Molecular ion peak is intense in unsaturated compounds. Molecular ion represents the most stable cation formed by the removal of an electron from the parent molecule. In conjugated alkene, molecular ion is formed by the removal of one of the π electrons. The molecular ion, thus formed, is stabilized by the resonance. It means the burden of positive charge is shared between all atoms. Here we will study normal alkene. Let's try to solve the spectra of 3-heptne.
The other intense peak corresponds to the ion formed by McLafferty rearrangement. Let’s see what McLafferty rearrangement is. Molecules those have gama γ Hydrogen can undergo this rearrangement.
What does it mean by γ Hydrogen? Carbon atoms are labeled on the basis of their position in molecule with respect to the functional group. Carbon atom which bears functional group is called α carbon and Carbon atom placed next to the α carbon is called β carbon and the second next carbon is called the γ Carbon. The Hydrogen atoms attached to these carbon atoms are respectively called as α, β and γ Hydrogen. In McLafferty rearrangement, molecular ion splits from β Carbon and functional group gains the γ Hydrogen. Thus formed cation is stabilized by resonance and is represented by an intense peak in mass spectra.
The next stable cation is allylic carbocation. Allyl group is three carbon molecule with one double bond. It is formed when alkene undergoes allylic cleavage that means a cleavage which results in formation of allyl carbocation. This allyl cation is stabilized by resonance and appears as next highest peak to the molecular ion peak.
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