《有机化学》课程PPT教学课件（Organic Chemistry with Biological Applications, 3th Edition, John McMurry, 2016）Chapter 10 Structure Determination - Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet Spectroscopy

CNGNGE JOHN MCMURRY CHAPTER 10 Structure Determination: Mass Spectrometry, Infrared Spectroscopy, H IRD EDITION and Ultraviolet Organic Chemistry with Biological applications Spectroscopy

CHAPTER 10 Structure Determination: Mass Spectrometry, Infrared Spectroscopy, and Ultraviolet Spectroscopy

Introduction Modern techniques for structure determination of organic compounds include: Mass spectrometry n Size and formula of the compound Infrared spectroscopy Functional groups present in the compound Ultraviolet spectroscopy Conjugated z electron system present in the compound Nuclear magnetic resonance spectroscopy Carbon-hydrogen framework of the compound

Modern techniques for structure determination of organic compounds include: ▪ Mass spectrometry ▪ Size and formula of the compound ▪ Infrared spectroscopy ▪ Functional groups present in the compound ▪ Ultraviolet spectroscopy ▪ Conjugated p electron system present in the compound ▪ Nuclear magnetic resonance spectroscopy ▪ Carbon-hydrogen framework of the compound Introduction

10-1 Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments Mass spectrometry (MS)measures the mass and molecular weight (MW)of a molecule Provides structural information by finding the masses of fragments produced when molecules break apart Three basic parts of mass spectrometers: Sample Display Ionization source Mass analyzer Detector Electron impact(EI),or Magnetic sector,or Photomultiplier,or Electrospray ionization(ESI),or Time-of-flight(TOF),or Electron multiplier,or Matrix-assisted laser Quadrupole(Q) Micro-channel plate desorption ionization(MALDI) Cengge Leaming.All Rights Reserved

Mass spectrometry (MS) measures the mass and molecular weight (MW) of a molecule ▪ Provides structural information by finding the masses of fragments produced when molecules break apart ▪ Three basic parts of mass spectrometers: 10-1 Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments

Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments Electron-impact,magnetic- sector instrument Sample is vaporized into ionization source Magnet Ionsdeflected Bombarded by electron beam according to m/z (70 eV)dislodging valence Heated filament electron of sample producing cation-radical RH 。 RH+.+e- Sample Organic Cation molecule radical beam LCD display Most cation-radicals fragment and are separated in magnetic field according to their mass-to- charge ratio (m/z) Since z 1 for most ions the value of mz is mass of ion

Electron-impact, magnetic￾sector instrument ▪ Sample is vaporized into ionization source ▪ Bombarded by electron beam (70 eV) dislodging valence electron of sample producing cation-radical ▪ Most cation-radicals fragment and are separated in magnetic field according to their mass-to￾charge ratio (m/z) ▪ Since z = 1 for most ions the value of m/z is mass of ion Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments

Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments Mass spectrum of propane(C3Ha;MW=44) 100 80 -m/2=29 60 40 -m2=44 20 0 1020 40 60 80 100 120 140 m/2 Base peak Tallest peak Assigned intensity of 100% Base peak at m/z=29 in propane mass spectrum -Parent peak Unfragmented cation radical-molecular ion(M) Parent peak only 30%of base peak for propane

Mass spectrum of propane (C3H8 ; MW = 44) ▪ Base peak ▪ Tallest peak ▪ Assigned intensity of 100% ▪ Base peak at m/z = 29 in propane mass spectrum ▪ Parent peak ▪ Unfragmented cation radical – molecular ion (M+ ) ▪ Parent peak only 30% of base peak for propane Mass Spectrometry of Small Molecules: Magnetic-Sector Instruments

10-2 Interpreting Mass Spectra Molecular weight determined from molecular ion peak -High resolution double-focusing mass spectrometers are accurate to about 0.0005 amu M+1 peak results from presence of 13C and 2H Fragmentation occurs when high-energy cation radical falls apart One fragment retains positive charge and is a carbocation One fragment is a neutral radical fragment

Molecular weight determined from molecular ion peak ▪ High resolution double-focusing mass spectrometers are accurate to about 0.0005 amu ▪ M+1 peak results from presence of 13C and 2H ▪ Fragmentation occurs when high-energy cation radical falls apart ▪ One fragment retains positive charge and is a carbocation ▪ One fragment is a neutral radical fragment 10-2 Interpreting Mass Spectra

Interpreting Mass Spectra Mass spectrum of 2,2-dimethylpropane (MW =72) 100 80 60 m/2=41 ◆m=57 40 m/2=29 20 0 10 20 40 60 80 100 120 140 m/z 2,2-dimethylpropane fragments so easily that no M*peak observed when electron-impact ionization is used "Soft"ionization methods can prevent fragmentation of molecular ion

Mass spectrum of 2,2-dimethylpropane (MW = 72) ▪ 2,2-dimethylpropane fragments so easily that no M+ peak observed when electron-impact ionization is used ▪ “Soft” ionization methods can prevent fragmentation of molecular ion Interpreting Mass Spectra

Interpreting Mass Spectra Base peak in mass spectrum of 2,2-dimethylpropane is at m/z 57 m/z=57 corresponds to t-butyl cation Molecular ion fragments to give most stable carbocation CH3 CH3 H3C-C-CH3 H3C-C+ .CH3 CH3 CH3 m/z=57

▪ Base peak in mass spectrum of 2,2-dimethylpropane is at m/z = 57 ▪ m/z = 57 corresponds to t-butyl cation ▪ Molecular ion fragments to give most stable carbocation Interpreting Mass Spectra

Interpreting Mass Spectra Mass spectrum of hexane(CH14;MW=86) 100 8 80 m/z=43 60 -m/2=57 40 m/2=29 20 一M+=86 0 10 20 40 60 80 100 120 140 m/2 All carbon-carbon bonds of hexane are electronically similar and break to a similar extent Mass spectrum contains mixture of ions

Mass spectrum of hexane (C6H14; MW = 86) ▪ All carbon-carbon bonds of hexane are electronically similar and break to a similar extent ▪ Mass spectrum contains mixture of ions Interpreting Mass Spectra

Interpreting Mass Spectra M+86 for hexane m/z=71 arises from loss of methyl radical from hexane cation radical m/z=57 arises from loss of CH3CH2CH2CH2CH2CH3 ethyl radical from hexane cation radical Hexane m/z=43 arises from loss of propyl radical from hexane [CH3CH2CH2CH2CH2CH3]+. cation radical Molecular ion,M+ m/z=29 arises from loss of (m/2=86) butyl radical from hexane cation radical CH3CH2CH2CH2CH2+ CH3CH2CH2CH2+ CH3CH2CH2+ CH3CH2+ miz: 71 43 29 Relative abundance (%) 10 100(base peak) 75 40

M+ = 86 for hexane ▪ m/z = 71 arises from loss of methyl radical from hexane cation radical ▪ m/z = 57 arises from loss of ethyl radical from hexane cation radical ▪ m/z = 43 arises from loss of propyl radical from hexane cation radical ▪ m/z = 29 arises from loss of butyl radical from hexane cation radical Interpreting Mass Spectra