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Advanced Organic Chemistry: Mass spectrum of 3,3-dimethylpentane

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The mass spectrum of 3,3-dimethylpentane

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 3,3-dimethylpentane

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Mass spectroscopy - spectra index

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

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

 3,3-dimethylpentane 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 3,3-dimethylpentane

[M]+ is the molecular ion peak (M) with an m/z of 100 corresponding to [C7H16]+, the original 3,3-dimethylpentane molecule minus an electron, [CH3CH2C(CH3)2CH2CH3]+.

It must be very unstable as it is a very tiny peak!

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.

3,3-dimethylpentane 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 3,3-dimethylpentane - though the ratio will be greater (in fact here the M peak is almost absent, so even less chance of observing the M+1 peak).

e.g. m/z 44 ion could be [13C12C2H7]+, m/z 72 ion [13C12C4H11]+, and m/z 86 ion [13C12C5H13]+

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

i.e. [C3H8]+, [C5H12]+ and [C6H14]+

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

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

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 3,3-dimethylpentane.

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 2,3-dimethylpentane.

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

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

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

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

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

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

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

Ethene is often eliminated to give a smaller fragment

e.g. the m/z 71 ion gives the m/z ion 43

[C5H11]+  ===>  [C3H7]+  +  C2H4

Mass change = 71 - 28 = 43.

The m/z 57 ion can be formed by loss of a ethene from the m/z 85 ion.

[C6H13]+  ===>  [C4H9]+  +  C2H4

Mass change = 85 - 28 = 57

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

Formation of m/z 85 ion:

[CH3CH2C(CH3)2CH2CH3]+  ===>  [C6H13]+  +  CH3

C-C bond fission in the molecular ion of 3,3-dimethylpentane.

Here a methyl group is broken off, mass change = 100 - 15 = 81.

Formation of m/z 71 ion:

[CH3CH2C(CH3)2CH2CH3]+  ===>  [(CH3)2CHCHCH3]+  +  CH2CH3

C-C bond fission in the molecular ion of 3,3-dimethylpentane.

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

Formation of m/z 70 ion:

[CH3CH2C(CH3)2CH2CH3]+  ===>  [(CH3)2CHCHCH3]+  +  CH3

C-C bond fission in the molecular ion of 3,3-dimethylpentane.

A methyl group is broken off from the m/z 85 ion, mass change = 85 - 15 = 70.

Formation of m/z 57 ion:

[CH3CH2C(CH3)2CH2CH3]+  ===>  [C4H9]+  +  C3H7

C-C bond fission in the molecular ion of 3,3-dimethylpentane.

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

Formation of m/z 43 ion:

[(CH3)2CHCH(CH3)CH2CH3]+  ===>  [C3H7]+  +  C4H9

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

The m/z 43 ion intensity is the base peak ion.

Formation of m/z 29 ion:

[(CH3)2CHCH(CH3)CH2CH3]+  ===>  [CH2CH3]+  +  C5H11

C-C bond fission in the molecular ion of 3,3-dimethylpentane.

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 3,3-dimethylpentane m/z m/e base peaks, image and diagram of the mass spectrum of 3,3-dimethylpentane, details of the mass spectroscopy of 3,3-dimethylpentane,  low and high resolution mass spectrum of 3,3-dimethylpentane, prominent m/z peaks in the mass spectrum of 3,3-dimethylpentane, comparative mass spectra of 3,3-dimethylpentane, the molecular ion peak in the mass spectrum of 3,3-dimethylpentane, analysing and understanding the fragmentation pattern of the mass spectrum of 3,3-dimethylpentane, characteristic pattern of peaks in the mass spectrum of 3,3-dimethylpentane, relative abundance of mass ion peaks in the mass spectrum of 3,3-dimethylpentane, revising the mass spectrum of 3,3-dimethylpentane, revision of mass spectroscopy of 3,3-dimethylpentane, most abundant ions in the mass spectrum of 3,3-dimethylpentane, how to construct the mass spectrum diagram for abundance of fragmentation ions in the mass spectrum of 3,3-dimethylpentane, how to analyse the mass spectrum of 3,3-dimethylpentane, how to describe explain the formation of fragmented ions in the mass spectra of 3,3-dimethylpentane equations for explaining the formation of the positive ions in the fragmentation of the ionised molecule of 3,3-dimethylpentane recognising the base ion peak of 3,3-dimethylpentane interpreting interpretation the mass spectrum of 3,3-dimethylpentane CH3CH2C(CH3)2CH2CH3 Stick diagram of the relative abundance of ionised fragments in the fingerprint pattern of the mass spectrum of 3,3-dimethylpentane. Table of the m/e m/z values and formula of the ionised fragments in the mass spectrum of 3,3-dimethylpentane. The m/e m/z value of the molecular ion peak in the mass spectrum of 3,3-dimethylpentane.  The m/e m/z value of the base ion peak in the mass spectrum of 3,3-dimethylpentane. Possible examples of equations showing the formation of the ionised fragments in 3,3-dimethylpentane. Revision notes on the mass spectrum of 3,3-dimethylpentane. Matching and deducing the structure of the 3,3-dimethylpentane molecule from its mass spectrum. Mass spectroscopy of aliphatic alkanes, mass spectra of 3,3-dimethylpentane, 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

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

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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