Chem 243 Chapter 13: Mass Spectrometry
Homework 13: 41, 50, 67, 69
I.
Mass Spectrometry (MS)
A. How does it work? (not for memorization!)
1. Molecule bombarded with high energy electrons and falls apart
2. charged pieces collected
3. analyzed for mass to nearest whole amu
4. fragments may continue to break up (more than once)
5. some very expensive MS machines do more significant figures
B. Terms
1. fragments: charged pieces collected
2. m/z: mass/charge charge is typically +1
3. molecular ion peak (M): mass of the molecule
4. M+1 peak: small peak 1 higher mass unit than M due to 13C
5. M+2 peak: peak 2 higher mass units than M due to isotopes of
Br and Cl
6. heterolytic cleavage: both electrons go with more
electronegative atom as bond breaks
7. hemolytic cleavage: electrons split between two atoms
C. Fragmentation
1. After having an electron removed, molecule is unstable and
fragments into a cation and a radical
2. more stable fragments usually formed in higher abundance
3. weaker bonds usually break more often
4. lone pair electron (e-) usually dislodged over bonding e5. C-C, C-H bonds break heterolytically
6. C-O, C-Br, C-Cl bonds break homolytically
7. M+1 peak of increasing intensity as # carbons increase
a. intensity of M+1 = #C x 0.011 x intensity of M
b. not necessary to memorize!
8. Example
D. By functional group
1. Bromides
a. M+2 peak of equal intensity
b. 79Br abundance = 81Br
c. C-Br bond is weakest, so usually breaks first
2. Chlorides
a. M+2 peak = a third of M peak
b. 35Cl abundance = three times 37Cl
c. C-Cl bond str. ~ equal to C-C, sometimes you get α
cleavage (C-C bond next to C-Cl breaks)
d. Example
3. Ethers
a. usually broken heterolytically, with O taking both e-,
leaving behind carbocation
b. or homolytically at α C-C bond
c. Examples: p523
4. Alcohols
a, pretty unstable, typically have small M peaks
b. typically break at alpha C-C
c. if gamma (γ) H, may break off H2O, mw 18
5. Ketones
a. typically break at α C-C
b. McLafferty rearrangement (p526) with γ H
i. beta C-C bond breaks after ketone snags γ H
Objectives
Knowledge
Describe m/z (mass to charge), base peak, parent or molecular ion (M) peak, M +1, M+2
peaks
Comprehension
Understand what a McLafferty rearrangement is and its implications are for mass spec
Application
Determine the presence or absence of Br or Cl from M+2 peak
Do not memorize formula, but if given, use it to calculate how many C’s from M+1 peak
(p518)
Analysis
Interpret mass spectrum and find molecular fragments
Inventory data available from the formula, IR, and mass spec to determine molecular
structure