Mass Spectroscopy

Mass SpectroscopyIntroduction2Mass SpectrometryMolecular weight can be obtained from a very small sample.It does not involve the absorption or emission of light.A beam of high-energy electrons breaks the molecule apart.The masses of the fragments and their relative abundance reveal information about the structure of the molecule. =>Mass SpectrometryTHE MAIN USE OF MS IN ORG CHEM IS: DETERMINE THE MOLECULAR MASS OFORGANIC COMPOUNDS DETERMINE THE MOLECULAR FORMULA OF ORGANIC COMPOUNDSHOW DO WE ACHIEVE THIS? PERSUADE THE MOLECULE TO ENTER THE VAPOR PHASE (CAN BE DIFFICULT) PRODUCE IONS FROM THE MOLECULES THAT ENTER THE GAS PHASE SEPARATE THE IONS ACCORDING TO THEIR MASS-TO-CHARGE RATIOS (m/z)) MEASURE AND RECORD THESE IONSIONIZING METHODS ELECTRON IMPACT - HIGH ENERGY ELECTRONS ABOUT 70 EV!! CHEMICAL IONIZATION LOW ENERGY ELECTRON IMPACT

RADICAL CATION

ONLY CATIONS ARE CARRIED TO DETECTOR

CATIONRADICALBOND-BREAKINGMS of METHANE

So what does a typical ms looks like????m/zTypical Mass Spectrum

molecular ion

Isotope peaks - P+1, P+2, etc

Alarm Pheromone of Honey BeeNOTE: 114-71 = 43NOTE: 43 = mass of radical9Electron Impact IonizationA high-energy electron can dislodge an electron from a bond, creating a radical cation (a positive ion with an unpaired e-).

=>10Separation of IonsOnly the cations are deflected by the magnetic field.Amount of deflection depends on m/z.The detector signal is proportional to the number of ions hitting it.By varying the magnetic field, ions of all masses are collected and counted. =>11Mass Spectrometer=>

12The Mass SpectrumMasses are graphed or tabulated according to their relative abundance.

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13The GC-MS=>A mixture of compounds is separatedby gas chromatography, then identifiedby mass spectrometry.

14High Resolution MSMasses measured to 1 part in 20,000.A molecule with mass of 44 could be C3H8, C2H4O, CO2, or CN2H4.If a more exact mass is 44.029, pick the correct structure from the table:C3H8 C2H4O CO2 CN2H4 44.0626044.0262043.9898344.03740 =>15Molecules with HeteroatomsIsotopes: present in their usual abundance.Hydrocarbons contain 1.1% C-13, so there will be a small M+1 peak.If Br is present, M+2 is equal to M+.If Cl is present, M+2 is one-third of M+.If iodine is present, peak at 127, large gap.If N is present, M+ will be an odd number.If S is present, M+2 will be 4% of M+. =>16Isotopic Abundance=>

81Br17Mass Spectrum with Sulfur

=>18Mass Spectrum with Chlorine=>

19Mass Spectrum with Bromine=>

20Mass Spectra of AlkanesMore stable carbocations will be more abundant.=>

Chapter 1221Mass Spectra of AlkenesResonance-stabilized cations favored.=>

22Mass Spectra of AlcoholsAlcohols usually lose a water molecule.M+ may not be visible.=>