has it after the second carbon, while for the second compound it's placed after the third carbon.

Potential Alpha-cleavage

Alpha-cleavage occurs when a bond adjacent to the carbonyl group is broken. For the first molecule \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{COOCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\), an alpha-cleavage will yield two fragments of \(m/z=59\) and \(m/z=67\), while for the second \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOCH}_{2} \mathrm{CH}_{3}\), the fragments will be of \(m/z=73\) and \(m/z=53\).

Potential Beta-cleavage

Beta-cleavage occurs when a bond two positions away from the carbonyl group is broken. For the first compound, beta-cleavage will create ions with \(m/z=45\) and \(m/z=81\), while for the second compound, the fragments will be of \(m/z=59\) and \(m/z=67\).

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Identify the Compounds

Potential Alpha-cleavage

Potential Beta-cleavage

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