[Author ©  Dr WP 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 spectrum of 2-methylpropan-2-ol [updated Nov 4th 2025]

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Mass spectroscopy - spectra index

Introductory note on the mass spectrum of 2-methylpropan-2-ol

Students and teachers please note my explanation of the mass spectrum of 2-methylpropan-2-ol is designed for advanced, but pre-university, chemistry courses.

If M represents the 2-methylpropan-2-ol 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 2-methylpropan-2-ol and only the formation of singly charged positive are considered for the mass spectrum of 2-methylpropan-2-ol.

I've included a stick diagram and table of m/z ions for the mass spectrum of 2-methylpropan-2-ol 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 2-methylpropan-2-ol.

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, but the mass spectrometer software does!

2-methylpropan-2-ol   C₄H₁₀O     

The molecular structure and naming of aliphatic alcohols and ethers

Interpreting the fragmentation pattern of the mass spectrum of 2-methylpropan-2-ol

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₄H₁₀O]⁺, the original 2-methylpropan-2-ol molecule minus an electron, [(CH₃)₃COH]⁺

In the case of 2-methylpropan-2-ol, it is tiny, indicating the molecular ion is very unstable.

The most abundant ion of the molecule under mass spectrometry investigation (2-methylpropan-2-ol) 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 2-methylpropan-2-ol.

The parent molecular ion of 2-methypropan-1-ol m/z 59 [C₃H₇O]⁺

Unless otherwise indicated, assume the carbon atoms in 2-methylpropan-2-ol are the ¹²C isotope.

The parent molecular ion for 2-methylpropan-2-ol is the m/z ion 74 corresponding to

[C₄H₁₀O]⁺ or  [(CH₃)₃COH]⁺

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 2-methylpropan-2-ol.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropan-2-ol

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; O = 16

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  O-H = 463

Possible equations to explain the most abundant ion peaks of 2-methylpropan-2-ol (tabulated above)

Formation of m/z 59 and 60 ions:

[(CH₃)₃COH]⁺  ===>  [(CH₃)₂COH]⁺  +  CH₃

C-C bond scission in the parent molecular ion to lose a methyl group,

mass change 74 - 15 = 59 (M-15 ion peak)

The m/z 59 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The [C₃H₇O]⁺ ion has an m/z of 60 if one of the carbon atoms is a ¹³C isotope i.e. [¹³C¹²C₂H₇O]⁺

Carbon-13 only accounts for ~1% of all carbon atoms (¹²C ~99%), but the more carbon atoms in the molecule/fragment, the greater the probability of observing this ¹³C M+1 peak.

The [C₃H₇O]⁺ fragment ion has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a ¹³C atom.

Formation of m/z 57 ion:

[(CH₃)₃COH]⁺  ===>  [(CH₃)₃C]⁺  +  OH

Scission of C-O bond in the parent molecular ion,

mass change 74 - 17 = 57 (M-17 ion)

This is a tertiary carbocation, relatively stable.

Formation of m/z 43 ion:

[(CH₃)₃COH]⁺  ===>  [C₃H₇]⁺  +  CH₂OH

C-C bond scission and proton loss the CH₂OH fragment?

Mass change = 74 - 31 = 43 (M-31 ion)

Formation of m/z 31 ion:

[(CH₃)₃COH]⁺  ===>  [CH₂OH]⁺  +  C₃H₇

Mass change = 74 - 43 = 31 (M-43 ion)

Note this is the same C-C bond scission in the m/z 43 ion formation - quite often either fragment can be ionised, but only one of the two fragments can carry the positive charge.

Formation of m/z ions 27 and 29

There are all sorts of possibilities e.g.

m/z 29: [C₃H₇]⁺  ===> [C₂H₅]⁺ + CH₂

m/z 27:  [C₂H₅]⁺  ===> [C₂H₃]⁺ + H + H (as H₂ ?)

Formation of m/z 17 ion

Any ionised fragment, or the original molecular ion, with an OH group can give this ion from a C-O bond scission.

[R-OH]⁺  ===>  [OH]⁺  +  R  (R = rest of original ion or fragment with OH)

Formation of m/z 15 ion

Any ionised fragment, or the original molecular ion, with a methyl group can give this ion from a C-C bond scission.

[R-CH₃]⁺  ===>  [CH₃]⁺  +  R  (R = rest of original ion or fragment with CH₃ group)

The mass spectrum of 2-methylpropan-2-ol shows a molecular ion at m/z 74 and prominent fragment peaks at m/z 59, 57, and 43 due to loss of OH and alkyl groups.

Key Mass Spectral Features of 2-Methylpropan-2-ol

2-Methylpropan-2-ol (tert-butanol) has the molecular formula C₄H₁₀O and a molecular ion (M⁺) of 74.

Its fragmentation pattern reflects the stability of tertiary carbocations and common neutral losses:

Sources: NIST Chemistry WebBook

Common Misconceptions in Exams

• Expecting a strong molecular ion peak: Tertiary alcohols often show weak or absent M⁺ due to fragmentation.
• Confusing m/z 59 and 57: but arise from different neutral losses (OH versus CH₃).
• Assuming all fragments are radicals: Only ions are detected; radicals are neutral and not seen in spectra.
• Overinterpreting low m/z peaks: Peaks like m/z 15 (CH₃⁺) are common but not diagnostic for compound identification.

Exam Revision Tips

• Learn common fragmentation routes: Alcohols often lose OH or H₂O; tertiary alcohols favour stable carbocations.
• Practice deducing fragments: Work backward from m/z values to possible neutral losses and structures.
• Use isotopic patterns for confirmation: Though not prominent in this case, they help with halogenated compounds.
• Compare with similar alcohols: Contrast spectra of ethanol, propan-2-ol, and 2-methylpropan-2-ol to spot trends.
• Annotate spectra in practice: Label peaks with fragment structures and neutral losses to reinforce understanding.

The infrared spectrum of 2-methylpropan-2-ol

The H-1 NMR spectrum of 2-methylpropan-2-ol

The C-13 NMR spectrum of 2-methylpropan-2-ol

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