Mechanism of Phenylurethane 1-butanol and phenyl isocyanate

Mass Spectrometry Interpretation 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

A Mass Spectrum tells you the molecular mass 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

Molecular Ion Peak: Odd or Even Mass? aniline

Isotope Peaks: 35Cl and 37Cl

Isotope Peaks: 79Br and 81Br

Molecular Ion Peak: Alcohols … 1-butanol

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..) PhC=O+ (m/z 105…) Alpha (a) cleavage in alcohols, amines, ethers Aromatic rings do NOT fragment easily

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

Fragmentation of alkanes

Characteristic Fragment Ions Look for aryl ions at 77 (phenyl), 91 (benzyl), and 105 (benzoyl)

Fragmentation in Aromatics Tropylium Ion

Characteristic Fragment Ions Acyl ions are stable (m/z 43, 57..)

Acyl Fragment acetophenone

Characteristic Neutral Losses 15 can only be methyl 17 usually OH 18 always H2O 28 CO or CH2=CH2 29 CH3CH2 or CHO 31 CH3O 35/37 Cl (special isotope pattern too!) 42 CH2=C=O 43 CH3CO or C3H7 45 CH3CH2O or COOH 79/81 Br (special isotope pattern too!)

Weights of common fragments or neutral losses

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

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