Chapter G, Problem G.10P

Interpretation Introduction

Interpretation:

A mass peak at m/z = $\text{59}$ appears in the mass spectrum of an amide, ${\text{C}}_{\text{5}}{\text{H}}_{\text{11}}\text{NO}$. Draw the structure of a molecule that is consistent with this result is to be drawn.

Concept introduction:

The mass of amide ${\text{C}}_{\text{5}}{\text{H}}_{\text{11}}\text{NO}$ is $\text{101}$ u. The peak at m/z = $\text{59}$ is due to the loss of a propene, ${\text{CH}}_{\text{3}}{\text{CH=CH}}_{\text{2}}$, and the formation of the enol radical cation ${\text{CH}}_{\text{2}}{\text{=C(}}^{\text{+}}{\text{OH)NH}}_{\text{2}}$. This is the McLafferty rearrangement of the amide’s molecular ion. A carbonyl-containing compound can undergo a McLafferty rearrangement if an alkyl group attached to the carbonyl carbon possesses a γ-carbon with at least one hydrogen atom. A $\text{H}$- atom from the $\text{γ}$-carbon shifts to the carbonyl $\text{O}$-atom, and the bond joining the $\text{α}$ and $\text{β}$ carbons is broken. The result is the ejection of an uncharged alkene molecule. The McLafferty rearrangement is not limited to just ketones, it is characteristic of several compound classes containing the carbonyl group, including aldehydes, carboxylic acids, esters, and amides.