Wize University Chemistry Textbook > Structure Elucidation (IR Spectroscopy, NMR, Mass Spectrometry)

Mass Spectrometry

Mass Spectrometry (MS)

Key Definitions

1. Mass/Charge Ratio – Weight of each ion fragment produced by ionization

2. Ion Abundance/Intensity – Amount of each ion produced (%)

3. Base Peak – Tallest peak in spectrum (most stable)

4. Molecular Ion – Peak of original starting compound

5. Isotope Peaks – Ions containing heavier isotopes (12C vs. 13C)

Mass Spectrum of Benzoic Acid

Mass Spectrum of an Alcohol - 2-propanol

Alcohol molecular ions are rarely observed, they rapidily fragment. The oxygen atom will stabilize the radical via resonance.

Isotopes in Mass Spectrometry

Different isotopes of atoms will be seen in a mass spectrum. In some cases, these isotope peaks are fairly large and can serve as diagnostic tools. For example, Boron, Chlorine and Bromine have very distinct isotope patterns.

Carbon: 12C (99%), 13C (1%)

Boron: 10B (20%), 11B (80%)

Chlorine: 35Cl (76%), 37Cl (24%)

Bromine: 79Br (51%), 81Br (49%)

Mass Spectrum of Tert-butyl Bromide

Using MS to determining # of Carbons

We can use MS to estimate the number of carbon atoms in a compound. It uses the ratio of the mass peak for the 13C isotope to the mass peak of the 12C isotope. The equation we use is as follows:

Max # of Carbons = [M+1]+/M+ x 100

We can explore this relationship looking at the MS spectrum of n-hexane:

Peak, m/z (% abundance): M+, 86.0 (22.4); [M+1]+, 87.0 (1.4); [M-(C2H5)]+, 57.0 (100); [(M+1)-(C2H5)]+, 58.0 (4.4)

$\frac{1.4}{22.4}\cdot100\ =\ 6.3;\ \frac{4.4}{100}\cdot100\ =\ 4.4$

The mass spectrum of (E)-1,2-dibromoethene is shown below. Rationalize the major spectrum peaks at 187.9 (49%), 185.9 (100%), 183.9 (51%), 106.9 (97%), and 104.9 (100%)

Image illustrating Example: Mass Spectrometry 1

Carbon-halogen bonds will often cleave in a mass spectrometer because the bromide radical is stable. Bromide containing compounds often have a distinct isotope pattern, and multiple bromine atoms further complicate the spectrum:

Below is the mass spectrum of 2-pentene. Label the molecular ion and base peak. Provide the structure of the base peak and rationalize its formation.

Image illustrating Example: Mass Spectrometry 2

The mass spectrum of tert-butyl chloride (2-chloro-2-methylpropane) does not show it's molecular ion. There is three key diagnostic signals at m/z = 79, 77, and 57. What are the structures of the compounds that give rise to these signals (hint: be specific with any heteroatom isotopes).

Image illustrating Example: Mass Spectrometry 3

The Mass spectrum of 3-bromopentane is shown below. Explain what each of the labeled peaks represents. Note: 81Br and 79Br exist naturally in a 50:50 ratio

Image illustrating Mass Spectrometry Practice

152: 81:

150: 79:

123: 71:

121: 29:

$152\ :\ M^+with\ ^{81}Br$

$150\ :\ M^+with\ ^{79}Br$

$123:\ [CH_3CH_2CH_2Br]^+with\ ^{81}Br$

$121\ :\ [CH_3CH_2CH_2Br]^+\ with\ ^{79}Br$

$81\ :\ ^{81}Br$

$79\ :\ ^{79}Br$

$71:[CH_3CH_2CHCH_2CH_3]^+$

$29\ :\ [CH_2CH_3]^+$

Mark Yourself Question

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Explain what each peak represents on the following mass spectrum:

Illustration for practice question: Mass Spectrum Practice Problem

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Mark Yourself Question

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The following is the spectra for 2-methyl-2-hexene. Which fragment of the molecule is represented by the peak at 69m/z?

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Extra Practice

IR Spectroscopy, NMR and Mass spectrometry

From the spectra provided, identify the structure of the unknown molecule.

MS:

IR:

Mass Spectrometry

In the following EI MS Spectrum, we can confirm that what functionality is present based on the pattern?

Mass Spectrometry

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