Friedel-Crafts Alkylations (Exp.II)

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• Mechanism of Phenylurethane

1-butanol and phenyl isocyanate
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Mass SpectrometryInterpretation of Mass Spectra
Check Mass Spectrometry Chapters in Lab and
Lecture Texts

• Molecules of a sample are bombarded by a stream
  of high energy electrons
• Molecular ion peak M.
• Molecular ion bonds break to give fragment IONS
  and NEUTRAL radicals or molecules. Mass Spectra
  only show ions, not neutrals.
• m/z of M - m/z of F (fragment ion) mass of
  neutral
• Each peak can be interpreted in terms of both a
  fragment ion and/or a neutral loss

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Mass Spectrometry

• Molecular ion peak (M.) at m/z 32
• Base peak at m/z 31
• Fragment ions at 29 and 15

A Mass Spectrum tells you the molecular mass
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Molecular Ion Peak Odd or Even Mass?
aniline
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Isotope Peaks 35Cl and 37Cl
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Isotope Peaks 79Br and 81Br
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Molecular Ion Peak Alcohols
1-butanol
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Favored Fragmentations

• The greater the stability of the fragment ion,
  the more intense the peak
• Alkyl ion stability follows carbocation chemistry
  rules
• 3 and allyl more stable than 2
• 2 more stable than 1
• Acyl ions are stable (m/z 43, 57..)
• PhCO (m/z 105)
• Alpha (a) cleavage in alcohols, amines, ethers
• Aromatic rings do NOT fragment easily

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Characteristic Fragment Ions

• Look for alkyl ions at m/z 43, 57, 71, 85 (14 amu
  or CH2 series)

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Fragmentation of alkanes
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Characteristic Fragment Ions

• Look for aryl ions at 77 (phenyl), 91 (benzyl),
  and 105 (benzoyl)

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Fragmentation in Aromatics Tropylium Ion
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Characteristic Fragment Ions

• Acyl ions are stable (m/z 43, 57..)

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Acyl Fragment
acetophenone
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Characteristic Neutral Losses

• 15 can only be methyl
• 17 usually OH
• 18 always H2O
• 28 CO or CH2CH2
• 29 CH3CH2 or CHO
• 31 CH3O
• 35/37 Cl (special isotope pattern too!)
• 42 CH2CO
• 43 CH3CO or C3H7
• 45 CH3CH2O or COOH
• 79/81 Br (special isotope pattern too!)

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Weights of common fragments or neutral losses
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Mass SpectrometrySummary

• Molecular weight
• odd-numbered (N ...) ?
• isotope peaks (Cl, Br)?
• Alcohols
• Recognize typical fragments
• H2O, alkyl, acyl, tropylium ion

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• Next time
• Bring Spectra of your Unknown 3!
• Brief Review of Mass Spectroscopy
• Discussion/Review of H-NMR, CMR