Help with mass spec

[bobby5x]

Hello, my name is Rob and I am a part time college student, currently taking the lab portion of Organic Chemisty 2. I need help interpreting the mass spec of Isoamyl acetate (part of a homework assignment.)

 

The graph has a base peak at m/z = 43 which I understand to be the isopropyl portion and/or the ion consisting of a methyl attached to the carbonyl (can't think of the name of this.)

 

The graph also has two large peaks at m/z = 55 and 70. These, I do not understand. I assume that this molecule would undergo a McLafferty rearrangement, and the resulting alkene would have a weight of 70, but I thought that this alkene was always neutral. I suppose that if it's not, the 55 would be this ion minus a methyl, but I didn't think any of this happened, let alone formed the second and third biggest peaks.

 

In any case, can anyone help me with this?

[CharonY]

Were the ions generated by PI or EI?

Anyway you are not completely off-track. m/z 43 is the acylium ion, and m/z 70 is the result from a heterolytic cleavage from the distonic ion.

So you have a gamma-H rearrangement, followed by the cleavage.

 

What also happens is a double H rearrangement (also called McLafferty+1), resulting in a weaker peak at m/z 61.

[bobby5x]

Thanks for the help!