(1-butanol, prim-butanol, prim-butyl alcohol, n-butyl alcohol) CH₃CH₂CH₂CH₂OH

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Introductory note on the mass spectrum of butan-1-ol

Students and teachers please note my explanation of the mass spectrum of butan-1-ol is designed for advanced, but pre-university, chemistry courses.

If M represents the butan-1-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 butan-1-ol and only the formation of singly charged positive are considered for the mass spectrum of butan-1-ol.

I've included a stick diagram and table of m/z ions for the mass spectrum of butan-1-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 butan-1-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 for butan-1-ol, but the mass spectrometer software does!

Butan-1-ol C₄H₁₀O , . primary alcohol

The molecular structure and naming of aliphatic alcohols and ethers

Interpreting fragmentation pattern: mass spectrum of butan-1-ol (1-butanol, n-butyl alcohol)

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₄H₁₀O]⁺, the original butan-1-ol molecule minus an electron, [CH₃CH₂CH₂CH₂OH]⁺

The molecular ion peak for butan-1-ol is relatively small, suggesting it a very unstable ion.

The tiny M+1 peak at m/z 75, corresponds to an ionised butan-1-ol molecule with one ¹³C atom in it i.e. an ionised butan-1-ol molecule of formula ¹³C¹²C₃H₁₀O

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.

Butan-1-ol has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (butan-1-ol) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

 The base ion peak for butan-2-ol is m/z 56 ion [C₄H₈]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of butan-1-ol.

The parent molecular ion for butan-1-ol is the m/z 74 ion [C₄H₁₀O]⁺

The m/z ion 19 is due to the formation of the oxonium (hydronium) ion [H₃O]⁺, characteristic of alcohol mass spectra.

Analysing-explaining the principal ions in the fragmentation pattern of the mass spectrum of butan-1-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 (~1% 13);  O = 16

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

Suggested equations to explain the most abundant ion peaks of butan-1-ol

Formation of m/z 73 ion:

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [C₄H₉O]⁺  +  H

C-H (or O-H) bond scission with loss of proton,

mass change 74 - 1 = 73 (M-1 ion peak)

Formation of m/z 57 ion:

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [C₄H₉]⁺  +  OH

C-O bond scission in the parent molecular ion, low probability,

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

m/z 57 could be the [C₃H₅O]⁺ ion?

[?]⁺  ===>  [C₃H₅O]⁺  +  ?

Note that an accurate mass spectrometer can sort out ions with the same integer m/z value because they can measure relative fragment ion masses to four decimal places.

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹⁶O = 15.9949, from which you can calculate (predict) that the accurate relative ion masses are:

For m/z 57: [C₄H₉]⁺ = 57.0702 * [C₃H₅O]⁺  = 57.0339, relative ion mass difference of 0.0363.

Formation of m/z 56 ion:

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [C₄H₈]⁺  +  H₂O

mass change 74 - 18 = 56 (M-18 ion peak)

The m/z 56 ion is the base peak, the most abundant 'stable' ion fragment, formed by loss of water from the parent molecular ion of butan-1-ol to give an alkene.

Formation of m/z 45 ion

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [C₂H₅O]⁺  +  C₂H₅

C-C bond scission in the parent molecular ion,

mass change = 74 - 29 = 45 (M-29 ion peak)

Formation of m/z 43 ion

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [CH₃CH₂CH₂]⁺  +  CH₂=OH

From C-C bond fission of the parent molecular ion

mass change 74 - 31 = 43 (M-31 ion peak)

Formation of m/z 31 ion:

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [CH₂=OH]⁺  +  CH₃CH₂CH₂

Characteristic of the fragmentation of primary alcohols like butan-1-ol and the 2nd most abundant ion.

mass change 74 - 43 = 31 (M-43 ion peak)

Formation of m/z 29 ion

[CH₃CH₂CH₂CH₂OH]⁺  ===>  [CH₃CH₂]⁺  +  CH₂CH₂=OH

from C-C bond scission of the parent molecular ion

mass change 74 - 45 = 29 (M-45 ion peak)

Formation of m/z 27 ion

[CH₂CH₂=OH]⁺  ==> [C₂H₃]⁺  +  H₂O

mass change 45 - 18 = 27 from the m/z 45 ion.

Formation of m/z 15 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₃]⁺  +  C₃H₇O

C-C bond scission to break off a the end methyl group

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

(see also formation of m/z 59, either fragment may be ionised)