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Advanced Organic Chemistry: Mass spectrum of 3-ethylpentane

The mass spectrum of 3-ethylpentane

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-ethylpentane

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

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

3-ethylpentane alkanes structure and naming (c) doc b 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-ethylpentane

[M]+ is the molecular ion peak (M) with an m/z of 100 corresponding to [C7H16]+, the original 3-ethylpentane molecule minus an electron, [(CH3CH2)3CH]+.

The very tiny M+1 peak at m/z 101, corresponds to an ionised 3-ethylpentane molecule with one 13C atom in it i.e. an ionised 3-ethylpentane 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-ethylpentane has 7 carbon atoms, so on average, ~1 in 14 molecules will contain a 13C atom.

This sort of argument also applies to fragment ions from the parent molecular ion of 3-ethylpentane - though the ratio will be greater:

e.g. m/z 44 ion could be [13C12C2H7]+or m/z 72 ion [13C12C4H11]+

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

i.e. [C3H8]+or [C5H12]+

Either way, for identification purposes, all these peaks add uniqueness to the fragmentation pattern of the mass spectrum of 3-ethylpentane

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

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 3-ethylpentane m/z 100: [C7H16]+

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

m/z value of [fragment]+ 71 70 57 56 55
[molecular fragment]+ [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 71 ion:

[(CH3CH2)3CH]+  ===>  [C5H11]+  +  CH2CH3

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

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

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 you 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 71 gives the m/z ion 43 ion.

[C5H11]+  ===>  [C3H7]+  +  C2H4

Mass change = 71 - 28 = 43.

Formation of m/z 70 ion:

[C5H11]+  ===>  [C5H10]+  +  H

Hydrogen atom loss from m/z 71 ion too?

Formation of m/z 57 ion: (see also above from m/z 85 ion)

[(CH3CH2)3CH]+  ===>  [C4H9]+  +  C3H7

C-C bond scission of the parent molecular ion of 3-ethylpentane.

Loss of a C3H7 group, mass change = 100 - 43 = 57

Formation of m/z 43 ion:

[(CH3CH2)3CH]+  ===>  [C3H7]+  +  C4H9

C-C bond scission of the parent molecular ion of 3-ethylpentane.

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.

Will also be formed by: [C5H11]+  ===>  [C3H7]+  +  C2H4

Formation of m/z 29 ion:

[(CH3CH2)3CH]+  ===>  [CH2CH3]+  +  C5H11

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

Again, an ethyl group is broken off, but this become 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 3-ethylpentane m/z m/e base peaks, image and diagram of the mass spectrum of 3-ethylpentane, details of the mass spectroscopy of 3-ethylpentane,  low and high resolution mass spectrum of 3-ethylpentane, prominent m/z peaks in the mass spectrum of 3-ethylpentane, comparative mass spectra of 3-ethylpentane, the molecular ion peak in the mass spectrum of 3-ethylpentane, analysing and understanding the fragmentation pattern of the mass spectrum of 3-ethylpentane, characteristic pattern of peaks in the mass spectrum of 3-ethylpentane, relative abundance of mass ion peaks in the mass spectrum of 3-ethylpentane, revising the mass spectrum of 3-ethylpentane, revision of mass spectroscopy of 3-ethylpentane, most abundant ions in the mass spectrum of 3-ethylpentane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 3-ethylpentane, how to analyse the mass spectrum of 3-ethylpentane, how to describe explain the formation of fragmented ions in the mass spectra of 3-ethylpentane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 3-ethylpentane recognising the base ion peak of 3-ethylpentane interpreting interpretation the mass spectrum of 3-ethylpentane (CH3CH2)3CH CH3CH2CH(CH2CH3)CH2CH3

Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 3-ethylpentane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 3-ethylpentane. The m/e m/z value of the molecular ion peak in the mass spectrum of 3-ethylpentane.  The m/e m/z value of the base ion peak in the mass spectrum of 3-ethylpentane. Possible examples of equations showing the formation of the ionised fragments in 3-ethylpentane. Revision notes on the mass spectrum of 3-ethylpentane. Matching and deducing the structure of the 3-ethylpentane molecule from its mass spectrum. Mass spectroscopy of aliphatic alkanes, mass spectra of 3-ethylpentane, a structural isomer of molecular formula C7H16


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