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What is the likeliest m/z value for the base peak in the mass spectrum of 3-methylpentane?

1 Answer
May 6, 2015

The most likely value for the base peak in the mass spectrum of 3-methylpentane is at #m"/"z = 57#.

The structure of 3-methylpentane is

www.introorganicchemistry.com

Mass spectroscopic fragmentation of hydrocarbons tends to occur at a branch.

So there are two possible fragmentation patterns:

  1. Loss of #"CH"_3 ("M" -15)#.This will give a peak at #m"/"z =71#, corresponding to #"CH"_3"CH"_2"CH"^+"CH"_2"CH"_3#.
  2. Loss of #"CH"_3"CH"_2 ("M" - 29)#. This will give a peak at #m"/"z = 57#, corresponding to #"CH"_3"CH"^+"CH"_2"CH"_3#.

The peaks at 57 and 29 should be more intense than those at 71 and 15, because the ethyl cation (29) is more stable than the methyl cation (15).

The 2-butyl cation is more stable than the ethyl cation, so the base peak is probably at #m"/"z = 57#.