[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectra of phenol [spectra page updated April 4th 2026 *]

 Mass spectrometry - spectra index

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

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

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and 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 phenol and only the formation of singly charged positive are considered for the mass spectrum of phenol.

I've included a stick diagram and table of m/z ions for the mass spectrum of phenol 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 phenol.

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 and compared the accurate ion masses if appropriate for phenol. 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 phenol, but the mass spectrometer software does!

The molecular structure and naming of aromatic compounds

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 molecular 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 (See note above data table of ions).

The base peak ion for the mass spectrum of phenol is m/z 94 ion [C₆H₆O]⁺

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

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

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

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

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

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.

Atomic masses: H = 1;  C = 12 (~1% 13);  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 77 ion:

[C₆H₅OH]⁺  ===>  [C₆H₅]⁺  +  OH

mass change 94 - 17 = 77, the phenyl cation

Scission of the C-O bond.

A characteristic ion in the mass spectra of mono-substituted benzene ring compounds.

Formation of m/z 66 ion:

[C₆H₅OH]⁺  ===>  [C₅H₆]⁺  +  CO

A complex reaction (from the internet),

mass change 94 - 28 = 66 (M-28)

Loss of CO fragment from parent molecular ion.

The tiny peak at m/z 67 is probably formed in the same way, but containing a ¹³C carbon isotope atom i.e. [¹³C¹²C₅H₅OH]⁺

Formation of m/z 65 ion:

[C₆H₅OH]⁺  ===>  [C₅H₅]⁺  +  CHO

A complex reaction (from the internet),

mass change 94 - 29 = 65 (M-29 ion peak)

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 also be formed by H₂ loss from the m/z 65 ion.

Formation of 'some'  of the ions with m/z <56 (see diagram and data table)

The m/z 63 to 66 ions (of five carbon atoms) will lose e.g. CH or CO fragments to give the range of m/z 50 to 55 ions of four carbon atoms.

BUT there are ions of similar m/z values containing an oxygen atom e.g. m/z 55 too.

The m/z 47 to 55 ions (of five carbon atoms) will lose e.g. CH or CO fragments to give the range of m/z 37 to 40 ions of four carbon atoms.

Formation of m/z 17 and 77 ions:

[C₆H₅OH]⁺  ===>  [OH]⁺  +  C₆H₅

Tiny peak, a very low probability of this particular fragmentation reaction.

You can also see a similar sized tiny peak for m/z 77, a common ion in the mass spectra of aromatic benzene ring compounds like phenol, but of very low intensity here in the mass spectrum of phenol.

[C₆H₅OH]⁺  ===>  [C₆H₅]⁺  +  OH

Physical & chemical properties of phenol and some of its derivatives & uses

The chemistry of AROMATIC COMPOUNDS revision notes INDEX