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Advanced Organic Chemistry: Mass spectrum of heptane

The mass spectrum of heptane

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 heptane

See also comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16

C7H16 mass spectrum of heptane fragmentation pattern of m/z m/e ions for analysis and identification of heptane image diagram doc brown's advanced organic chemistry revision notes 

Heptane, C7H16 , CH3(CH2)5CH3 , alkanes structure and naming (c) doc b , alkanes structure and naming (c) doc b

For more see The molecular structure, classification and naming of alkanes

Interpreting the fragmentation pattern of the mass spectrum of heptane

[M]+ is the molecular ion peak (M) with an m/z of 100 corresponding to [C7H16]+, the original heptane molecule minus an electron, [CH3(CH2)5CH3]+.

The small M+1 peak at m/z 101, corresponds to an ionised heptane molecule with one 13C atom in it i.e. an ionised heptane molecule of formula 13C12C6H16

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.

Heptane has 7 carbon atoms, so on average, ~1 in 14 molecules of will contain a 13C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of heptane - though the ratio will be greater:

e.g. m/z 44 ion could be [13C12C2H7]+, m/z 58 ion [13C12C3H9]+, and m/z 72 ion [13C12C4H11]+

These ions might be more likely than those containing only 12C isotope atoms

i.e. [C3H8]+, [C4H10]+ and [C5H12]+

Either way, for identification purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of heptane

The most abundant ion of the molecule under mass spectrometry investigation (heptane) 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 heptane.

Unless otherwise indicated, assume the carbon atoms in heptane are the 12C isotope.

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

The parent molecular ion of heptane m/z 100: [C7H16]+

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of heptane.

m/z value of [fragment]+ 85 71 70 57 56 55
[molecular fragment]+ [C6H13]+ [C5H11]+ [C5H10]+ [C4H9]+ [C4H8]+ [C4H7]+
m/z value of [fragment]+ 43 42 41 39 29 27
[molecular fragment]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [C2H5]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of heptane

Examples of equations to explain some of the most abundant ion peaks in the mass spectrum of heptane

Atomic masses: H = 1;  C = 12 (13 for ~1 in 100)

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

Formation of m/z 85 ion:

[CH3(CH2)5CH3]+  ===>  [CH3(CH2)5]+  +  CH3

Most fragmentation in alkanes arises from C-C bond scission (C-C bond weaker than C-H).

In this case and end methyl group is broken off by C-C bond scission in the parent molecular ion, mass change = 100 - 15 = 85.

Note in the equations below that both fragments are capable of being ionised, but only one at a time.

Many other fragments are formed by hydrogen atom/molecule loss, so get m/z ion sequences for heptane (and its isomers) like 71 ==>70, 57 ==> 56 ==> 55, 43 ==> 39 and 29 ==> 27 etc. (see examples below).

Ethene is often eliminated to give a smaller fragment e.g. the m/z ion 85 gives the m/z ion 57

[CH3(CH2)5]+  ===>  [CH3(CH2)3]+  +  C2H4

Mass change 85 - 28 = 57.

Formation of m/z 71 ion:

[CH3(CH2)5CH3]+  ===>  [CH3(CH2)4]+  +  CH2CH3

C-C bond scission in the parent molecular ion of 3-methylhexane.

Here an end ethyl group is broken off, mass change = 100 - 29 = 71.

Formation of m/z 57 ion:

[CH3(CH2)5CH3]+  ===>  [CH3(CH2)3]+  +  CH2CH2CH3

C-C bond scission in the parent molecular ion.

Loss of propyl group, mass change = 100 - 43 = 57

Formation of m/z 43 ion:

[CH3(CH2)5CH3]+  ===>  [CH2CH2CH3]+  +  CH3(CH2)3

C-C bond scission in the parent molecular ion.

Loss of butyl group, mass change = 100 - 57 = 43

The m/z 43 ion is the base ion peak, the most abundant and 'stable' ion fragment.

Formation of m/z 29 ion:

[CH3(CH2)5CH3]+  ===>  [CH2CH3]+  +  CH3(CH2)4

C-C bond scission in the parent molecular ion.

Here an end ethyl group is broken off and becomes ionised, mass change = 100 - 71 = 29.

Comparing the 1H NMR and 13C NMR spectra of the nine alkane structural isomers of C7H16

You can distinguish all 9 isomers from a data combination of their number of 1H NMR chemical shifts,

and their resulting integrated 1H proton ratios, plus, their number of 13C chemical shifts.

