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

May 6, 2015

The most likely value for the base peak in the mass spectrum of 3-methylpentane is at $m \text{/} z = 57$.

The structure of 3-methylpentane is

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 $m \text{/} z = 71$, corresponding to ${\text{CH"_3"CH"_2"CH"^+"CH"_2"CH}}_{3}$.
2. Loss of "CH"_3"CH"_2 ("M" - 29). This will give a peak at $m \text{/} z = 57$, corresponding to ${\text{CH"_3"CH"^+"CH"_2"CH}}_{3}$.

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 $m \text{/} z = 57$.