CH₃CH(OH)CH₂CH₃ (2-butanol, sec-butanol, sec-butyl alcohol)

 email doc brown  Re-edit mass spectrum of CH₃CH(OH)CH₂CH₃

 Links associated with butan-2-ol  *  [privacy policy, cookies and disclaimer]

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

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

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

I've included a stick diagram and table of m/z ions for the mass spectrum of butan-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 butan-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 for butan-2-ol, but the mass spectrometer software does!

Butan-2-ol    C₄H₁₀O             

a secondary alcohol The molecular structure and naming of aliphatic alcohols and ethers

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

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₄H₁₀O]⁺, the original butan-2-ol molecule minus an electron, [CH₃CH(OH)CH₂CH₃]⁺

This is a tiny peak, indicating the molecular ion is very unstable.

You might see an even smaller M+1 peak at m/z 75, corresponding to an ionised butan-2-ol molecule with one ¹³C atom in it i.e. an ionised butan-2-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-2-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-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.

The base ion peak for butan-2-ol is the m/z 45 ion [CH₃CHOH]⁺ or just [C₂H₅O]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of butan-2-ol.

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

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

The parent molecular ion for butan-2-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 and explaining the principal ions in the fragmentation pattern of the mass spectrum of butan-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 (~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-2-ol (tabulated above)

Formation of m/z 73 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [C₄H₉O]⁺  +  H

C-H (or O-H) bond scission in the parent molecular ion with loss of proton,

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

Formation of m/z 59 ion:

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

C-C bond scission an loss of methyl group from either end of the parent molecular ion.

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

The tiny peak at m/z 60 is probably due to the same ion formed in the same way, but containing a ¹³C atom i.e. it's structure is [¹³C¹²C₂H₇O]⁺ and unlikely to be

Formation of m/z 57 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [C₄H₉]⁺  +  OH

C-O bond scission of the parent molecular ion,

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

Could be a [C₃H₅O]⁺ ion ?

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:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [C₄H₈]⁺  +  H₂O

Elimination of water from the parent molecular ion to give a positively ionised molecule of a butene.

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

It could also be formed by H loss from the m/z 57 ion above.

Formation of m/z 55 ion:

[C₄H₉O]⁺  ===>  [C₄H₇]⁺  +  H₂O

Elimination of water from the m/z 73 ion.

Mass change 73 - 18 = 55

It could also be formed by H loss from the m/z 57 or 56 ions.

Formation of m/z 45 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₃CHOH]⁺  +  CH₂CH₃

C-C bond scission of the parent molecular ion, mass change 74 - 29 = 45 (M-29 ion peak)

The m/z 45 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The small peak for the m/z 46 ion will be formed in the same way, but containing a ¹³C atom.

The m/z 45 ion could eliminate water and give the m/z 27 ion.

[CH₃CHOH]⁺  ===>  [C₂H₃]⁺  +  H₂O

The m/z 45 ion can lose hydrogen atoms to give the m/z 43 and 44 ions (see table).

The m/z 45 ion could eliminate water to give the m/z 27 ion

[CH₃CHOH]⁺  ===>  [C₂H₃]⁺  +  H₂O

Formation of m/z 43 ion:

Two possibilities, but mode of formation?

[?]⁺  ===>  [C₃H₇]⁺  +  ?

[?]⁺  ===>  [C₂H₃O]⁺  +  ?

Note again 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 43: [C₂H₃O]⁺ = 43.0183  and [C₃H₇]⁺ = 43.0546, a difference of 0.0363 in relative ion mass.

Formation of m/z 31 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₂=OH]⁺  +  C₃H₇

Characteristic of the fragmentation of primary alcohols like butan-2-ol.

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

Formation of m/z 29 ion:

[CH₃CH(OH)CH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  CH₃CHOH

C-C bond scission of the parent molecular ion, mass change 74 - 45 = 29 (M-45 ion peak)

Formation of m/z 15 ion:

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

C-C bond scission to break off a methyl group (from either end of molecule?).

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

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