mass spectrum of phenol C6H6O C6H5OH fragmentation pattern of m/z m/e ions for analysis and identification of phenol image diagram doc brown's advanced organic chemistry revision notes

Advanced Organic Chemistry: Mass spectrum of phenol

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Interpreting the mass spectrum of phenol

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 phenol

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Mass spectroscopy - spectra index

mass spectrum of phenol C6H6O C6H5OH fragmentation pattern of m/z m/e ions for analysis and identification of phenol image diagram doc brown's advanced organic chemistry revision notes

Phenol, C₆H₆O, C₆H₅OH, ,

Interpreting the fragmentation pattern of the mass spectrum of phenol

[M]⁺ is the molecular ion peak with an m/z of 94 corresponding to [C₆H₆O]⁺, the original phenol molecule minus an electron, [C₆H₅OH]⁺.

The small M+1 peak at m/z 95, corresponds to an ionised phenol molecule with one ¹³C atom in it i.e. an ionised phenol molecule of formula [¹³C¹²C₅H₆O]⁺

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.

Phenol has 6 carbon atoms, so on average, ~1 in 17 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (phenol) 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 phenol.

Unless otherwise indicated, assume thecarbon atoms in phenol are the ¹²C isotope.

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

Parent molecular ion of phenol: [C₆H₅OH]⁺, m/z 94, which in this case, is also the base peak ion.

m/z value of [fragment]⁺	66	65	63	55 ?	53 ?
[molecular fragment]⁺	[C₅H₆]⁺	[C₅H₅]⁺	[C₅H₃]⁺	[C₃H₃O]⁺	[C₄H₅]⁺

m/z value of [fragment]⁺	51 ?	50 ?	47 ?	40	39	38
[molecular fragment]⁺	[C₄H₃]⁺	[C₄H₂]⁺	[?]⁺	[C₃H₄]⁺	[C₃H₃]⁺	[C₃H₂]⁺

I'm not sure on the identity of some of these ions?

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

Atomic masses: H = 1; C = 12; O = 16

Bond enthalpies = kJ/mol: = 518 (benzene); C-H = 412; C-O = 360; O-H = 463

Possible equations to explain the most abundant ion peaks of phenol (tabulated above)

Formation of m/z 66 ion:

[C₆H₅OH]⁺ ===> [C₅H₆]⁺ + CO

A complex reaction (from the internet), mass change 94 - 28 = 66.

Loss of CO fragment from parent molecular ion.

Formation of m/z 65 ion:

[C₆H₅OH]⁺ ===> [C₅H₅]⁺ + CHO

A complex reaction (from the internet), mass change 94 - 29 = 65.

Loss of CHO fragment from parent molecular ion.

The m/z 65 ion is also formed by hydrogen loss from the m/z 66 ion.

The m/z 63 ion may be formed by H₂ loss from the m/z 65 ion.

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