C8H18 mass spectrum of octane fragmentation pattern of m/z m/e ions for analysis and identification of n-octane image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of octane CH₃(CH₂)₆CH₃

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Interpreting and explaining the mass spectrum of octane

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Mass spectrometry - spectra index

Introductory note on the mass spectrum of octane

Students and teachers please note my explanation of the mass spectrum of octane is designed for advanced, but pre-university, chemistry courses.

If M represents the octane molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^- ==> [M•]⁺_(g) + 2e^- and for fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺ + R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of octane and only the formation of singly charged positive are considered for the mass spectrum of octane.

I've included a stick diagram and table of m/z ions for the mass spectrum of octane and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including octane.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated the accurate ion masses, BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for octane, but the mass spectrometer software does!

C8H18 mass spectrum of octane fragmentation pattern of m/z m/e ions for analysis and identification of n-octane image diagram doc brown's advanced organic chemistry revision notes

Octane, C₈H₁₈, CH₃(CH₂)₆CH₃, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃,

an alkane The molecular structure and naming of alkanes

Interpreting the fragmentation pattern of the mass spectrum of octane

[M]⁺ is the molecular ion peak (M) with an m/z of 114 corresponding to [C₈H₁₈]⁺, the original octane molecule minus an electron, [CH₃(CH₂)₆CH₃]⁺

The tiny M+1 peak at m/z 115, corresponds to an ionised octane molecule with one ¹³C atom in it i.e. an ionised octane molecule of formula ¹³C¹²C₇H₁₈

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.

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.

The base ion peak for the mass spectrum of octane is m/z 43 ion [C₃H₇]⁺

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

Unless otherwise indicated, assume the carbon atoms in octane or fragments are the ¹²C isotope.

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

The parent molecular ion is the m/z 114 ion [C₈H₁₈]⁺ or [CH₃(CH₂)₆CH₃]⁺

m/z value of [fragment]⁺	85	84	71	70	57	56	55
[molecular fragment]⁺	[C₆H₁₃]⁺	[C₆H₁₂]⁺	[C₅H₁₁]⁺	[C₅H₁₀]⁺	[C₄H₉]⁺	[C₄H₈]⁺	[C₄H₇]⁺

m/z value of [fragment]⁺	43	42	41	39	29	28	27
[molecular fragment]⁺	[C₃H₇]⁺	[C₃H₆]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[C₂H₅]⁺	[C₂H₄]⁺	[C₂H₃]⁺

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

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Some possible equations to explain the most abundant ion peaks of octane

Atomic masses: H = 1; C = 12 (~1% 13)

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

Formation of m/z 85 ion

[CH₃(CH₂)₆CH₃]⁺ ===> [CH₃(CH₂)₅]⁺ + CH₂CH₃

C-C bond scission, parent molecular ion loses ethyl group,

114 - 29 = 85 (M-29 ion peak)

Subsequent proton loss gives the m/z 84 ion.

The m/z 86 ion is probably formed in the same way but contains a ¹³C atom i.e. it has the formula [¹³C¹²C₅H₁₃]⁺ rather than a [C₆H₁₄]⁺ ion.

Note that an accurate mass spectrometer can sort out (resolve) pairs of ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places,

e.g. using accurate relative isotopic masses:

¹H = 1.0078 ¹²C = 12.0000 ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

m/z 86: [C₆H₁₄]⁺ = 86.1092 * [¹³C¹²C₅H₁₃]⁺ = 86.1048, a relative ion mass difference of 0.0044

Formation of m/z 71 ion

[CH₃(CH₂)₆CH₃]⁺ ===> [CH₃(CH₂)₄]⁺ + CH₂CH₂CH₃

C-C bond scission, molecular ion loses propyl group, 114 - 43 = 71 (M-43 ion peak)

or from other fragments e.g.: [CH₃(CH₂)₅]⁺ ===> [CH₃(CH₂)₄]⁺ + CH₂

Formation of m/z 57 ion

[CH₃(CH₂)₆CH₃]⁺ ===> [CH₃(CH₂)₃]⁺ + CH₂CH₂CH₂CH₃

C-C bond scission splitting the parent molecular ion in half,

mass change 114 - 57 = 57 (M-57 ion peak)

Formation of m/z 43 ion

[CH₃(CH₂)₆CH₃]⁺ ===> [CH₃CH₂CH₂]⁺ + C₅H₁₁

or from other fragments e.g.: [CH₃(CH₂)₄]⁺ ===> [CH₃CH₂CH₂]⁺ + CH₂=CH₂

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

Formation of m/z 29 ion

[CH₃(CH₂)₆CH₃]⁺ ===> [CH₂CH₃]⁺ + CH₃(CH₂)₅

mass change 114 - 85 = 29

or from other fragments e.g.: [CH₃(CH₂)₅]⁺ ===> [CH₂CH₃]⁺ + C₄H₈

All involving C-C bond scission of the alkyl chain.

Note:

(i) For alkanes that you often get a series of peaks separated by 14 mass units, due to successive loss of a CH₂ grouping or different points of C-C bond scission in the parent molecular ion e.g. m/z ions 85, 71, 57, 43 and 29.

(ii) You also get ionised fragments giving rise to the m/z ion series of 84, 70, 56 and 42

These are formed by elimination of hydrogen from other fragment ions or from C-C bond scission and proton rearrangement e.g.

e.g. formation of the m/z 56 ion from the m/z 85 ion

[CH₃(CH₂)₅]⁺ ===> [C₄H₈]⁺ + CH₂CH₃

mass change 85 - 29 = 86

(iii) With alkane molecules the largest fragment in each group of peaks is usually formed by C-C chain bond scission, rather than proton loss (bond enthalpies: C-C 336 kJ/mol and C-H 416 kJ/mol).

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What next? links associated with octane

The infrared spectrum of octane

The H-1 NMR spectrum of octane

The C-13 NMR spectrum of octane

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