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INTERPRETATION OF THE MASS SPECTRUM OF SALICYLIC ACIDWM43 Figure 2 gives a more accurate mass spectrum for salicylic acid. Thisspectrum shows any isotope peaks which exist. The intensities of themolecular ion peaks have been accurately measured and are given on theInformation Sheet.1008091.9868120.0211Intensity / %60402039.023253.003764.037000 20 40 60 80 100 120 140 160MassFigure 2 A more accurate version of the mass spectrum of salycylic acidUsing a database to establish which isomerit isA web site, which supplies mass spectra of compounds, indicates that there arethree isomers with the molecular formula C 7H 6O 3. The mass spectra of thesecompounds provides the following information.138.03172-hydroxybenzoic acidPeak 120 92 138 64 39 63Intensity 100 73 39 19 14 143-hydroxybenzoic acidPeak 138 121 93 65 39 91Intensity 100 75 23 16 12 124-hydroxybenzoic acidPeak 121 138 65 93 39 63Intensity 100 83 24 22 18 104 Which of these matches the spectrum of salicylic acid? Write the chemicalname and structure of salicylic acid on the Information Sheet.Table 1 Mass spectra of the three hydroxybenzoicacidsConfirming the structure using thefragmentation patternMass spectrometrists can find out a lot about the structure of a molecule bylooking at the fragmentation pattern in the mass spectrum. This provides anadditional way of distinguishing 2-hydroxybenzoic acid from its two isomers, 3-and 4-hydroxybenzoic acid. It may help with this activity to build models of thethree molecules.5 Using Table 1, write down the peaks in the mass spectrum of2-hydroxybenzoic acid that are not shared by 3- and 4-hydroxybenzoic acid.6 Which well-known molecule with M r= 18 might be lost in the conversion:138 (the whole molecule) Æ 120in 2-hydroxybenzoic acid?7 What are the two most likely ways in which the molecule in 6 can arise from2-hydroxybenzoic acid? Show this by ringing the relevant atoms on drawingsof the structure of the molecular ion of 2-hydroxybenzoic acid (Figure 3).8 3- and 4-hydroxybenzoic acid do not have the same fragmentation patternbecause they cannot form an ion of mass = 120. Explain why this ion cannotform.AS LEVEL„ Salters Advanced Chemistry 2000 – see Copyright restrictionsC OH +OOHFigure 3 The molecular ion of 2-hydroxybenzoicacid145
WM4INTERPRETATION OF THE MASS SPECTRUM OF SALICYLIC ACIDInformation Sheet: Mass spectrum dataApproximate molecular mass(found by fast, low-resolution scan)Number of possible molecular formulaeAccurate molecular mass(measured by slower, high-resolution scan)Possible molecular formulaeIsotope peaksA group of peaks starting at the parent ion peak, due to isotopes of carbon. These peaks may besmall on the spectrum, but are given here scaled up so that the largest peak in this group has abundance 100.Molecular ion peak = 100% Indicated number of carbon atoms:M + 1 peak = 8.2%Six largest peaks in the spectrumMassRelativeabundanceIdentity of this sample146AS LEVEL„ Salters Advanced Chemistry 2000 – see Copyright restrictions
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