[Author © Dr Phil Brown GRIC, 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 spectrometry analysis of ethanol [spectra page updated RE-EDIT]

 Mass spectrometry - spectra index

Introductory note on the mass spectrum of ethanol

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

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

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

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 ethanol, but the mass spectrometer software does!

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]⁺.

Unless otherwise stated, C means a ¹²C atom, if not, the isotopic carbon atom ¹³C will be indicated.

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 (ethanol) 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 ethanol is the m/z 31 ion [CH₂OH]⁺

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

Unless otherwise indicated, assume the carbon atoms in ethanol molecular ion and fragment ions 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.

Suggested equations explaining the principal fragments of the mass spectrum of ethanoic acid

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.

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

Atomic masses: H = 1;  C = 12 (1 in ~100 is 13);  O = 16

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

Suggested equations 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 (M-1 ion peak)

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

[C₂H₅O]⁺  ===>  [C₂H₃O]⁺  +  H₂

Note that the ionised ethanol molecule [C₂H₆O]⁺ can be written as [C₂H₅OH]⁺  or  [CH₃CH₂OH]⁺.

BUT, the ionised original molecule, or, any other ionised fragment, may not necessarily have the same structure as it had in the original ethanol molecule.

This is a general situation in the mass spectra of organic compounds like ethanol.

this is why mass spectrum fragments are often listed as simple molecular formulae as well as showing the actual bonding arrangement in the fragment, which may not fit in with the 'usual' organic molecular structures you are used to!

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 (M-15 ion peak)

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 (M-18 ion peak)

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 (M-17 ion peak)

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]⁺  +  H₂

Mass change 29 - 2 = 27

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.

Note the m/z 19 ion is [H₃O]⁺ and occurs in the spectra of alcohols.

The H-1 NMR spectrum of Ethanol (ethyl alcohol)