[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 analysis of cyclopentane [spectra page updated Mar 22nd 2026 *]

email doc brown  Re-edit mass spectrum of cyclo-C₅H₁₀

Mass spectrometry - spectra index

Introductory note on the mass spectrum of cyclopentane

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

If M represents the cyclopentane 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 cyclopentane and only the formation of singly charged positive are considered for the mass spectrum of cyclopentane.

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

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 cyclopentane, but the mass spectrometer software does!

Cyclopentane, C₅H₁₀ , , , all bond angles ~109^o, but it is NOT planar, pentagon is slightly bent.

Interpreting the fragmentation pattern of the mass spectrum of cyclopentane

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

The small M+1 peak at m/z 71, corresponds to an ionised cyclopentane molecule with one ¹³C atom in it i.e. an ionised cyclopentane 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.

Cyclopentane 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 (cyclopentane) 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 peak ion for the mass spectrum of cyclopentane is the m/z 42 ion [C₃H₆]⁺

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

The parent molecular ion of cyclopentane is m/z 70: [C₅H₁₀]⁺

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

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)

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

Possible equations to explain the most abundant ion peaks of cyclopentane

Formation of m/z 55 ion

 [C₅H₁₀]⁺  ===>  [C₄H₇]⁺  +  CH₃

mass loss 70 - 15 = 55, from the molecular ion (M-15 ion peak)

C-C bond scission and proton rearrangement

Formation of m/z 42 ion:

[C₅H₁₀]⁺  ===>  [C₃H₆]⁺  +  C₂H₄

The m/z 42 ion is the base peak ion, the most abundant and 'stable' ion fragment.

This equates to a loss of two CH₂ units from a broken cyclopentane ring, the elimination of an ethene molecule

mass change 70 - 28 = 42 to give the M-28 ion peak

The m/z 43 ion could be formed in the same way but containing a ¹³C atom i.e. it has the structure  [¹³C¹²C₂H₆]⁺ rather the [C₃H₇]⁺ ion.

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 43: [C₃H₇]⁺  = 43.0546  and  [¹³C¹²C₂H₆]⁺ =  42.0424, a difference of 0.0044 in relative ion mass.

Formation of m/z 41 ion

 [C₄H₇]⁺  ===>  [C₃H₅]⁺  +  CH₂

mass loss 55 - 14 = 41

C-C bond scission

_{and you can construct lots of other possibilities!}

_{The m/z 42 ion can lose hydrogen atoms to give the m/z 41 down to 39 (see data table)}

Formation of m/z 27 and 29 ions e.g.

 [C₃H₆]⁺  ===>  [C₂H₃]⁺  +  CH₃

 [C₃H₆]⁺  ===>  [C₂H₅]⁺  +  CH

_{There are lots of possibilities for most of these fragmentations!}

_{The m/z 29 ion can lose hydrogen atoms to give the m/z 28 down to 26.}