Consider the following information from the mass spectrum of an unknown compound.(M)+• at m/z = 72, relative height = 38.3% of base peak(M+1)+• at m/z = 73, relative height = 1.7% of base peakPropose a formula for the molecule:Select answer from the options belowC5H9O2C4H10OC5H9OC4H8OC4H8O2
Question
Consider the following information from the mass spectrum of an unknown compound.(M)+• at m/z = 72, relative height = 38.3% of base peak(M+1)+• at m/z = 73, relative height = 1.7% of base peakPropose a formula for the molecule:Select answer from the options belowC5H9O2C4H10OC5H9OC4H8OC4H8O2
Solution
The molecular ion peak (M)+• is at m/z = 72, which represents the molecular weight of the compound. This is the most important clue to the molecular formula.
The (M+1)+• peak at m/z = 73 is due to the presence of isotopes. The most common isotope effects are due to the presence of carbon-13, which is about 1.1% of natural carbon. If the (M+1)+• peak is 1.7% of the base peak, this suggests that there are about 1.7 / 1.1 = 1.54, or approximately 2 carbons in the molecule.
However, this seems to contradict the molecular weight of 72. If we consider the formulas given:
C5H9O2: The molecular weight is 5(12.01) + 9(1.01) + 2(16.00) = 101.09 C4H10O: The molecular weight is 4(12.01) + 10(1.01) + 16.00 = 74.06 C5H9O: The molecular weight is 5(12.01) + 9(1.01) + 16.00 = 85.08 C4H8O: The molecular weight is 4(12.01) + 8(1.01) + 16.00 = 72.06 C4H8O2: The molecular weight is 4(12.01) + 8(1.01) + 2(16.00) = 88.06
The only formula that fits the molecular weight of 72 is C4H8O. Therefore, the formula of the molecule is likely C4H8O.
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