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Advanced Organic Chemistry: Mass spectrum of 2-chloro-2-methylpropane

The mass spectrum of 2-chloro-2-methylpropane

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 2-chloro-2-methylpropane

C4H9Cl (CH3)3CCl mass spectrum of 2-chloro-2-methylpropane fragmentation pattern of m/z m/e ions for analysis and identification of  tert-butyl chloride image diagram doc brown's advanced organic chemistry revision notes 

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Interpreting the fragmentation pattern of the mass spectrum of 2-chloro-2-methylpropane

[M]+ is the molecular ion peak (M) with an m/z of 92 corresponding to [C4H9Cl]+, the original 2-chloro-2-methylpropane molecule minus an electron, [(CH3)3C35Cl]+.

Not here, but at a lower electron beam energy, you might see a tiny peak at m/z 94 (M+2) corresponding to another molecular ion containing the heavier isotope of chlorine,  [(CH3)3C37Cl]+.

Since chlorine has two common isotopes of 35Cl and 37Cl in the ratio 3 : 1, you should observe double peaks in the intensity ratio 3 : 1, two mass units apart for molecular fragments containing a chlorine atom from the fragmentation of 1-chlorobutane.

An example of this are quoted in the table below for m/z values of 79 and 77, you can see they are roughly in the ratio 3 : 1 in the mass spectrum diagram above.

You 'might' see another very tiny M+1 peak at m/z 92, corresponds to an ionised 2-chloro-2-methylpropane molecule with one 13C atom in it i.e. an ionised 2-chloro-2-methylpropane molecule of formula [13C12C3H935Cl]+

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.

2-chloro-2-methylpropane 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 (2-chloro-2-methylpropane) 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 2-chloro-2-methylpropane.

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

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

m/z value of [fragment]+ 79 77 57 56 55
[molecular fragment]+ [(CH3)2C37Cl]+ [(CH3)2C35Cl]+ [(CH3)3C]+ [C4H8]+ [C4H7]+
m/z value of [fragment]+ 42 41 39 29 28 27 15
[molecular fragment]+ [C3H6]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H4]+ [C2H3]+ [CH3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-chloro-2-methylpropane

Atomic masses: H = 1; C = 12; Cl = 35 or 37 (3:1)

Bond enthalpies kJ/mol: C-C = 348;  C-Cl = 338; C-H = 412

Possible equations to explain the most abundant ion peaks of 2-chloro-2-methylpropane

Note the molecular ion peak is very small indicating the molecule fragments very easily.

Formation of m/z 77 and 79 ions:

[(CH3)3C35Cl]+  ===>  [(CH3)2C35Cl]+  or  [(CH3)2C37Cl]+  +  CH3

Scission of the C-C bond to lose a methyl group, mass loss 92 - 15 = 77, 94 - 15 = 79.

Low probability due to strength of C-C bond, scission of the weaker C-Cl bond more likely.

Note the expected 3:1 ratio of intensities expected for chlorine containing fragment ions.

Formation of m/z 57 ion:

[(CH3)3CCl]+  ===>  [(CH3)3C]+  +  Cl

Formed by the scission of the C-Cl bond, the weakest bond in the 2-chloro-2-methylpropane molecule.

The m/z 57 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 57 ion is a tertiary carbocation, the most stable type of alkyl based ion, the positive charge is stabilised by the +I (inductive) effect of the three methyl groups.

Another reason why the ionised fragments, not containing chlorine, are more likely to be formed, is the more electronegative chlorine tends to make the the chlorine containing fragment retain the electrons.

Formation of m/z 41 ion:

[?]+  ===>  [C3H5]+  +  ?

Formation of m/z 39 ion:

[?]+  ===>  [C3H3]+  +  ?

Formation of m/z 29 ion:

[?]+  ===>  [C2H5]+  +  ?

Formation of m/z 27 ion:

[?]+  ===>  [C2H3]+  +  ?

Formation of m/z 15 ion:

[(CH3)3CCl]+  ===>  [CH3]+  +  (CH3)2CCl

C-C bond scission of the parent molecular ion (or other fragment) to free a positively charged methyl group

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