Mass Spectrometry: Alcohol Fragmentation

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.