Organic Chemistry

516 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

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Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance

O

CH 2 CCH 2 CH 3

Figure 13.33

The IR spectrum of Compound 5.

Summary

Mass spectrometryallows us to determine the molecular
massand the molecular formulaof a compound, as well as
certain structural features. In mass spectrometry, a small
sample of the compound is vaporized and then ionized as a
result of an electron’s being removed from each molecule,
producing a molecular ion—a radical cation. Many of the
molecular ions break apart into cations, radicals, neutral
molecules, and other radical cations. The bonds most like-
ly to break are the weakest ones and those that result in the
formation of the most stable products. The mass spectrom-
eter records a mass spectrum—a graph of the relative
abundance of each positively charged fragment, plotted
against its value.
The molecular ion (M) peak is due to the fragment that re-
sults when an electron is knocked out of a molecule; the m>z

m>z

value of a molecular ion gives the molecular mass of the

compound. The “nitrogen rule”states that if a compound has

an odd-mass molecular ion, the compound contains an odd

number of nitrogen atoms. Peaks with smaller values—

fragment ion peaks—represent positively charged frag-

ments of the molecule. The base peakis the peak with the

greatest intensity. High-resolution mass spectrometers deter-

mine the exact molecular mass, which allows a compound’s

molecular formula to be determined.

The peak occurs because there are two naturally

occurring isotopes of carbon. The number of carbon atoms in

a compound can be calculated from the relative intensities of

the M and peaks. A large peak is evidence of

a compound containing either chlorine or bromine; if it is

one-third the height of the M peak, the compound contains

M+ 1 M+ 2

M+ 1

m>z

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Wavelength (μm)

Wavenumber (cm−^1 )

% Transmittance

PROBLEM 29

A compound with molecular formula gives the infrared spectrum shown in

Figure 13.34. Identify the compound.

C 4 H 6 O

Figure 13.34

The IR spectrum for Problem 29.