Espectrometria de Massa e NMR

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Suggested solutions for Chapter 3 3 Problem 1 How does the mass spectrum gives evidence of isotopes in the compounds of bromine, and carbon? Assuming the molecular ion of each of these compounds is of 100% abundance, what peaks (and in what intensity) would appear around that mass number? (a) (b) (c) Give in cases (a) and (c) a possible structure for the compound. What compound is (b)? Purpose of the problem To give you practice in spotting the important atoms that have isotopes and in interpreting multiple ions. The molecular ion is the most important peak in the mass spectrum and is often the only peak to interest us. Suggested solution Bromine has two isotopes, and 81 Br, in about a 1:1 ratio, chlorine has two, ³⁵Cl and ³⁷Cl, in about a 3:1 ratio and there is about 1.1% ¹³C in normal compounds. Hence the molecular ions of three compounds will be as follows. MW 124/126. There will also be weak peaks (2.2% of each main peak) at 125 and Compounds containing Br or CI 127 for the same ions with ¹³C. should have their molecular ions b has a molecular ion at 720 with a strong peak at 721. The chance that one C atom is is specified like this. The first figure gives with and the second simply 60 X 1.1% = 66% so the M + 1 peak is more than half as strong as itself. The with chance of two ¹³C atoms is small. C₆H₄BrCl is more complicated as the molecular ion will contain a 1:1 ratio of 79 Br and 81 Br, and a 3:1 ratio of and The molecular ion consists of four peaks (ratios in brackets): (3), 35 (3), (1), and (1). The masses of these peaks are 190, 192, 192, and 194. The molecular ion will therefore have three peaks at 190, 192, and 194 in a ratio of 3:4:1 with peaks at 191, 193, and 195 at 6 X 1.1% = 6.6% of the peak before it. So the complete picture is 190 (75%), 191 (5%), 192 (100%), 193 (6.6%), 194 (25%), and 195 (1.7%). Compound (b) is, of course, and the other compounds might be isomers such as these. (a) (c) one of these: Br CI Br Br OH Problem 2 The NMR spectrum for ethyl benzoate contains these peaks: 17.3, 61.1, 100-150 p.p.m. (four peaks), and 166.8 p.p.m. Which peak belongs to which carbon atom? Purpose of the problem To familiarize you with the four main regions of the ¹³C NMR spectrum: saturated carbons at 0-50 p.p.m., saturated carbons next to oxygen at 50-100 p.p.m., alkenes and aromatic compounds