[wsytnik]

The molecular ion in the mass spectrum of 3-methyl-2-pentene appears at m/z 84.
Propose a structural formula for the prominent peak at m/z 55.

I am having problem with this question in my studies.  While I understand how to draw the compound and what the 84 represents, how do I segment a m/z 55.  I tried moving around the double bond and that doesn't seem to be working.  Is there a charge I am missing?  Thanks for any help.

[arit]

The decrease in m/z implies that the molecule has lost a mass fragment weighing 84-55= 29.
What do you think has happened to the molecule?

[wsytnik]

Thanks for the help.  I took an ethyl group off and left a positive charge on the carbon and got the right answer.

I'll be posting more questions as this course gets harder and harder I'm sure