Chemistry 231
Mass Spectrometry Notes
Mass Spectrometry
Mass Spectrometry
„ Mass Spectrometry is another technique used by organic chemists to analyze the structure of
compounds.
„ Mass Spectrometry (MS) is distinctly different from IR, UV-VIS, and NMR, which all rely on
characteristic absorptions and emissions of EM by a sample.
„ In a mass spectrometer, a sample of a compound is vaporized and ionized, literally ripping the molecule
apart into distinct fragments.
„ The most common method of ionization is to bombard the vaporized molecules with high energyelectrons.
„ The result is the formation of a radical cation also known as the molecular ion.
Mass Spectrometry
„ Consider the fragmentation of pentane:
„ The way a particular molecular ion fragments depends on the strength of its bonds and the stability of
the fragments.
„ Always remember the relative stabilities of carbocations!
m/z Values
„ The m/z value for a fragment is simply the molecular mass of the fragment divided by the charge of the
fragment.
„ Since z = +1 for most ionic fragments, the m/z is usually the mass of the fragment.
„ Note that non-charged fragments (uncharged radicals) are not detected by the spectrometer, and
therefore do not appear in the spectrum.
Peaks in an MS Spectrum
„ The most important peak in the MS spectrum from a diagnostic standpoint is the molecular ion peak
(M).
„ This peak is often the heaviest significant peak in the spectrum, though other very small heavier peaks
may be observed (more on these later…)
„ The m/z value of this peak generally tells us the approximate molecular mass of the compound. The
more sensitive the spectrometer, the more useful the result.
„ The base peak is the most intense (i.e. highest peak) in the spectrum.
„ It is arbitrarily assigned a relative abundance of 100; the intensity of all other peaks are scaled against it.
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Chemistry 231
Mass Spectrometry Notes
„ Peaks are commonly observed with m/z values 1 or 2 units smaller than the m/z for a given peak, usually
resulting from a loss of a hydrogen atom.
Another Example: 2-Methylbutane
„ 2-methylbutane has the same molecular formula as pentane, hence the m/z value for the molecular ion is
the same
„ The mass spectrum of 2-methylbutane is very similar to that of pentane, with the exception that the peak
at m/z = 57 is much more intense.
„ 2-Methylbutane easily loses a methyl group to form a secondary carbocation.
Fragmentation at Functional Groups: Halides
„ If a molecule contains nonbonding electrons, electron bombardment generally dislodges one of these
nonbonding electrons (they are not held as tightly as bonding electrons).
„ An electron is ejected from the bromine atom in1-bromopropane
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„ Two molecular ions are formed because Br is a mixture of Br and Br.
„ These isotopes are found in nearly equal abundance in nature, so with bromides you will often observe
pairs of peaks of equal height, separated by exactly 2.
„ The easiest bond to break in the resulting molecular ion is the C–Br bond; it breaks heterolytically.
Fragmentation at Functional Groups: Halides
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„ Chlorine also has two common isotopes: Cl and Cl, which are found in nature in about a 3 to 1 ratio.
„ Like bromides, you will see twins of peaks with chlorides, but the one with the lower m/z value will be
about three times taller than the one at the higher value.
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Chemistry 231
Mass Spectrometry Notes
Fragmentation at Functional Groups: Ethers
„ Electron bombardment dislodges a nonbonding electron from the oxygen of an ether
This may follow two distinct pathways
„ Heterolytic cleavage of the C–O bond occurs with electrons going to the more electronegative atom, the
oxygen.
„ Homolytic cleavage of the C–C bond also takes place at the α−position.
Fragmentation at Functional Groups: Alcohols
„ The molecular ions of alcohols fragment so readily that they rarely are observed.
„ Primary & secondary alcohols show weak molecular ion peaks.
„ Tertiary alcohols show no molecular ion peak.
„ Alcohols, like alkyl halides and ethers, undergo α-cleavage, with the largest α-substituent being the
most readily cleaved.
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Chemistry 231
Mass Spectrometry Notes
„ Alcohols also eliminate water molecules, undergoing intramolecular abstraction of a gamma (γ) proton,
assuming one exists.
Fragmentation at Functional Groups: Ketones
„ The mass spectrum of a ketone generally has an intense molecular ion peak.
„ Ketones fragment via α-cleavage with the largest α-substituent being the most readily cleaved
„ Ketones also tend to eliminate an alkene via a rearrangement known as the McLafferty
rearrangement.
„ Esters and alkenes will also often do this.
„ Like alcohols, this involves the intramolecular abstraction of a gamma proton.
Fragmentation at Functional Groups: Benzene Rings
„ Benzene rings which are substituted with alkyl groups often give a diagnostic peak at 91, indicating the
formation of the tropylium cation.
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Br
Spectrum 1
OH
Spectrum 2
O
Spectrum 3
Spectrum 4
O
O
Spectrum 5