Interpreting the fragmentation pattern of the mass spectrum of butan-2-ol

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₄H₁₀O]⁺, the original butan-2-ol molecule minus an electron, [CH₃CH(OH)CH₂CH₃]⁺.

This is a tiny peak, indicating the molecular ion is very unstable.

You might see an even smaller M+1 peak at m/z 75, corresponding to an ionised butan-2-ol molecule with one ¹³C atom in it i.e. an ionised butan-2-ol molecule of formula [¹³C¹²C₃H₁₀O]⁺

Carbon-13 only accounts for ~1% of all carbon atoms (¹²C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this ¹³C M+1 peak.

Butan-2-ol has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (butan-2-ol) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of butan-2-ol.

Unless otherwise indicated, assume the carbon atoms in butan-2-ol are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of butan-2-ol.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of butan-2-ol

Atomic masses: H = 1; C = 12; N = 14; O = 16;  Cl = 35 or 37 (3:1);  Br = 79 or 81 (1:1);

Bond enthalpies kJ/mol: C-C = 348;  C-Cl = 338; C-Br 276; C-I = 238; C-H = 412;  C-O = 360;  O-H = 463

Possible equations to explain the most abundant ion peaks of butan-2-ol (tabulated above)

Formation of m/z 73 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [C₄H₉O]⁺  +  H

C-H bond scission an loss of proton, mass change 74 - 1 = 73.

Formation of m/z 59 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [C₃H₇O]⁺  +  CH₃

C-C bond scission an loss of methyl group from either end (?) of the parent molecular ion.

Formation of m/z 57 ion:

[?]⁺  ===>  [C₃H₅O]⁺  +  ?

Could be a [C₄H₉]⁺ ion ?

Formation of m/z 56 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [C₄H₈]⁺  +  H₂O

Elimination of water from the parent molecular ion to give a positively ionised molecule of a butene.

Formation of m/z 55 ion:

[C₄H₉O]⁺  ===>  [C₄H₇]⁺  +  H₂O

Elimination of water from the m/z 73 ion. Mass change 73 - 18 = 55

OR C-C bond scission of the parent molecular ion, mass change 74 - 29 = 55.

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₃CH(OH)]⁺  +  CH₂CH₃

Formation of m/z 45 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₃CHOH]⁺  +  CH₂CH₃

C-C bond scission of the parent molecular ion, mass change 74 - 29 = 45.

The m/z 45 ion is the base peak ion, the most abundant and 'stable' ion fragment.

Formation of m/z 29 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  CH₃CHOH

C-C bond scission of the parent molecular ion, mass change 74 - 45 = 29.

Formation of m/z 15 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₃]⁺  +  C₃H₇O

C-C bond scission to break off a methyl group (from either end of molecule?).

Mass change 74 - 59 = 15 (see also formation of m/z 59, either fragment may be ionised.

The chemistry of ALCOHOLS revision notes INDEX

Mass spectroscopy index

ALL SPECTROSCOPY INDEXES

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