mass spectrum of propanal fragmentation pattern of m/z m/e ions for analysis and identification of propionaldehyde image diagram doc brown's advanced organic chemistry revision notes

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Advanced Organic Chemistry: Mass spectrum of propanal

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The mass spectrum of propanal (propionaldehyde)

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 propanal

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Mass spectroscopy - spectra index

mass spectrum of propanal fragmentation pattern of m/z m/e ions for analysis and identification of propionaldehyde image diagram doc brown's advanced organic chemistry revision notes

propanal (propionaldehyde), C₃H₆O, , , ,

Interpreting the fragmentation pattern of the mass spectrum of propanal (propionaldehyde)

[M]⁺ is the molecular ion peak (M) with an m/z of 58 corresponding to [C₃H₆O]⁺, the original propanal molecule minus an electron, [CH₃CH₂CHO]⁺.

The small M+1 peak at m/z 59, corresponds to an ionised propanal molecule with one ¹³C atom in it i.e. an ionised propanal molecule of formula ¹³C¹²C₂H₆O

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.

Propanal has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a ¹³C atom.

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

The parent molecular m/z 58 ion is [CH₃CH₂CHO]⁺

m/z value of [fragment]⁺	57	39	31	30	29	29
[molecular fragment]⁺	[CH₃CH₂C=O]⁺	[C₃H₃]⁺	[H₃CO]⁺	[H₂C=O]⁺	[CHO]⁺	[CH₃CH₂]⁺

m/z value of [fragment]⁺	28	28	27	26	18	15	14
[molecular fragment]⁺	[CO]⁺	[C₂H₄]⁺	[C₂H₃]⁺	[C₂H₂]⁺	[H₂O]⁺	[CH₃]⁺	[CH₃]⁺

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

Atomic masses: H = 1; C = 12; O = 16;

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

Possible equations to explain some of the most abundant ion peaks of propanal

Formation of m/z 57 ion:

[CH₃CH₂CHO]⁺ ===> [CH₃CH₂C=O]⁺ + H

C-H bond scission in the parent molecular ion, mass change 58 - 1 = 57.

Elimination of water from this ion gives the m/z 39 ion of [C₃H₃]⁺, mass change 57 - 18 = 39.

Formation of m/z 29 ion:

(i) [CH₃CH₂CHO]⁺ ===> [CHO]⁺ + CH₃CH₂

(ii) [CH₃CH₂CHO]⁺ ===> [CH₃CH₂]⁺ + CHO

C-C bond fission of the parent molecular ion, mass change 58 - 29 = 29.

The m/z 29 ion is the base peak ion, the most abundant ion fragment from two sources, both formed from alternative ionisation of fragments from a C-C bond fission in the parent molecular ion of propanal.

Hydrogen atom loss from either gives the m/z 28 ion [C₂H₄]⁺ or [CO]⁺.

Formation of m/z 15 ion:

Formed by C-C bond scission in the parent molecular ion or the m/z 29 ion.

[CH₃CH₂CHO]⁺ ===> [CH₃]⁺ + C₂H₃O

[CH₃CH₂]⁺ ===> [CH₃]⁺ + CH₂

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Links associated with propanal

The chemistry of ALDEHYDES and KETONES revision notes INDEX

The infrared spectrum of Propanal (propionaldehyde)

The H-1 NMR spectrum of Propanal (propionaldehyde)

The C-13 NMR spectrum of Propanal (propionaldehyde)

Mass spectroscopy index

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