Transcript Mass Spectrometry

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).
Today:
Mass Spectrometry
Next time:
H-NMR, CMR
Following Tuesday (May 11):
Your questions
Final Exam in Lab
Mass Spectrometry
Interpretation of Mass Spectra
Study 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
+
+
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
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 (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
•
•
•
•
•
•
•
•
•
•
•
•
1 is H (e.g. from aldehydes)
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
O
15 = CH3 18 = H2O 29 = C2H5 43 = C3H7 or
O
31 = CH3O
O
57 = C4H9 or
71 = C5H11 or
45 = CH3CH2O
O
79Br 81Br
85 = C6H13 or 35Cl
77 =
O
C
CH2
91 =
105 =
37Cl
Mass Spectrometry
Summary
Molecular weight:
•
odd-numbered (N ...) ?
•
isotope peaks (Cl, Br)?
•
Alcohols …
Recognize typical fragments:
H2O, alkyl, acyl, tropylium ion …
Next time:
Bring Spectra of your Unknown #3!
• Brief Review of Mass Spectroscopy
• Discussion/Review of H-NMR, CMR