Alcohols (R-OH) ionize to lose one non-bonded electron from the oxygen atom, forming molecular ions. Due to their tendency to fragment rapidly, the intensity of the molecular ion peak in the mass spectrum is weak or sometimes absent. The fragmentation patterns for alcohols occur in two ways, i.e. ⍺-cleavage and dehydration. During ⍺-cleavage, the bond at the ⍺-position adjacent to the hydroxyl group cleaves to give a resonance-stabilized cation and a radical. However, intramolecular dehydration involves losing a water molecule from an alcohol, forming an alkene cation, which shows a peak at M−18. Consider the fragmentation of butanol as shown below. ⍺-cleavage produces a base peak at a mass-to-charge ratio of 31. In the case of dehydration, the fragmentation forms an alkene radical cation at a mass-to-charge ratio of 56.
Alcohols, upon ionization, lose a non-bonded electron from oxygen to form molecular ions.
However, as evident from the mass spectrum of 1-butanol, the molecular ions from alcohols exhibit very weak peak intensity because they readily fragment.
The commonly observed alcohol fragmentation patterns are ⍺ cleavage and dehydration.
In ⍺ cleavage, a C–C bond at the ⍺ carbon next to the hydroxyl group cleaves to form a resonance-stabilized cation and a radical.
Alternatively, during dehydration, alcohols undergo intramolecular elimination of a water molecule, yielding an alkene radical cation that generates a peak at M−18.
As seen in the mass spectrum of 1-butanol, the prominent peak at a mass-to-charge ratio of 56 corresponds to dehydration.
The base peak at a mass-to-charge ratio of 31 corresponds to ⍺ cleavage.
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