Question

Draw a rearrangement like reaction D in figure 22-12 to show how m/z 58 arises from 4-methyl-2-pentanone.

Short answer

The structure of the 4-methyl-2-pentanone is shown below :

Step-by-step solution

Mc- Lafferty Rearrangement :

The Mc- Lafferty Rearrangement is a characteristic fragmentation of the

molecular ion of a carbonyl compound containing at least one γ- Hydrogen.

It is an example of intramolecular rearrangement.

     Step 2 :  Showing Mc- Lafferty Rearrangement in  4-methyl-2-pentanone :

In the given compound a γ-Hydrogen or 4-H (on numbering from carbonyl carbon) is present. So, there is a possibility of a Mc- Lafferty Rearrangement . After fragmentation we get [a radical cation, $\frac{\mathrm{m}}{\mathrm{z}}$= 58 . [ ${\mathrm{C}}_3\mathrm{H}0:(12\times 3)+\left(1\times 6\right)+16=36+6+16=58$].

Hence , due to Mc- Lafferty Rearrangement $\frac{\mathrm{m}}{\mathrm{z}}$= 58 arises from 4-methyl-2-pentanone.