CHEMISTRY DEPARTMENT MASS SPECTROMETRY EXERCISE

1 In the mass spectrum of the following compounds, which is the tallest—the peak at m/z = 57 or the peak at m/z = 71? a) 3-methylpentane b) 2-methylpentane 2 Draw structures for a saturated hydrocarbon that has a molecular ion with an m/z value of 128 3 A mass spectrum shows significant peaks at m/z = 87, 115, 140, and 143. Which of the following compounds is responsible for that mass spectrum? 4 What hydrocarbons that contain a six-membered ring will have a molecular ion peak at m/z = 112? 5 What peaks in their mass spectra can be used to distinguish between the following compounds? 6 The mass spectrum for a compound with molecular weight of 102 is shown below. Its IR spectrum has a broad, strong absorption at 3600 cm–1 and a medium absorption at 1360 cm–1. a. Identify the compound. b. Show the mechanism for formation of the peak at m/z = 84. 7 Draw a structure for a carboxylic acid that has a molecular ion with an m/z value of 116. 8 Halogenated compounds are particularly easy to identify by their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant isotopes. Chlorine occurs as 35Cl (75.8%) and 37Cl (24.2%); bromine occurs as 79Br (50.7%) and 81Br (49.3%). At what masses do the molecular ions occur for the following formulas? What are the relative percentages of each molecular ion? (a) Bromomethane, CH3Br (b) 1-Chlorohexane, C6H13Cl 9 Propose structures for compounds that fit the following data: (a) A ketone with M+=86 and fragments at m/z = 71 and m/z = 43 (b) An alcohol with M+=88 and fragments at m/z = 73, m/z = 70, and m/z =59 10 2-Methylpentane (C6H14) has the mass spectrum shown. Which peak represents M+? Which is the base peak? Propose structures for fragment ions of m/z 5 71, 57, 43, and 29. Why does the base peak have the mass it does? 11 What fragments might you expect in the mass spectra of the following compounds? 12 4-Methyl-2-pentanone and 3-methylpentanal are isomers. Explain how you could tell them apart, both by mass spectrometry and by infrared spectroscopy. 13 Predict the masses and the structures of the most abundant fragments observed in the mass spectra of the following compounds. 14 Give logical fragmentation reactions to account for the following ions observed in these mass spectra. 15 The mass spectrum of n-octane shows a prominent molecular ion peak (m/z 114). There is also a large peak at m/z 57, but it is not the base peak. The mass spectrum of 3,4-dimethylhexane shows a smaller molecular ion, and the peak at mass 57 is the base peak. Explain these trends in abundance of the molecular ions and the ions at mass 57, and predict the intensities of the peaks at masses 57 and 114 in the spectrum of 2,2,3,3-tetramethylbutane. 16 An unknown, foul-smelling hydrocarbon gives the mass spectrum and infrared spectrum shown. (a) Use the mass spectrum to propose a molecular formula. How many elements of unsaturation are there? (b) Use the IR spectrum to determine the functional group(s), if any. (c) Propose one or more structures for this compound. What parts of the structure are uncertain? If you knew that hydrogenation of the compound gives n-octane, would the structure still be uncertain? (d) Propose structures for the major fragments at 39, 67, 81 and 95 in the mass spectrum. Explain why the base peak is so strong. 17 A graduate student was following a procedure to make 3-propylcyclohexa-l,4-diene. During the workup procedure, his research adviser called him into her office. By the time the student returned to his bench, the product had warmed to a higher temperature than recommended. He isolated the product, which gave the appropriate =C-H stretch in the IR, but the C=C stretch appeared around 1630 cm-I as opposed to the literature value of