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Advanced Organic Chemistry: Mass spectrum of 2-methylpropanal

Interpreting the mass spectrum of 2-methylpropanal (isobutyraldehyde)

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing mass spectra of 2-methylpropanal

mass spectrum of 2-methylpropanal C4H8O (CH3)2CHCHO fragmentation pattern of m/z m/e ions for analysis and identification of isobutyraldehyde image diagram doc brown's advanced organic chemistry revision notes 

2-methylpropanal   C4H8O   aldehydes and ketones nomenclature (c) doc b    aldehydes and ketones nomenclature (c) doc b    aldehydes and ketones nomenclature (c) doc b

Interpreting the fragmentation pattern of the mass spectrum of 2-methylpropanal

[M]+ is the molecular ion peak (M) with an m/z of 72 corresponding to [C4H8O]+, the original 2-methylpropanal molecule minus an electron, [(CH3)2CHCHO]+.

The small M+1 peak at m/z 73, corresponds to an ionised 2-methylpropanal molecule with one 13C atom in it i.e. an ionised 2-methylpropanal molecule of formula [13C12C3H8O]+

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.

2-methylpropanal 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 (2-methylpropanal) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of 2-methylpropanal.

Unless otherwise indicated, assume the carbon atoms in 2-methylpropanal are the 12C isotope.

Some of the possible positive ions, [molecular fragment]+, formed in the mass spectrometry of 2-methylpropanal.

m/z value of [fragment]+ 71 57 43 42 41 39 29 28 27
[molecular fragment]+ [C4H7O]+ [C3H5O]+ [C3H7]+ [C3H6]+ [C3H5]+ [C3H3]+ [CHO]+ [C2H4]+ [C2H3]+

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of 2-methylpropanal

Atomic masses: H = 1; C = 12; N = 14; 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 2-methylpropanal (tabulated above)

Formation of m/z 71 ion:

[(CH3)2CHCHO]+  ===>  [C4H7O]+  +  H

Scission of C-H bond in the aldehyde group, mass change 72 - 1 = 71.

Formation of m/z 57 ion:

[(CH3)2CHCHO]+  ===>  [CH3CH2CO]+  +  CH3

C-C bond scission, loss of methyl group, mass change 72 - 15 = 57.

Structure of ion?: [CH3CH2CO]+  or  [CH3CHCHO]+  ?

Formation of m/z 43 ion:

[(CH3)2CHCHO]+  ===>  [(CH3)2CH]+  +  CHO

Scission of C-C bond, mass change 72 - 29 = 43, secondary carbocation formed.

The CHO can also be ionised to give the m/z 29 ion (see below).

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

Could the m/z ion also be due to [CH3C=O]+ ?

Formation of m/z 29 ion:

[(CH3)2CHCHO]+  ===>  [CHO]+  +  (CH3)2CH

Scission of C-C bond, mass change 72 - 43 = 29.


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