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Advanced Organic Chemistry: Mass spectrum of 3-methylhexane

The mass spectrum of 3-methylhexane

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 3-methylhexane

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

C7H16 mass spectrum of 3-methylhexane fragmentation pattern of m/z m/e ions for analysis and identification of 3-methylhexane image diagram doc brown's advanced organic chemistry revision notes 

 3-methylhexane   C7H16   alkanes structure and naming (c) doc b

 alkanes structure and naming (c) doc b alkanes structure and naming (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of 3-methylhexane

[M]+ is the molecular ion peak (M) with an m/z of 100 corresponding to [C7H16]+, the original 3-methylhexane molecule minus an electron, [CH3CH2CH(CH3)CH2CH2CH3]+.

The very tiny M+1 peak at m/z 105, corresponds to an ionised 3-methylhexane molecule with one 13C atom in it i.e. an ionised 3-methylhexane 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.

3-methylhexane has 7 carbon atoms, so, on average, ~1 in every 14 molecules will contain a 13C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of 3-methylhexane - 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 3-methylhexane.

The most abundant ion of the molecule under mass spectrometry investigation (3-methylhexane) 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 3-methylhexane.

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 3-methylhexane.

The parent molecular ion of 3-methylhexane m/z 100: [C7H16]+

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 3-methylhexane.

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 3-methylhexane

Examples of equations to explain some of the most abundant ion peaks in the mass spectrum of 3-methylhexane

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:

[CH3CH2CH(CH3)CH2CH2CH3]+  ===>  [C6H13]+  +  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, 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 proton loss so get sequences like 71 => 70, 57 => 56 => 55, 43 => 42 => 41 => 40 => 39 and 29 => 28 => 27 etc.

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

[C6H13]+  ===>  [C4H7]+  +  C2H4

Mass change 85 - 28 = 57.

Formation of m/z 71 ion:

[CH3CH2CH(CH3)CH2CH2CH3]+  ===>  [C5H11]+  +  CH2CH3

C-C bond scission of 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:

[CH3CH2CH(CH3)CH2CH2CH3]+  ===>  [CH3CH2CHCH3]+  +  CH2CH2CH3

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

Formation of m/z 43 ion:

[CH3CH2CH(CH3)CH2CH2CH3]+  ===>  [CH2CH2CH3]+  +  C4H9

Loss of a C4H9 group, mass change = 100 - 57 = 43

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

Loss of a C4H9 group, mass change = 100 - 57 = 43

Formation of m/z 29 ion:

[CH3CH2CH(CH3)CH2CH2CH3]+  ===>  [CH2CH3]+  +  C5H11

Here an end ethyl group is broken off, 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 3-methylhexane m/z m/e base peaks, image and diagram of the mass spectrum of 3-methylhexane, details of the mass spectroscopy of 3-methylhexane,  low and high resolution mass spectrum of 3-methylhexane, prominent m/z peaks in the mass spectrum of 3-methylhexane, comparative mass spectra of 3-methylhexane, the molecular ion peak in the mass spectrum of 3-methylhexane, analysing and understanding the fragmentation pattern of the mass spectrum of 3-methylhexane, characteristic pattern of peaks in the mass spectrum of 3-methylhexane, relative abundance of mass ion peaks in the mass spectrum of 3-methylhexane, revising the mass spectrum of 3-methylhexane, revision of mass spectroscopy of 3-methylhexane, most abundant ions in the mass spectrum of 3-methylhexane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 3-methylhexane, how to analyse the mass spectrum of 3-methylhexane, how to describe explain the formation of fragmented ions in the mass spectra of 3-methylhexane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 3-methylhexane recognising the base ion peak of 3-methylhexane interpreting interpretation the mass spectrum of 3-methylhexane  CH3CH2CH(CH3)CH2CH2CH3

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 3-methylhexane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 3-methylhexane. The m/e m/z value of the molecular ion peak in the mass spectrum of 3-methylhexane.  The m/e m/z value of the base ion peak in the mass spectrum of 3-methylhexane. Possible examples of equations showing the formation of the ionised fragments in 3-methylhexane. Revision notes on the mass spectrum of 3-methylhexane. Matching and deducing the structure of the 3-methylhexane molecule from its mass spectrum. Mass spectroscopy of aliphatic alkanes, mass spectra of 3-methylhexane, 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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