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Advanced Organic Chemistry: Mass spectrum of 2-chlorobutane

The mass spectrum of 2-chlorobutane

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-chlorobutane

See also Comparing infrared, mass, 1H NMR & 13C NMR spectra of the 4 structural isomers of C4H9Cl

C4H9Cl CH3CHClCH2CH3 mass spectrum of 2-chlorobutane fragmentation pattern of m/z m/e ions for analysis and identification of sec-butyl chloride image diagram doc brown's advanced organic chemistry revision notes 

(c) doc b, (c) doc b, (c) doc b, (c) doc b, 2-chlorobutane

For more see Molecular structure, classification and naming of halogenoalkanes (haloalkanes)

Interpreting the fragmentation pattern of the mass spectrum of 2-chlorobutane

[M]+ is the molecular ion peak (M) with an m/z of 92 corresponding to [C4H9Cl]+, the original 2-chlorobutane molecule minus an electron, [CH3CH35ClCH2CH3]+. Since this ion is so unstable, there is even less chance of observing the m/z 94 M+2 ion [CH3CH37ClCH2CH3]+ (see note below on isotopes).

Since chlorine has two common isotopes of 35Cl and 37Cl in the approximate ratio of 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.

Two examples of this are quoted in the table below for m/z values of 79 and 77, 65 and 63, and 64 and 62, you can see they are roughly in the ratio 3 : 1 in the mass spectrum diagram above.

You might, but not here, see a very tiny M+1 peak at m/z 93, corresponds to an ionised 2-chlorobutane molecule with one 13C atom in it i.e. an ionised 2-chlorobutane 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-chlorobutane 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-chlorobutane) 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-chlorobutane.

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

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

m/z value of [fragment]+ 79 77 65 64 63 62
[molecular fragment]+ [CH3CH37ClCH2]+ [CH3CH35ClCH2]+ [CH2CH237Cl]+ [CH2CH37Cl]+ [CH2CH235Cl]+ [CH2CH35Cl]+
m/z value of [fragment]+ 58 ? 57 56 55 51 50 ? 49
[molecular fragment]+ [13CC3H8]+ [CH3CHCH2CH3]+ [C4H8]+ [C4H7]+ [CH237Cl]+ [CH335Cl]+ [CH235Cl]+
m/z value of [fragment]+ 43 42 41 39 29 28 27
[molecular fragment]+ [CH3CH2CH2]+ [C3H6]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H4]+ [C2H3]+

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

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 some of the most abundant ion peaks in the mass spectrum of 2-chlorobutane

Note the molecular ion peaks (M and M+2) are very small indicating the parent molecular ion of 2-chlorobutane fragments very easily.

Formation of m/z 77 and 79 ions:

[CH3CHClCH2CH3]+  ===>  [CH3CH37ClCH2]+  or   [CH3CH35ClCH2]+  +  CH3

C-C bond scission to free an end methyl group.

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

The ions could also be [CH37ClCH2CH3]+  and  [CH35ClCH2CH3]+, either way the m/z 77 and 79 ions correspond to [C3H6Cl]+.

Mass loss 92 - 15 = 77  and  94 - 15 = 79.

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

Formation of m/z 63 and 65 ions:

[CH3CHClCH2CH3]+  ===>   [CH3CH37Cl]+  or  [CH3CH37Cl]+  +  CH2CH3

C-C bond scission to free ethyl group.

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.

Mass loss 92 - 29 = 63  and  94 - 15 = 65.

Formation of m/z 62 and 64 ions:

[CHClCH2CH3]+  ===>  [CH2CH35Cl]+  or  [CH2CH37Cl]+  +  CH3

C-C bond scission to free a methyl group from the m/z 77 and 79 ions.

Mass losses: 77 - 15 = 62  and  79 - 15  =  64 for the fragment ion [C2H3Cl]+.

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:

[CH3CHClCH2CH3]+  ===>  [CH3CHCH2CH3]+  +  Cl

Formed by the scission of the C-Cl bond, the weakest bond in the 2-chlorobutane 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 secondary carbocation, a stable type of alkyl based ion, the positive charge is stabilised by the +I (inductive) effect of the two alkyl groups.

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

Note the m/z peak of 58 could correspond with the ion [13C12C3H9]+.

Formation of m/z 56 ion:

[CH3CHClCH2CH3]+  ===>  [C4H8]+  +  HCl

Elimination of hydrogen chloride from the parent molecular ion.

A favourable reaction, since the m/z 56 ion intensity is almost the same as the m/z 57 base ion peak.

Formation of m/z 41 ion:

[C4H8]+  ===>  [C3H5]+  +  CH3

Formation of m/z 39 ion:

[C3H5]+  ===>  [C3H3]+  +  H2

Formation of m/z 29 ion:

[CH3CHClCH2CH3]+  ===>  [C2H5]+  +  CH3CHCl

C-C bond scission in the parent molecular ion.

