MiniQuiz 6 Answers: Which of the compounds below do you expect to absorb light in the UV-region? (circle) Hexane Ethanol Benzene Naphthalene Explain your answer: Both benzene and naphthalene have several conjugated double bonds (none in hexane nor ethanol).

Mass Spectrometry Next time: H-NMR, CMR Following Tuesday (May 11): Your questions Final Exam in Lab Today:

Mass Spectrometry Interpretation of Mass Spectra 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 Study Mass Spectrometry Chapters in Lab and Lecture Texts

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

Molecular Ion Peak: Odd or Even Mass? aniline

Isotope Peaks: 35 Cl and 37 Cl

Isotope Peaks: 79 Br and 81 Br

Molecular Ion Peak: Alcohols … 1-butanol MW: 74

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 (  ) 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 CH 2 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 1 is H (e.g. from aldehydes) 15 can only be methyl 17 usually OH 18 always H 2 O 28 CO or CH 2 =CH 2 29 CH 3 CH 2 or CHO 31 CH 3 O 35/37 Cl (special isotope pattern too!) 42 CH 2 =C=O 43 CH 3 CO or C 3 H 7 45 CH 3 CH 2 O or COOH 79/81 Br (special isotope pattern too!)

Weights of common fragments or neutral losses

Molecular weight: odd-numbered (N...) ? isotope peaks (Cl, Br)? Alcohols … Recognize typical fragments: H 2 O, alkyl, acyl, tropylium ion … Mass Spectrometry Summary

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