Associated links for pent-1-ene (1-pentene)

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Introductory note on the mass spectrum of pent-1-ene

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

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

I've included a stick diagram and table of m/z ions for the mass spectrum of pent-1-ene 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 pent-1-ene.

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 pent-1-ene, but the mass spectrometer software does!

Pent-1-ene, , , (1-pentene)

an alkene The molecular structure and naming of alkenes

Interpreting the mass spectrum of pent-1-ene

[M]⁺ is the molecular ion peak (M) with an m/z of 70 corresponding to [C₅H₁₀]⁺, the original molecule minus an electron, [CH₃CH₂CH₂CH=CH₂]⁺

The small M+1 peak at m/z 71, corresponds to an ionised pent-1-ene molecule with one ¹³C atom in it i.e. an ionised molecule of formula ¹³C¹²C₄H₁₀

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.

Pent-1-ene has 5 carbon atoms, so on average, ~1 in 20 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (pent-1-ene) 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 the mass spectrum of pent-1-ene is m/z 42 ion [C₃H₆]⁺

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

Unless otherwise indicated, assume the carbon atoms in pent-1-ene or fragments are the ¹²C isotope.

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

The parent molecular ion is the m/z 70 ion   [C₅H₁₀]⁺  or  [CH₃CH₂CH₂CH=CH₂]⁺

Some suggested possible equations explaining the principal fragments

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=C = 612;  C-H = 412

Many of the fragments from alkenes ar C_nH_2n-1 and C_nH_2n ions.

Formation of the m/z 56 ion

Formed by loss of methyl group from C-C bond scission of the molecular ion

 [C₅H₁₀]⁺ ===>  [C₄H₈]⁺  +  CH₂

mass change 70 - 14 = 56 (M-15 ion peak)

BUT, it could be the m/z 55 ion with a ¹³C atom in it (see below)

Formation of the m/z 55 ion

Formed by loss of methyl group from C-C bond scission of the parent molecular ion

 [C₅H₁₀]⁺ ===>  [C₄H₇]⁺  +  CH₃

mass change 70 - 15 = 55 (M-15 ion peak)

The m/z 56 ion is probably formed in the same way i.e the [¹³C¹²C₃H₈]⁺ ion rather than the  [C₄H₈]⁺

Note that an accurate mass spectrometer can sort out (resolve) pairs of 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   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 56: [¹³C¹²C₃H₇]⁺ = 56.0580, [C₄H₈]⁺ = 56.0624, a difference of 0.0044 in relative ion mass.

Formation of the m/z 43 ion

Formed by C-C bond fission of the m/z 70 parent molecular ion.

 [C₅H₁₀]⁺ ===>  [C₃H₇]⁺  +  C₂H₃

mass change 70 - 27 = 43  (M-27 ion peak), but see below for an alternative ion.

Formation of the m/z 42 ion

Formed by elimination of ethene from the m/z 70 parent ion.

 [C₅H₁₀]⁺ ===>  [C₃H₆]⁺  +  C₂H₄

mass change 70 - 28 = 42  (M-28 ion peak)

m/z 42 ion is the base ion peak

The m/z 43 ion is probably formed in the same way but contains a ¹³C atom i.e. it has the formula [¹³C¹²C₂H₆]⁺ rather than [C₃H₇]⁺

So again, using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 43: [C₃H₇]⁺ = 43.0546 and [¹³C¹²C₂H₆]⁺ = 43.0502 relative ion mass difference of 0.0044.

Formation of the m/z 41 ion

Formed by loss of ethyl group from C-C bond scission of the molecular ion

 [C₅H₁₀]⁺ ===>  [C₃H₅]⁺  +  CH₂CH₃

mass change 70 - 29 = 41 (M-29 ion peak)

Formation of the m/z 39 ion

Can be formed by elimination of hydrogen from the m/z 41 ion.

 [C₃H₅]⁺ ===>  [C₃H₃]⁺  +  H₂

mass change 41 - 2 = 39

(this is a triangular ion carrying a positive charge)

Formation of the m/z 29 ion

Formed by loss of ethyl group from C-C bond scission of the m/z 70 parent molecular ion

 [C₅H₁₀]⁺ ===>  [C₂H₅]⁺  +  C₃H₅

mass change 70 - 41 = 29 (M-41 ion peak)

Formation of the m/z 28 ion

Formed by elimination of propene from the m/z 70 parent ion, involving C-C bond scission and proto rearrangement.

 [C₅H₁₀]⁺ ===>  [C₂H₄]⁺  +  C₃H₆

mass change 70 - 42 = 28  (M-42 ion peak)

Formation of the m/z 27 ion

Formed by elimination of hydrogen from the m/z 29 ion.

 [C₂H₅]⁺ ===>  [C₂H₃]⁺  +  H₂

mass change 29 - 2 = 27

Formation of the m/z 15 ion

Formed by C-C bond scission in the parent molecular ion, though small peak indicates a low probability of the methyl group breaking off.

 [C₅H₁₀]⁺ ===>  [CH₃]⁺  +  C₄H₇

mass change 70 -  55 = 15

This could happen to any fragment with a methyl group.

The infrared spectrum of pent-1-ene (1-pentene)

The H-1 NMR spectrum of Pent-1-ene (1-pentene)

The C-13 NMR spectrum of Pent-1-ene (1-pentene)

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