[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 - analysing the mass spectra of 1-methylethyl methanoate (isopropyl methanoate) [spectra page updated April 4th 2026 *]

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Students and teachers please note my explanation of the mass spectrum of propan-2-yl methanoate (isopropyl formate) is designed for advanced, but pre-university, chemistry courses.

If M represents the propan-2-yl methanoate (isopropyl formate) molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and 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 propan-2-yl methanoate (isopropyl formate) and only the formation of singly charged positive are considered for the mass spectrum of propan-2-yl methanoate (isopropyl formate).

I've included a stick diagram and table of m/z ions for the mass spectrum of propan-2-yl methanoate (isopropyl formate) 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 propan-2-yl methanoate (isopropyl formate).

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 and compared the accurate ion masses if appropriate for this ester. 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 this ester, but the mass spectrometer software does!

HCOOCH(CH₃)₂  1-methylethyl methanoate, propan-2-yl methanoate,

is an example of an ester, some old names: isopropyl methanoate, isopropyl formate

Interpreting the fragmentation pattern of the mass spectrum of 1-methylethyl methanoate

[M]⁺ is the molecular ion peak (M) with an m/z of 88 corresponding to [C₄H₈O₂]⁺, the original 1-methylethyl methanoate molecule minus an electron, [HCOOCH(CH₃)₂]⁺

The peak is tiny, suggesting the molecular ion is relatively unstable compared to many of the fragmentation ions.

The most abundant ion of the molecule under mass spectrometry investigation (1-methylethyl methanoate) 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 1-methylethyl methanoate is the m/z 45 ion [HCOO]⁺

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

Unless otherwise indicated, assume the carbon atoms in 1-methylethyl methanoate are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of 1-methylethyl methanoate.

The parent molecular ion of 1-methylethyl methanoate is m/z 88: [C₄H₈O₂]⁺  or  [HCOOCH(CH₃)₂]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 1-methylethyl methanoate

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;  C=O  743

Possible equations to explain the most abundant ion peaks of 1-methylethyl methanoate (tabulated above)

Formation of m/z 73 ion:

[HCOOCH(CH₃)₂]⁺  ===>  [HCOOCHCH₃]⁺  +  CH₃

C-C bond scission of the parent molecular ion, loss of methyl group,

mass change 88 - 15 = 73 (M-1 ion peak)

Note the 'connection' between the m/z 73 and 15 ion equations, either fragment can be ionised, but only one of the two fragments can be ionised to carry the positive charge.

Formation of m/z 59 ion:

[HCOOCH(CH₃)₂]⁺  ===>  [OCH(CH₃)₂]⁺  +  HCO

C-O bond scission of the parent molecular ion, loss of HCO group,

mass change 88 - 29 = 59 (M-29 ion peak)

Formation of m/z 45 ion:

[HCOOCH(CH₃)₂]⁺  ===>  [HCOO]⁺  +  CH(CH₃)₂

C-O bond scission of the parent molecular ion, loss of HCO group,

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

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

The m/z 46 ion [H¹³COO]⁺ could be formed in the same way, but containing a 13C isotope ion

Formation of m/z 43 ion:

[HCOOCH(CH₃)₂]⁺  ===>  [CH(CH₃)₂]⁺  +  HCOO

C-O bond scission of the parent molecular ion, loss of HCOO group,

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

A secondary carbocation is formed.

Note the 'connection' between the m/z 43 and 45 ion equations, either fragment can be ionised, but only one of the two fragments can be ionised to carry the positive charge.

The m/z 43 ion can lose hydrogen atoms/molecule to give a succession of lighter fragment ions e.g. m/z values of 42, 41 and 39

Formation of m/z 29 ion:

[?]⁺  ===>  [C₂H₅]⁺  +  ?

or more likely

[HCOOCH(CH₃)₂]⁺  ===>  [CHO]⁺  +  OCH(CH₃)₂

C-O bond scission of the parent molecular ion

I'm not sure which fragment species predominates for these pairs, which might be formed by other fragmentation reactions, BUT an accurate mass spectrometer can sort them out - 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, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 29: [CHO]⁺ = 29.0027  and [C₂H₅]⁺ = 29.0390, a difference of 0.0363 in relative ion mass,

Formation of m/z 28 ion:

[?]⁺  ===>  [C₂H₄]⁺  +  ?

or

[CHO]⁺  ===>  [CO]⁺  +  H

I'm not sure which fragment species predominates for these pairs or how they are formed,

but (see note above), using accurate relative isotopic masses:

¹H = 1.0078 ¹²C = 12.0000 ¹⁶O = 15.9949, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 28: [CO]⁺ = 27.9949  and [C₂H₄]⁺ = 28.0312, difference of 0.0363 in relative ion mass.

Formation of m/z 15 ion:

[HCOOCH(CH₃)₂]⁺  ===>  [CH₃]⁺  +  HCOOCHCH₃

The methyl ion can be formed by C-C bond scission of the parent molecular ion or from several of the larger fragment ions with at least two carbon atoms.

The infrared spectrum of 1-methylethyl methanoate, (propan-2-yl methanoate) (isopropyl formate)

The H-1 NMR spectrum of 1-methylethyl methanoate, (propan-2-yl methanoate) (isopropyl formate)

C-13 NMR spectrum of 1-methylethyl methanoate, (propan-2-yl methanoate) (isopropyl formate)

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Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on spectroscopy (mass spectra of 1-methylethyl methanoate (IUPAC Propan-2-yl methanoate, isopropyl methanoate, isopropyl formate)) are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.