email doc brown  Re-edit mass spectrum of CH₃COOCH₂CH₃

 Mass spectrometry - spectra index

Introductory note on the mass spectrum of ethyl ethanoate

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

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

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

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 ethyl ethanoate, but the mass spectrometer software does!

Interpreting the fragmentation pattern of the mass spectrum of ethyl ethanoate

Ethyl ethanoate C₄H₈O₂  , , ,  

The molecular structure and naming of carboxylic acids and derivatives

See also Isomers of molecular formula C₄H₈O₂ for other isomeric structures

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original ethyl ethanoate molecule minus an electron, [CH₃COOCH₂CH₃]⁺.

You might see a tiny M+1 peak at m/z 89, corresponding to an ionised ethyl ethanoate molecule with one ¹³C atom in it i.e. an ionised ethyl ethanoate molecule of formula ¹³C¹²C₃H₈O₂

Carbon consists of ~1% ¹³C atoms, ethanoic acid has 4 carbon atoms, so about 1 in 50 molecules or fragments may contain a carbon-13 atom.

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.

Ethyl ethanoate acid 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 (ethyl ethanoate) 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 ethyl ethanoate is the m/z 43 ion [CH₃CO]⁺

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

Parent molecular ion m/z 88   [C₄H₈O₂]⁺  or  [CH₃COOCH₂CH₃]⁺

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

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

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 (13 for ~1 in 100);  O = 16

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  C=O  = 743

Suggested equations to explaining the principal fragments of the mass spectrum of ethyl ethanoate

The most abundant ion peaks

Formation of m/z 73 ion: 

[CH₃COOCH₂CH₃]⁺  ===>  [C₂H₅O-C=O]⁺  +  CH₃

C-C bond scission in the parent molecular ion,

mass change 88 - 15 = 73 (M-15 ion peak)

Formation of m/z 70 ion: 

[CH₃COOCH₂CH₃]⁺  ===>  [C₄H₆O]⁺  +  H₂O

Elimination of water from the parent molecular ion?,

mass change 88 - 18 = 70 (M-18 ion peak)

Formation of m/z 45 ion: 

[CH₃COOCH₂CH₃]⁺  ===>  [C₂H₅O]⁺  +  CH₃CO

C-O bond scission in the parent molecular ion,

mass change 88 - 43 = 45 (M-43 ion peak)

Formation of m/z 43 ion: 

[CH₃COOCH₂CH₃]⁺  ===>  [CH₃C=O]⁺  +  C₂H₅O

C-O bond scission in the parent molecular ion,

mass change 88 - 45 = 43 (M-45 ion peak)

The m/z 43 ion is the base peak ion, the most abundant 'stable' ion fragment.

The m/z 44 ion is probably formed in the same way, but contains a ¹³C atom i.e. it has the formula [¹³C¹²CH₃O]⁺ but it could be [C₂H₄O]⁺?

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  ¹⁶O = 15.9949: you can then calculate (predict) that the accurate relative ion masses are:

[C₂H₄O]⁺ = 44.0261  whereas  [¹³C¹²CH₃O]⁺ = 44.0217, a difference of 0.0044 in relative ion mass, no problem for a modern very accurate mass spectrometer.

Formation of m/z 29 ion: 

[CH₃COOCH₂CH₃]⁺  ===>  [C₂H₅]⁺  +  CH₃COO

C-O bond scission in the parent molecular ion, mass change 88 - 59 = 29 (M-59 ion peak)

Formation of m/z 15 ion: 

[CH₃COOCH₂CH₃]⁺  ===>  [CH₃]⁺  +  C₃H₅O₂

C-C bond scission in the parent molecular ion,

mass change 88 - 73 = 15 (M-73 ion peak)

The infrared spectrum of Ethyl ethanoate (ethyl acetate)

The H-1 NMR spectrum of Ethyl ethanoate (ethyl acetate)

The C-13 NMR spectrum of Ethyl ethanoate (ethyl acetate)

The chemistry of CARBOXYLIC ACIDS and DERIVATIVES revision notes INDEX

Mass spectrometry index

ALL SPECTROSCOPY INDEXES

All Advanced Organic Chemistry Notes

Use My Google search site box

Email doc b: chem55555@hotmail.com