Name of the alkane structural isomer of molecular formula C7H16 Abbreviated structural formulae of the nine isomers of molecular formula C7H16 (interpretation complications with 3-methylhexane and 2,3-dimethylpentane because they exhibit R/S isomerism due to a chiral carbon) Skeletal formula of the nine alkane isomers of  molecular formula C7H16 Number of 1H NMR chemical shifts (δ) and proton ratio (links to spectrum) Number of 13C chemical shifts (δ) (links to spectrum)
heptane structural formula skeletal formula alkanes molecular structure naming (c) doc b heptane skeletal formula alkanes molecular structure naming (c) doc b 4 δ: proton ratio: 3:2:2:1 (6:4:4:2 in the molecule) 4 δ shifts
2-methylhexane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2-methylhexane skeletal formula alkanes molecular structure naming (c) doc b 6 δ: proton ratio : 6:3:2:2:2:1 6 δ shifts
3-methylhexane structural formula skeletal formula alkanes molecular structure naming (c) doc b 3-methylhexane skeletal formula alkanes molecular structure naming (c) doc b 7 δ: proton ratio: 3:3:3:2:2:2:1 (simplification) !!! 7 δ shifts
3-ethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 3-ethylpentane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 9:6:1 3 δ shifts
2,2-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,2-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 4 δ: proton ratio: 9:3:2:2 5 δ shifts
2,3-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,3-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 6 δ: proton ratio: 6:3:3:2:1:1 (simplification) !!! 6 δ shifts (simplification) !!!
2,4-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,4-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 12:2:2 3 δ shifts
3,3-dimethylpentane structural formula skeletal formula alkanes molecular structure naming (c) doc b 3,3-dimethylpentane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 3:3:2 (6:4:4 in the molecule) 4 δ shifts
2,2,3-trimethylbutane structural formula skeletal formula alkanes molecular structure naming (c) doc b 2,2,3-trimethylbutane skeletal formula alkanes molecular structure naming (c) doc b 3 δ: proton ratio: 9:6:1 4 δ shifts

Key words & phrases: C7H16 image diagram on how to interpret and explain the mass spectrum of heptane m/z m/e base peaks, image and diagram of the mass spectrum of heptane, details of the mass spectroscopy of heptane,  low and high resolution mass spectrum of heptane, prominent m/z peaks in the mass spectrum of heptane, comparative mass spectra of heptane, the molecular ion peak in the mass spectrum of heptane, analysing and understanding the fragmentation pattern of the mass spectrum of heptane, characteristic pattern of peaks in the mass spectrum of heptane, relative abundance of mass ion peaks in the mass spectrum of heptane, revising the mass spectrum of heptane, revision of mass spectroscopy of heptane, most abundant ions in the mass spectrum of heptane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of heptane, how to analyse the mass spectrum of heptane, how to describe explain the formation of fragmented ions in the mass spectra of heptane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of heptane recognising the base ion peak of heptane interpreting interpretation the mass spectrum of heptane

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of heptane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of heptane. The m/e m/z value of the molecular ion peak in the mass spectrum of heptane.  The m/e m/z value of the base ion peak in the mass spectrum of heptane. Possible examples of equations showing the formation of the ionised fragments in heptane. Revision notes on the mass spectrum of heptane. Matching and deducing the structure of the heptane molecule from its mass spectrum. Mass spectroscopy of aliphatic alkanes, mass spectra of heptane, a structural isomer of molecular formula C7H16


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Infrared spectra of the isomers of C7H16

The infrared spectrum of heptane

The infrared spectrum of 2-methylhexane

The infrared spectrum of 3-methylhexane

The infrared spectrum of 3-ethylpentane

The infrared spectrum of 2,2-dimethylpentane

The infrared spectrum of 2,3-dimethylpentane

The infrared spectrum of 2,4-dimethylpentane

The infrared spectrum of 3,3-dimethylpentane

The infrared spectrum of 2,2,3-trimethylbutane

Mass spectra of the isomers of C7H16

The mass spectrum of heptane

The mass spectrum of 2-methylhexane

The mass spectrum of 3-methylhexane

The mass spectrum of 3-ethylpentane

The mass spectrum of 2,2-dimethylpentane

The mass spectrum of 2,3-dimethylpentane

The mass spectrum of 2,4-dimethylpentane

The mass spectrum of 3,3-dimethylpentane

The mass spectrum of 2,2,3-trimethylbutane

H-1 proton NMR spectra of ALKANES

1H NMR spectra of the isomers of C7H16

The H-1 NMR spectrum of heptane

The H-1 NMR spectrum of 2-methylhexane

The H-1 NMR spectrum of 3-methylhexane

The H-1 NMR spectrum of 3-ethylpentane

The H-1 NMR spectrum of 2,2-dimethylpentane

The H-1 NMR spectrum of 2,3-dimethylpentane

The H-1 NMR spectrum of 2,4-dimethylpentane

The H-1 NMR spectrum of 3,3-dimethylpentane

The H-1 NMR spectrum of 2,2,3-trimethylbutane

C-13 carbon-13 NMR spectra of ALKANES

13C NMR spectra of the isomers of C7H16

The C-13 NMR spectrum of heptane

The C-13 NMR spectrum of 2-methylhexane

The C-13 NMR spectrum of 3-methylhexane

The C-13 NMR spectrum of 3-ethylpentane

The C-13 NMR spectrum of 2,2-dimethylpentane

The C-13 NMR spectrum of 2,3-dimethylpentane

The C-13 NMR spectrum of 2,4-dimethylpentane

The C-13 NMR spectrum of 3,3-dimethylpentane

The C-13 NMR spectrum of 2,2,3-trimethylbutane

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