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

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Advanced Organic Chemistry: Mass spectrum of heptane

The mass spectrum of heptane

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 heptane

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

Heptane, C₇H₁₆, CH₃(CH₂)₅CH₃ , ,

Interpreting the fragmentation pattern of the mass spectrum of heptane

[M]⁺ is the molecular ion peak (M) with an m/z of 100 corresponding to [C₇H₁₆]⁺, the original heptane molecule minus an electron, [CH₃(CH₂)₅CH₃]⁺.

The small M+1 peak at m/z 101, corresponds to an ionised heptane molecule with one ¹³C atom in it i.e. an ionised heptane 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.

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

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 heptane

Equations to explain the most abundant ion peaks of heptane (Atomic masses: C = 12, H = 1)

Formation of m/z 85 ion: [CH₃(CH₂)₅CH₃]⁺ ===> [CH₃(CH₂)₅]⁺ + 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 loss = 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 ===> 39 and 29 ===> 27 etc.

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

[CH₃(CH₂)₅]⁺ ===> [CH₃(CH₂)₃]⁺ + C₂H₄

Mass change 85 - 28 = 57.

Formation of m/z 71 ion: [CH₃(CH₂)₅CH₃]⁺ ===> [CH₃(CH₂)₄]⁺ + CH₂CH₃

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

Formation of m/z 57 ion: [CH₃(CH₂)₅CH₃]⁺ ===> [CH₃(CH₂)₃]⁺ + CH₂CH₂CH₃

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

Formation of m/z 43 ion: [CH₃(CH₂)₅CH₃]⁺ ===> [CH₂CH₂CH₃]⁺ + CH₃(CH₂)₃

Loss of butyl group, mass loss = 100 - 57 = 43

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)₅CH₃]⁺ ===> [CH₂CH₃]⁺ + CH₃(CH₂)₄

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

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