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Advanced Organic Chemistry: Mass spectrum of ethanol

The mass spectrum of ethanol (ethyl alcohol)

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

mass spectrum of ethanol fragmentation pattern of ions for analysis and identification of ethyl alcohol image diagram doc brown's advanced organic chemistry revision notes 

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Revision notes on the structure and naming (nomenclature) of aliphatic ALCOHOLS and ETHERS

Interpreting the mass spectrum of ethanol

[M]+ is the molecular ion peak (M) with an m/z of 46 corresponding to [C2H6O]+, the original ethanol molecule minus an electron, [CH3CH2OH]+.

The tiny M+1 peak at m/z 47, corresponds to an ionised ethanol molecule with one 13C atom in it i.e. an ionised ethanol molecule of formula [13C12CH6O]+.

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.

Ethanol has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a 13C atom.

The most abundant ion of the molecule under mass spectrometry investigation (methoxymethane) 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 methoxymethane.

Unless otherwise indicated, assume the carbon atoms in methoxymethane are the 12C isotope.

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

m/z value of [fragment]+ 45 43 31 30 29 28 27 26 15
[molecular fragment]+ [C2H5O]+ [C2H3O]+ [CH2OH]+ [CHOH]+ [C2H5]+ [C2H4]+ [C2H3]+ [C2H2]+ [CH3]+

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

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

Bond enthalpies = kJ/mol: C-C = 348;  C-H = 412;  C-O = 360;  743;  O-H = 463

Explaining the principal fragments of the mass spectrum of ethanol

Formation of m/z 43 and 45 ions:

The prominent peak at m/z 45 corresponds to the loss of a hydrogen atom (actually a hydrogen radical H•) from the ionised ethanol molecule - the parent molecular ion.

[C2H6O]+  ===>  [C2H5O]+  +  H

Mass change 46 - 1 = 45.

Loss of H2 from this ion forms the m/z 43 ion.

[C2H6O]+  ===>  [C2H3O]+  +  H2

Formation of m/z 31 ion:

The m/z 31 ion is formed by scission of the C-C bond in the parent molecular ion.

[CH3CH2OH]+  ===>   [CH2OH]+  +  CH3

 Mass change 46 - 15 = 31.

The m/z 31 ion is the base peak, the most stable fragment.

Formation of m/z 28 ion:

The m/z 28 ion is formed by the elimination of water from the parent molecular ion of ethanol.

[CH3CH2OH]+  ===>   [C2H4]+  +  H2O

Mass change 46 - 18 = 28.

Formation of m/z 29 ion:

The m/z 29 ion is formed by scission of the C-O bond in the parent molecular ion of ethanol.

[CH3CH2OH]+  ===>   [C2H5]+  +  OH

Mass change 46 - 17 = 29.

Formation of m/z 27 ion:

The m/z 27 ion can be formed by elimination of hydrogen from the m/z 29 ion.

[C2H5]+  ===>   [C2H3]+  +  OH

Formation of m/z 15 ion:

The m/z 15 ion formed by scission of [CH3CH2OH]+  ===>  [CH3]+  +  CH2OH

Scission of the C-C bond in the parent molecular ion of ethanol.

 

See also comparing the IR, mass, 1H NMR and 13C NMR spectra of isomers of C2H6O below.

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Comparing the infrared, mass, 1H NMR and 13C NMR spectra of the 2 isomers of C2H6O

NOTE: The images are linked to their original detailed spectral analysis pages AND can be doubled in size with touch screens to increase the definition to the original ethanol (ethyl alcohol) and methoxymethane (dimethyl ether) image sizes.

INFRARED SPECTRA: Apart from the significant differences in the fingerprint region at wavenumbers 1500 to 400 cm-1, the most striking difference is the broad O-H stretching band ~3400 cm-1, found in the infrared spectrum of alcohols, but absent in the infrared spectrum of ethers.

MASS SPECTRA: Both ethanol and methoxymethane show some similarities in their mass spectra, their but their base ion peaks are quite different - for ethanol it is m/z 31 and for methoxymethane it is m/z 45. Ethanol gives the m/z 29 [C2H5]+ ion, which can be distinguished from the m/z 29 [C2H5]+ ion by high resolution spectroscopy. Ethanol has more abundant peaks for m/z ions 26, 27 and 43.

1H NMR SPECTRA: The 1H NMR spectra of ethanol and methoxymethane are quite significantly different. Ethanol gives 3 peaks in the proton ratio 3:2:1 (3 different chemical environments), whereas methoxymethane only gives one 1H chemical shift peak (all 6 protons in the same chemical environment).

13C NMR SPECTRA: The 13C NMR spectra of ethanol and methoxymethane are different. Ethanol gives two 13C resonances, but methoxymethane only one (2 different 13C chemical environments and a 13C single chemical environment).


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