Formation of m/z 28 ion:

[C2H5]+  ===>  [C2H4]+  +  H

Ionised ethene molecule formed.

Formation of m/z 27 ion:

[C2H5]+  ===>  [C2H3]+  +  H2

Formation of m/z 15 ion:

[(CH3)3C35Cl]+  ===>  [CH3]+  +  (CH3)2CCl

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

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 4 halogenoalkane isomers of C4H9Cl

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane and 2-chloro-2-methylpropane image sizes.  These four molecules are structural isomers of molecular formula C4H9Cl and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic halogenoalkanes (haloalkanes, alkyl halides, chloroalkanes, alkyl chlorides).

INFRARED SPECTRA (above): Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, there are no other great striking differences, but each could be identified from its infrared spectrum. The infrared spectrum of 2-chloro-2-methylpropane is noticeably simpler in the fingerprint region, perhaps due to the greater symmetry of the molecule.

MASS SPECTRA (above): Theoretically, all four can give the parent molecular ions of m/z 92 and 94, but they are all relatively tiny peaks. 2-chlorobutane and 2-chloro-2-methylpropane give a base ion peak of m/z 57. The base ion peak for 1-chlorobutane is m/z 56 and that of 1-chloro-2-methylpropane is m/z 43. Each gives different patterns of pairs of m/z values two mass units apart, in the peak height ratio of 3:1, if the positive fragment contains a chlorine atom (35Cl or 37Cl) e.g look for m/z pairs 49/51, 63/65 and 77/79 in their mass spectra.

1H NMR SPECTRA (above): The 1H NMR spectra of all four molecules give different integrated proton ratios i.e.1-chlorobutane four peaks of ratio 3:2:2:2; 2-chlorobutane four peaks of ratio 3:3:2:1, 1-chloro-2-methylpropane three peaks of ratio 6:2:1 and 2-chloro-2-methylpropane gives just one peak '1' (effectively no ratio involved), so all four molecular structures can be distinguished from each other by their 1H NMR spectra proton ratios, numbers of peaks and (n+1) rule splitting patterns.

13C NMR SPECTRA (above): The 13C NMR spectra of the four molecules show various numbers of carbon-13 chemical environments i.e 1-chlorobutane and 2-chlorobutane show four 13C NMR resonances, 1-chloro-2-methylpropane three 13C NMR resonances and 2-chloro-2-methylpropane only two 13C resonances (3 and 2 chemical environments respectively. Therefore 1-chloro-2-methylpropane and 2-chloro-2-methylpropane can be distinguished from the other three by their number of resonances in their 13C NMR spectra, but 1-chlorobutane and 2-chlorobutane cannot be distinguished from each other from their number of 13C NMR resonance lines - other data would be required.

Key words & phrases: C4H9Cl CH3CHClCH2CH3 image diagram on how to interpret and explain the mass spectrum of 2-chlorobutane m/z m/e base peaks, image and diagram of the mass spectrum of 2-chlorobutane, details of the mass spectroscopy of 2-chlorobutane,  low and high resolution mass spectrum of 2-chlorobutane, prominent m/z peaks in the mass spectrum of 2-chlorobutane, comparative mass spectra of 2-chlorobutane, the molecular ion peak in the mass spectrum of 2-chlorobutane, analysing and understanding the fragmentation pattern of the mass spectrum of 2-chlorobutane, characteristic pattern of peaks in the mass spectrum of 2-chlorobutane, relative abundance of mass ion peaks in the mass spectrum of 2-chlorobutane, revising the mass spectrum of 2-chlorobutane, revision of mass spectroscopy of 2-chlorobutane, most abundant ions in the mass spectrum of 2-chlorobutane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 2-chlorobutane, how to analyse the mass spectrum of 2-chlorobutane, how to describe explain the formation of fragmented ions in the mass spectra of 2-chlorobutane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 2-chlorobutane recognising the base ion peak of 2-chlorobutane interpreting interpretation the mass spectrum of 2-chlorobutane sec-butyl chloride

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 2-chlorobutane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 2-chlorobutane. The m/e m/z value of the molecular ion peak in the mass spectrum of 2-chlorobutane.  The m/e m/z value of the base ion peak in the mass spectrum of 2-chlorobutane. Possible examples of equations showing the formation of the ionised fragments in 2-chlorobutane. Revision notes on the mass spectrum of 2-chlorobutane. Matching and deducing the structure of the 2-chlorobutane molecule from its mass spectrum. Mass spectroscopy of  aliphatic halogenoalkanes haloalkanes alkyl halides alkyl chlorides chloroalkanes, mass spectra of 2-chlorobutane, an isomer of molecular formula C4H9Cl


Links associated with 2-chlorobutane

The chemistry of HALOGENOALKANES (haloalkanes) revision notes INDEX

The infrared spectrum of 2-chlorobutane (sec-butyl chloride)

The H-1 NMR spectrum of 2-chlorobutane (sec-butyl chloride)

The C-13 NMR spectrum of 2-chlorobutane (sec-butyl chloride)

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