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CHMB42H3 (27)
Lecture

Chapter 13.docx

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Department
Chemistry
Course
CHMB42H3
Professor
Wanda/ Lana
Semester
Winter

Description
13.1 Mass Spectrometry  Small amount of compound is introduced into spectrometer and is vaporized and ionized o Electron Ionization-mass spectrometry (EI-MS) is most common method for ionization  Produces a molecular ion, a radical cation – a species with an unpaired electron and positive charge  So much kinetic energy in molecular ions, often fragment into smaller cations, radicals, neutral molecules, and other radical cations  Positively charged fragments are accelerated and deflected in a curved path in analyzer tube according to the mass-to-charge ratio (m/z) of the fragment o Particles with same m/z can be separated from others  Mass spectrum – a graph of the relative abundance of each fragment plotted against its m/z value o Charge (z) on all fragments reaching collector relatively the same (+1), therefore m/z is the molecular mass of the fragment  A mass spectrum only records positively charged fragments  Neutral molecules, neutral fragment s and negative ions are pumped out 13.2 The Mass Spectrum – Fragmentation  Molecular ion and fragment ions recorded in mass spec. are unique for each compound – like a fingerprint  Molecular ion (M) = peak with highest m/z value o The m/z value of molecular ion gives molecular mass of compound and thus the molecular formula  This is not a fragment  *Know the difference between M and M+n peaks  Fragment ion peaks = peaks with smaller m/z values than molecular ion o Positively charged fragments of the molecular ion  Base peak = tallest peak o Fragment with greatest abundance  Assigned 100% relative abundance  Relative abundance of other fragments are compared to base peak  Mass spec. also gives structural information o m/z values and relative abundance of fragments depends on strength of molecular ion’s bonds and stability of fragments  Differ across compounds of different structure although may have same molecular formula  eg. Primary radical more stable than methyl radical  *Know relative stabilities of carbocation and radicals 13.3 Isotopes in Mass Spectrometry  M+n peak are due to presence of naturally occurring isotopes o n is number of neutrons in isotope   Can use this formula because the natural abundance of carbon isotopes are known  Formula can also be used to determine number of carbon atoms in compounds containing H, O, and the halogens because their isotopes make very small contributions to M+1 peak o Cannot be used for nitrogen and sulphur containing compounds because natural abundance of their isotopes is high  Knowing abundance of certain isotopes can be used to determine composition of compound o eg. If M and M+2 peak are relatively the same height, then that is a good indication that the compound has one bromine atom because that natural abundance of Br and Br81 are about the same 13.5 The Fragmentation Patterns of Functional Groups  Each functional group has characteristic fragmentation patterns that can help identify a compound  First, molecular ion (M) is obtained and then fragmentation of the molecular ion through homolytic and/or heterolytic breaking of bonds proceeds  Alkyl Halides (R-X) o Formation of M – Electron bombardment dislodges one of the lone-pair electrons from the halogen atom  Lone pair electrons are weakly held relative to bonding electrons o Alkyl bromides  Weakest bond is the C-Br bond  Fragmentation: o Breaks heterolytically  Electrons goes to more electronegative Br atom o Alkyl Chlorides  Fragmentation (two ways):  Breaks heterolytically  Breaks homolytically at the α-carbon (Carbon bonded to the chlorine) o Called α-Cleavage o α-Cleavage occurs in alkyl chlorides because C-Cl bond and C-C bond have same strength  Does not occur for alkyl bromides since C-C bond is stronger than C-Br bond  Cl is less electronegative than Br o Results in a less polar bond  Ethers (R-O-R) o Formation of M – Electron bombardment dislodges one of the lone-pair electrons from the O atom o Fragmentation (two ways):  C-O bond breaks heterolytically  Electrons go to more electronegative O atom  C-C bond breaks homolytically at the α-carbon (α-Cleavage)  The alkyl group most likely to be cleaved is the one that forms the most stable radical o 3° radical > 2° radical > 1° radical > methyl radical  Alcohols (R-OH) o Formation of M – Electron bombardment dislodges one of the lone-pair electrons from the O atom o Small molecular ion peaks because they fragment so readily o Fragmentation:  C-C bond breaks homolytically at the α-carbon (α-Cleavage)  The alkyl group that is most likely to be cleaved is the one that forms the most stable radical  Two bonds broken  Forms a stable water molecule o Composed of OH group and γ-hydrogen Alkyl Halides, ethers and alcohols have similar fragmentation behaviours: 1. Bond between carbons and a more electronegative atom (a halogen or O) breaks heterolytically 2. Bond between carbon and an atom of similar electronegativity (a carbon, a hydrogen or a weakly electronegative halogen such as Cl) breaks homolytically 3. The bond most likely to break are the weakest bonds and those that lead to formation of the most stable products (Look for a fragmentation that results in a cation with a positive charge shared by two atoms)  Ketones (R-CO-R) o Formation of M – Electron bombardment dislodges one of the lone-pair electrons from the O atom o Intense molecular ion peaks because they do not fragment readily o Fragmentation:  Breaks homolytically at the C-C bond adjacent to the C=O bond  Results in formation of a cation with a positive charge shared by two atoms  The alkyl group that is most likely to be cleaved is the one that forms the most stable radical  If one of the alkyl groups attached to the carbonyl carbon has a γ-hydrogen, a McLafferty rearrangement occurs  A six-membered ring transition state  α-carbon and β-carbon bond breaks homolytically and a hydrogen atom from the γ-carbon migrates to the oxygen o Positive charge is now shared by two atoms  More stable 13.6 Other Ionization Methods  Chemical ionization-mass Spectrometry (CI-MS) o Molecular ions produced this way are less apt to undergo fragmentations o Greater ability to determine molecular mass and therefore the molecular formula  EI-MS and CI-MS only useful for small molecules with low molecular weight since they must first be vaporized  Mass spec. of larger molecules carried out by desportion ionization (DI) 13.7 Spectroscopy and the Electromagnetic Spectrum  Spectroscopy is the study of the interaction of matter and electromagnetic radiation  A continuum of different types of electromagnetic radiation make up the electromagnetic spectrum  Different types of electromagnetic radiation: o Cosmic rays
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