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Advanced Organic Chemistry: Mass spectrum of butan-2-ol  (2-butanol)

Interpreting the mass spectrum of butan-2-ol  (2-butanol, sec-butyl alcohol)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of butan-2-ol

C4H10O CH3CH(OH)CH2CH3 mass spectrum of butan-2-ol fragmentation pattern of m/z m/e ions for analysis and identification of 2-butanol sec-butyl alcohol image diagram doc brown's advanced organic chemistry revision notes 

Butan-2-ol    C4H10O    alcohols and ether structure and naming (c) doc b    alcohols and ether structure and naming (c) doc b    alcohols and ether structure and naming (c) doc b 

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.

m/z value of [fragment]+ 73 59 57 56 55 ? 55? 45 44 ? 44 ?
[molecular fragment]+ [C4H9O]+ [C3H7O]+ [C3H5O]+ [C4H8]+ [C2H5O]+ [C4H7]+ [CH3CHOH]+ [C3H8]+ [C2H4O]+
m/z value of [fragment]+ 43 ? 43 ? 42 41 39 31 ? 29 28 27 15
[molecular fragment]+ [C3H7]+ [C2H3O]+ [C3H6]+ [C2H4O]+ [C3H3]+ [CH3O]+ [CH3CH2]+ [C2H4]+ [C2H3]+ [CH3]+

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.


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