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Advanced Organic Chemistry: Mass spectrum of 3-methylpentane

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The mass spectrum of 3-methylpentane

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-methylpentane

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Mass spectroscopy - spectra index

See also comparing infrared, mass, 1H NMR and 13C NMR spectra of the structural alkane isomers of C6H14

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

3-methylpentane C6H14, alkanes structure and naming (c) doc b , 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 3-methylpentane

[M]+ is the molecular ion peak (M) with an m/z of 86 corresponding to [C6H14]+, the original 3-methylpentane molecule minus an electron, [(CH3CH2)2CHCH3]+.

The small M+1 peak at m/z 87, corresponds to an ionised 3-methylpentane molecule with one 13C atom in it i.e. an ionised 3-methylpentane molecule of formula 13C12C5H14

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-methylpentane has 6 carbon atoms, so on average, ~1 in 17 molecules of will contain a 13C atom.

A similar argument applies to fragment ions from the breakdown of the parent molecular ion of 3-methylpentane - though the ratio will be greater e.g. the m/z 58 ion.

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

The most abundant ion of the molecule under mass spectrometry investigation is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

m/z value of [fragment]+ 71 58 57 56 55
[molecular fragment]+ [(CH3CH2)2CH]+ [C4H10]+ [C4H9]+ [C4H8]+ [C4H7]+
m/z value of [fragment]+ 43 42 41 39 29 27
[molecular fragment]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [CH3CH2]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 3-methylpentane

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

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

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

Formation of m/z 71 ion:

[(CH3CH2)2CHCH3]+  ===>  [(CH3CH2)2CH]+  +  CH3

C-C bond scission in the parent molecular ion, leading to loss of a methyl group from the parent molecular ion via C-C bond fission.

Mass change: 86 - 15 = 71, to give a secondary carbocation.

I've quoted a possible structure of the [C5H11]+ ion.

Formation of m/z 57 ion:

[(CH3CH2)2CHCH3]+  ===>  [CH3CH2CHCH3]+  +  CH3CH2

C-C bond scission of the parent molecular ion, leading to loss of an ethyl group from the parent molecular ion.

Mass change = 86 - 29 = 57, to give a secondary carbocation.

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

Formation of m/z 29 ion:

[(CH3CH2)2CHCH3]+  ===>  [CH3CH2]+  +  CH3CH2CHCH3

or from: [CH3CH2CHCH3]+  ===>  [CH3CH2]+  +  CH3CH

C-C bond scission of the parent molecular ion or larger fragment ion.

Mass change = 86 - 57 = 29 or 57 - 28 = 29

Sequences including m/z values of 42, 41, 40, 39 or 28, 28, 27, 26, indicate successive hydrogen atom/molecule loss from the m/z 43 or 29 ions.

Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the five structural alkane isomers of C6H14

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 hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane image sizes.  These five molecules are structural isomers of saturated alkanes of molecular formula C6H14 and exemplify the infrared, mass, 1H NMR and 13C NMR spectra of lower aliphatic alkanes (non-cyclic alkanes).

Infrared spectra below.

INFRARED SPECTRA:

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.

All the absorption bands are typical of molecules containing saturated alkyl structure and there are no characteristic infrared absorptions due to a specific functional group.

Infrared spectra above, mass spectra below.

MASS SPECTRA: Base ion peaks plus m/z comments.

Hexane: m/z 57, 42 and 56 prominent

2-methylpentane: m/z 43, 42 and 71 prominent

3-methylpentane: m/z 57, 41 and 56 prominent

2,2-dimethylbutane: m/z 43, 41, 57 and 71 prominent

2,3-dimethylbutane: m/z 43, 41, 42 and 71 prominent

Mass spectra above, 1H NMR spectra below.

1H NMR SPECTRA: They can all be distinguished by their different integrated proton ratios - need very high resolution.

Hexane: 3 1H δ shifts, H ratio 3:2:2 (6:4:4 in formula)

2-methylpentane: 5 1H δ shifts, H ratio 6:3:2:2:1

3-methylpentane: 4 1H δ shifts, H ratio 6:4:3:1

2,2-dimethylbutane: 3 1H δ shifts, H ratio 9:3:2

2,3-dimethylbutane: 2 1H δ shifts, H ratio 6:1 (12:2 in formula)

1H NMR spectra above, 13C NMR spectra below.

13C NMR SPECTRA: From the number of shifts, you can't distinguish (iii) and (iv) but you can distinguish them from (i), (ii) and (v). (i) Hexane: 3 13C δ shifts

(ii) 2-methylpentane: 5 13C δ shifts

(iii) 3-methylpentane: 4 13C δ shifts

(iv) 2,2-dimethylbutane: 4 13C δ shifts

(v) 2,3-dimethylbutane: 2 13C δ shifts

13C NMR spectra above.

Key words & phrases: image diagram on how to interpret and explain the mass spectrum of 3-methylpentane m/z m/e base peaks, image and diagram of the mass spectrum of 3-methylpentane, details of the mass spectroscopy of 3-methylpentane,  low and high resolution mass spectrum of 3-methylpentane, prominent m/z peaks in the mass spectrum of 3-methylpentane, comparative mass spectra of 3-methylpentane, the molecular ion peak in the mass spectrum of 3-methylpentane, analysing and understanding the fragmentation pattern of the mass spectrum of 3-methylpentane, characteristic pattern of peaks in the mass spectrum of 3-methylpentane, relative abundance of mass ion peaks in the mass spectrum of 3-methylpentane, revising the mass spectrum of 3-methylpentane, revision of mass spectroscopy of 3-methylpentane, most abundant ions in the mass spectrum of 3-methylpentane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 3-methylpentane, how to analyse the mass spectrum of 3-methylpentane, how to describe explain the formation of fragmented ions in the mass spectra of 3-methylpentane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 3-methylpentane recognising the base ion peak of 3-methylpentane interpreting interpretation the mass spectrum of 3-methylpentane Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 3-methylpentane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 3-methylpentane. The m/e m/z value of the molecular ion peak in the mass spectrum of 3-methylpentane.  The m/e m/z value of the base ion peak in the mass spectrum of 3-methylpentane. Possible examples of equations showing the formation of the ionised fragments in 3-methylpentane. Revision notes on the mass spectrum of 3-methylpentane. Matching and deducing the structure of the 3-methylpentane molecule from its mass spectrum. Mass spectroscopy of  aliphatic alkanes, mass spectra of 3-methylpentane, a structural isomer of molecular formula C6H14


Links associated with 3-methylpentane

The chemistry of ALKANES revision notes INDEX

The infrared spectrum of 3-methylpentane

The H-1 NMR spectrum of 3-methylpentane

The C-13 NMR spectrum of 3-methylpentane

Mass spectroscopy index

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