Additional Questions – Chapter 12 1 Propose a structure consistent with each set of data. a) A compound that contains a benzene ring and has a molecular ion at m/z = 107 b) A hydrocarbon that contains only sp3 hybridized carbons and a molecular ion at m/z = 84 c) A compound that contains a carbonyl group and gives a molecular ion at m/z = 114 d) A compound that contains C, H, N, and O and has an exact mass for the molecular ion at 101.0841 2 Compound A has the formula C8H7BrO2. The two most intense peaks in the mass spectrum are at m/z = 216 and 214, and they appear in almost equal amounts. Why? Compound A is an aldehyde. Explain why the second-most intense peaks in the mass spectrum come at m/z values of 215 and 213. 3 The mass spectrum of tetramethylsilane, (CH3)4Si has a base peak at m/z = 73. Calculate the relative abundances of the isotopic peaks at m/z = 74 and 75. 4 How would mass spectra help you distinguish these structures? 5 Calculate the degree of unsaturation indicated by each of the following molecular formulas. (a) C5H10 (b) C9H12O (c) C8H7ClO (d) C8H15N (e) C4H8Br2 6 Three unknown compounds containing only C, H and O gave rise to the following molecular weights. Draw as many reasonable structures as you can. (a) m/z = 46; (b) m/z = 30; (c) m/z = 56. 7 How many rings are present in each of the following compounds? Each consumes 2 moles of hydrogen on catalytic hydrogenation. (a) C10H18; (b) C8H8; (c) C8H8Cl2; (d) C8H8O; (e) C8H10O2; (f) C8H9ClO. 8 The mass spectrum of compound A shows the molecular ion at m/z = 85, an M+1 peak at m/z = 86 of approximately 6% abundance relative to M and an M+2 peak at m/z = 87 of less than 0.1% abundance relative to M. (a) Propose a molecular formula for compound A. (b) Draw at least 10 possible structural formulas for this molecular formula.9 A common lab experiment is the dehydration of cyclohexanol to cyclohexene. (a) Explain how could you tell from the IR spectrum whether your product was pure cyclohexene, pure cyclohexanol, or a mixture of cyclohexene and cyclohexanol. Give approximate frequencies for distinctive peaks. (b) Explain why mass spectrometry might not be a good way to distinguish cyclohexene from cyclohexanol. 10 Write equations showing the four principal fragmentation products expected in the mass spectrum of 2-methyl-4-heptanone. There are two different McLafferty rearrangement ions and two different -cleavages leading to oxonium ions. 11 Explain which of these bonds has the absorption for its stretching vibration at higher wavenumber: (a) C-H or C-D (b) C=C or C≡C (c) C-Cl or C-I 12 There are four isomers with molecular formula C4H9Cl. Only one of these isomers (compound A) has a chirality center. When compound A is treated with sodium ethoxide, three products are formed: compounds B, C and D. Compounds B and C are diastereomers, with compound B being the less stable diastereomer. Do you expect compound D to exhibit a signal at approximately 1650 cm-1 in the IR spectrum? Explain. 13 The sex attractant of the codling moth gives an IR spectrum with a broad signal between 3200 and 3600 cm-1 and two signals between 1600 and 1700 cm-1. In the mass spectrum of this compound, the molecular ion peak appears at m/z = 196, and the relative abundance of the molecular ion and (M+1) peak are 27.2% and 3.9%, respectively. (a) What functional groups are present in this compound? (b) How many carbon atoms are present in the compound? (c) Based on the information given, propose a molecular formula for the compound. 14 (a) Write the structural formulas for at least 14 stable compounds that have the formula C4H8O. (b) The IR spectrum of the unknown compound shows a strong peak near 1730 cm-1. Which structures now remain as possible formulas for the compound? 15 Which of the following compounds has a vibration that is infrared inactive? 1-butyne, 2-butyne, H2, H2O, and