Chem2462LectureNotes

Chem2462LectureNotes - Chemistry 2462 Spring 2009 1 Dr. G....

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Chemistry 2462 – Spring 2009 1 Dr. G. Vicente LECTURE NOTES FOR CHEMISTRY 2462 Chapters 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 and 22 of Paula Y. Bruice’s book "Organic Chemistry" , 5 th Edition
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Chemistry 2462 – Spring 2009 2 Dr. G. Vicente CHAPTER 12 : MASS SPECTROMETRY (MS), INFRARED (IR) AND ULTRAVIOLET/VISIBLE (UV-Vis) SPECTROSCOPY Mass Spectrometry (12.1 - 12.5) MS is used to determine the molecular weight and the molecular formula of an organic compound, as well as certain structural features. M electron beam M + e (molecular ion) fragmentation cations, radicals, cation radicals neutral molecules Mass spectrum : plot of the relative abundance of each positively charged fragment versus its m/z value. The m/z value of the molecular ion ( M , usually the peak with the largest m/z) gives the molecular weight. Peaks with smaller m/z values represent fragments of the molecule. M+1 peak results from molecular ions containing 13 C. The number of carbon atoms in a molecule can be calculated from the intensities of the M and M+1 peaks: No. carbon atoms = relative intensity of M+1 peak 0.011 x (relative intensity of M peak)
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Chemistry 2462 – Spring 2009 3 Dr. G. Vicente So, if there are 40 carbons in the molecule being examined, the M+1 peak will be 44% of the size of the M peak (because the natural abundance of 13 C is 1.1% of that of 12 C). M+2 peak is usually small unless the compound has Cl or Br. High-resolution mass spectrometry (HRMS) determines the exact molecular mass of a fragment with a precision of 0.0001 amu. Thus, allows determination of the molecular formula . 12 C = 12.000000 amu 1 H = 1.007825 amu (not 1.000000) 16 O = 15.994900 amu (not 16.000000) 14 N = 14.003100 amu (not 14.000000) For example: C 8 H 10 O = 122.0732 amu C 4 H 10 O 4 = 122.0579 amu C 9 H 14 = 122.1096 amu Fragmentation Patterns of functional groups (12.5) Alkanes : show M peak and characteristic fragments at m/z = 15, 29, 43,… due to methyl, ethyl, propyl, etc., cations. Alkyl bromides : same relative intensity of M and M+2 peaks. Characteristic fragments: M-79 or (M+2)-81.
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Chemistry 2462 – Spring 2009 4 Dr. G. Vicente Alkyl chlorides : relative intensity of M+2 peak is one-third that of M. Characteristic fragments: M-35 or (M+2)-37 and fragments from α -cleavage. Ethers : show M peak and fragments from cleavage of C-O bonds and from α -cleavage. Alcohols : 1 o and 2 o alcohols show weak M peak and 3 o alcohols show no M peak. Characteristic fragments: M-18 (loss of H 2 O) and fragments from α -cleavage. Ketones : usually show intense M peak and fragments from α -cleavage. Under “Ideal Fragmentation” conditions: M = [K-L-M-N] [K-L-M-N] K + [L-M-N] and K + [L-M-N] [K-L] + [M-N] and [K-L] + [M-N] [K-L-M] + N and [K-L-M] + N So, you see: [K-L-M-N] [K-L-M] [L-M-N] [M-N] [K-L] K "M " K L M N N
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Chemistry 2462 – Spring 2009 5 Dr. G. Vicente Spectroscopy and the electromagnetic spectrum (12.6) Review the electromagnetic spectrum: UV 200-400 nm 71-143 kcal/mole Bond DIssociation Energy RANGE ENERGY IR 625-4000 cm –1 1.8-11 kcal/mole Bond Deformation (stretch/bend) CORRESPONDS TO: NMR 100-1000 MHz 10 –5 - 10 –4 kcal/mole Nuclear Spin Flip Units and Conversion Factors Wavelength, λ (UV spectroscopy) 1 nm = 10 Å = 10 –9 m = 10 –7 cm Wavenumber (IR spectroscopy) ν : cm –1 Frequency (NMR spectroscopy) ν : Hz (s –1 ) Note that λ
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This note was uploaded on 08/04/2009 for the course CHEM 2462 taught by Professor Vicente during the Spring '09 term at LSU.

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Chem2462LectureNotes - Chemistry 2462 Spring 2009 1 Dr. G....

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