Question

Question:Suppose you have two bottles, labeled ketone A and ketone B. You know that one bottle contains ${\mathbf{CH}}_3\mathbf{CO}{\left({\mathbf{CH}}_2\right)}_5{\mathbf{CH}}_3$and one contains ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{CO}{\left({\mathbf{CH}}_2\right)}_4{\mathbf{CH}}_3$but you do not know which ketone is in which bottle. Ketone A gives a fragment at m/z= 99 and ketone B givesa fragment at m/z= 113. What are the likely structures of ketones A and B from these fragmentation data?

Short answer

Answer

Step-by-step solution

Step 1: α cleavage of carbonyl groups

Theα cleavage of aldehydes and ketones occurs along the carbon attached to the oxygen atoms. The cleavage leads to the formation of an acylium ion and a neutral radical.

 Determination of ketone A and B

The structure of the two ketones are given below:

Structure of the two given ketones

The m/z value of both the ketones is 128. Upon fragmentation, the following cations are formed.

Fragmentation of the two ketones

From the fragmentation, it can be said that ketone A is the one with the formula:${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CO}{\left({\mathrm{CH}}_2\right)}_4{\mathrm{CH}}_3$

And ketone B is:${\mathrm{CH}}_3\mathrm{CO}{\left({\mathrm{CH}}_2\right)}_5{\mathrm{CH}}_3$