C6H12 mass spectrum of cyclohexane fragmentation pattern of m/z m/e ions for analysis and identification of cyclohexane image diagram doc brown's advanced organic chemistry revision notes

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Advanced Organic Chemistry: Mass spectrum of cyclohexane

The mass spectrum of cyclohexane

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 cyclohexane

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

Cyclohexane, C₆H₁₂ , , , all bond angles ~109^o

Interpreting the fragmentation pattern of the mass spectrum of cyclohexane

[M]⁺ is the molecular ion peak (M) with an m/z of 84 corresponding to [C₆H₁₂]⁺, the original cyclohexane molecule minus an electron.

The small M+1 peak at m/z 85, corresponds to an ionised cyclohexane molecule with one ¹³C atom in it i.e. an ionised cyclohexane molecule of formula ¹³C¹²C₅H₁₂

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 cyclohexane.

m/z value of [fragment]⁺	69	57	56	55	43	42	41	39	29	27
[molecular fragment]⁺	[C₅H₉]⁺	[C₄H₉]⁺	[C₄H₈]⁺	[C₄H₇]⁺	[C₃H₇]⁺	[C₃H₆]⁺	[C₃H₅]⁺	[C₃H₃]⁺	[C₂H₅]⁺	[C₂H₃]⁺

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

Equations to explain the most abundant ion peaks of cyclohexane

Formation of m/z 56 ion: [C₆H₁₂]⁺ ===> [C₄H₈]⁺ + CH₂=CH₂

The m/z 56 ion is the base peak ion, the most abundant and 'stable' ion fragment.

It is formed by the elimination of ethene from the molecular ion.

The m/z values of 83, 69 and 55 are related by the loss of a mass unit of 14 (CH₂) from one fragment ion to the next.

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