The mass spectrum of 3-butyn-2-ol has a very strong fragment ion (base peak) at m/e = 55. Explain the formation and exceptional stability of this ion.
Hint: Consider the mass differences between this ion and the
molecular ion M.
The first thing to consider when given a structure to work with in a mass spectroscopy problem are the places the structure can break to give the most stable carbocations. Don't even consider the m/e ratio of the peaks you are trying to explain until you have looked at the structure. The following picture demonstrates the most stable carbocation that can be generated from 3-butyn-2-ol.
Once you have completed an analysis of what the structurally sensible fragments are, calculate their m/e ratios to see if you have answered the question. The m/e of the cation shown above is 55, the fragment the question asks about. If you are having trouble deciding what carbocations are the most stable, review carbocation stability starting on page 160 in your textbook and also starting on the bottom of page 244 (better consideration of resonance stabilization).
Last modified 1/20/97
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