Two main fragmentation patterns are seen most frequently in the mass spectrometry of alcohols:
- α-Cleavage – breaking the bond next to the carbon bearing the hydroxyl group.
- Loss of water (M-18) – dehydration of the molecular ion.
The reason for the tendency to undergo α-cleavage in alcohols is due to the resonance stabilization of the resulting carbocations by the lone pairs of the oxygen. Because of this, we often don’t see a strong molecular ion peak for alcohols and ethers.
For example, in the mass spectrum of 2-pentanol, the molecular ion peak is barely visible, while the base peak appears at m/z 45. This peak, together with the m/z 73, is generated from the signals of two carbocations formed by α-cleavage on both sides of the OH group. In each case, the resonance-stabilized CH₃CH₂OH⁺ and CH₃CH₂CH₂CH₂OH⁺ cations are formed, respectively:
Remember, α-cleavage is the most characteristic fragmentation of alcohols and ethers, and the presence of m/z 45 (CH₂OH⁺) is often used as a diagnostic signal to confirm their presence.
In this specific example, the peak generated as a result of dehydration (m/z 70) is very weak; however, the one at m/z 55 is most likely due to the further fragmentation of the radical carbocation formed during the loss of water:
Let’s also consider the mass spectrum and fragmentation of 2,3-dimethylbutan-1-ol. The carbocation generated from the loss of water gives a strong signal, but, like in the case of pentan-2-ol, it likely undergoes further fragmentation, forming a resonance-stabilized allylic carbocation (m/z 69).
The base peak is likely from the carbocation formed via α-cleavage. Although it is secondary, it is believed that hydride shifts and other rearrangements may convert it into a more stable tertiary carbocation. Notice that we also have a peak at m/z 43, which is a classic indicator of an acylium ion (CH₃CO⁺) or a propyl/isopropyl cation.
Check also our detailed Guide on Solving Mass Spectrometry Problems that includes the key McLafferty fragmentations of aldehydes, ketones, esters, acids, and nitriles. Once you master the key fragmentation, work on the comprehensive Mass Spectrometry Problems.
