Chapter12 - 12 Structure Determination Mass Spectrometry...

12. Structure Determination: Mass
Spectrometry and Infrared
Spectroscopy
Based on
McMurry’s Organic Chemistry , 6
th
edition
©2003 Ronald Kluger
Department of Chemistry
University of Toronto

McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 2
Determining the Structure of an
Organic Compound

The analysis of the outcome of a reaction requires
that we know the full structure of the products as well
as the reactants
 In the 19
th
and early 20
th
centuries, structures were
determined by synthesis and chemical degradation
that related compounds to each other

Physical methods now permit structures to be
determined directly. We will examine:

mass spectrometry (MS)

infrared (IR) spectroscopy

nuclear magnetic resonance spectroscopy (NMR)

ultraviolet-visible spectroscopy (VIS)

McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 3
12.1 Mass Spectrometry (MS)

Measures molecular weight
 Sample vaporized and subjected to bombardment by
electrons that remove an electron

Creates a cation-radical

Bonds in cation radicals begin to break (fragment)

Charge to mass ratio is measured (see Figure 12-1)

McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 4
The Mass Spectrum
 Plot mass of ions (m/z) (x-axis) versus the intensity of
the signal (roughly corresponding to the number of
ions) (y-axis)

Tallest peak is base peak (100%)

Other peaks listed as the % of that peak

Peak that corresponds to the unfragmented radical
cation is parent peak or molecular ion ( M
+
)

McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 5
MS Examples: Methane and Propane
 Methane produces a parent peak (m/z = 16) and
fragments of 15 and 14 (See Figure 12-2 a)

The MS of propane is more complex (Figure 12-2 b)
since the molecule can break down in several ways

McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 6
12.2 Interpreting Mass Spectra
 Molecular weight from the mass of the molecular ion

Double-focusing instruments provide high-resolution
“exact mass”

0.0001 atomic mass units – distinguishing specific
atoms

Example MW “72” is ambiguous: C
5
H
12
and C
4
H
8
O
but:

C
5
H
12
72.0939 amu exact mass C
4
H
8
O 72.0575 amu
exact mass

Result from fractional mass differences of atoms
16
O =
15.99491,
12
C = 12.0000,
1
H = 1.00783
 Instruments include computation of formulas for each
peak

McMurry Organic Chemistry 6th edition Chapter 12 (c) 20 7
Other Mass Spectral Features
 If parent ion not present due to electron