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Introductory note on the mass spectrum of ethoxyethane

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

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

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

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 ethoxyethane, but the mass spectrometer software does!

Ethoxyethane C₄H₁₀O, , , aliphatic ether

The molecular structure and naming of aliphatic alcohols and ethers

Interpreting the mass spectrum of ethoxyethane (diethyl ether)

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₄H₁₀O]⁺, the original ethoxyethane molecule minus an electron, [CH₃CH₂OCH₂CH₃]⁺

The small M+1 peak at m/z 75, corresponds to an ionised ethoxyethane molecule with one ¹³C atom in it i.e. an ionised ethoxyethane 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.

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

The same argument applies to ionised fragments for the original ionised molecule of ethoxyethane.

(In the mass spectrum of ethers, an M+1 ion can also be formed by a hydrogen atom radical combining with the molecular ion i.e. m/z 75 can originate from [M]⁺  +  H•  ===>  [•MH]⁺, but this is heading beyond pre-university level)

The most abundant ion of the molecule under mass spectrometry investigation (ethoxyethane) 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 ethoxyethane is the m/z 31 ion [CH₃O]⁺

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of ethoxyethane - identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of the mass spectrum of ethoxyethane.

Unless otherwise indicated, assume the carbon atoms in the ethoxyethane molecular ion and fragment ions are the ¹²C isotope.

Suggested equations to explain some of the most abundant ion mass spectrum peaks of ethoxyethane i.e. explaining the principal fragments of the mass spectrum of ethoxyethane.

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.

Formation of the m/z 59 ion

The m/z 59 ion is formed by the loss of a methyl group from the parent molecular ion.

[CH₃CH₂OCH₂CH₃]⁺  ===>  [CH₃CH₂OCH₂]⁺  +  CH₃

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

Formation of the m/z 29 or 45 ion

[CH₃CH₂OCH₂CH₃]⁺  ===>  [CH₃CH₂]⁺  +  CH₃CH₂O

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

[CH₃CH₂OCH₂CH₃]⁺  ===>  [CH₃CH₂O]⁺  +  CH₃CH₂

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

Both ionised fragments can arise from the scission of the C-O bond in the molecular ion gives m/z 45 [CH₃CH₂O]⁺  or m/z 29   [CH₃CH₂]⁺, both of which have prominent intensities in the mass spectrum of ethoxyethane.

The m/z 29 ion can further lose protons to give m/z ions 26 to 28, all of which show up in the mass spectrum of ethoxyethane.

Formation of the m/z 31 ion

The m/z 31 ion is the base peak ion, the most stable fragment [CH₃O]⁺

Structure and formation?

[CH₃CH₂O]⁺  ===>  [CH₃O]⁺  +  CH₂

e.g. from the m/z 45 ion, mass change 45 - 14 = 31, C-C bond scission and proton migration?

Infrared spectrum of ethoxyethane (diethyl ether)

The H-1 NMR spectrum of Ethoxyethane (diethyl ether)

The C-13 NMR spectrum of ethoxyethane

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

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