Chapter12 - 12. Structure Determination: Mass Spectrometry...

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12. Structure Determination: Mass Spectrometry and Infrared Spectroscopy Based on McMurry’s Organic Chemistry , 6 th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto
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McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 2 Determining the Structure of an Organic Compound The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants In the 19 th and early 20 th centuries, structures were determined by synthesis and chemical degradation that related compounds to each other Physical methods now permit structures to be determined directly. We will examine: mass spectrometry (MS) infrared (IR) spectroscopy nuclear magnetic resonance spectroscopy (NMR) ultraviolet-visible spectroscopy (VIS)
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McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 3 12.1 Mass Spectrometry (MS) Measures molecular weight Sample vaporized and subjected to bombardment by electrons that remove an electron Creates a cation-radical Bonds in cation radicals begin to break (fragment) Charge to mass ratio is measured (see Figure 12-1)
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McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 4 The Mass Spectrum Plot mass of ions (m/z) (x-axis) versus the intensity of the signal (roughly corresponding to the number of ions) (y-axis) Tallest peak is base peak (100%) Other peaks listed as the % of that peak Peak that corresponds to the unfragmented radical cation is parent peak or molecular ion ( M + )
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McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 5 MS Examples: Methane and Propane Methane produces a parent peak (m/z = 16) and fragments of 15 and 14 (See Figure 12-2 a) The MS of propane is more complex (Figure 12-2 b) since the molecule can break down in several ways
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McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 6 12.2 Interpreting Mass Spectra Molecular weight from the mass of the molecular ion Double-focusing instruments provide high-resolution “exact mass” 0.0001 atomic mass units – distinguishing specific atoms Example MW “72” is ambiguous: C 5 H 12 and C 4 H 8 O but: C 5 H 12 72.0939 amu exact mass C 4 H 8 O 72.0575 amu exact mass Result from fractional mass differences of atoms 16 O = 15.99491, 12 C = 12.0000, 1 H = 1.00783 Instruments include computation of formulas for each peak
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McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 7 Other Mass Spectral Features If parent ion not present due to electron
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Chapter12 - 12. Structure Determination: Mass Spectrometry...

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