Chapter 12A, Problem 15P

The mass spectrum of the following compound shows fragments at $m\text{/}z$= 127, 113, and 85. Propose structures for the ions that give rise to these peaks.

Interpretation Introduction

Interpretation: The structures of the ions of a given compound that shows fragments at $m\text{/}z=127,113$ and $85$ is to be proposed.

Concept introduction: Molecular ion is the radical cation which is formed by ejection of electrons from a molecule when a beam of high-energy electrons bombarded on a molecule. Mass of the molecule is equal to $m\text{/}z$ value of the molecular ion.

Answer to Problem 15P

The fragments of 3-ethyl, 3-methylheptane at $m\text{/}z=127$, $m\text{/}z=113$ and $m\text{/}z=85$ are shown below.

Explanation of Solution

The ball and stick model of the given compound is,

Figure 1

Black colored atoms have four bonds. So, these are the carbon atoms. The grey colored balls have one bond. So, these are the hydrogen atoms. The molecular structure is,

Figure 2

Hence, the ball and stick model of the given compound is 3-ethyl, 3-methylheptane. The mass of 3-ethyl, 3-methylheptane is follow.

$\begin{array}{rll}{\text{Mass of C}}_{\text{10}}{\text{H}}_{\text{22}}& =& \left(\text{10}\times \text{Mass of C atom + 22}\times \text{Mass of H atom}\right)\\ & =& \left(\text{10}\times \text{12+22}\times 1\right)\\ & =& \left(120+22\right)\\ & =& 142\text{ g/mol}\end{array}$

Thus, the molecular ion peak of 3-ethyl, 3-methylheptane is observed at $m\text{/}z=142$.

The molecular ion $\left(m\text{/}z=142\right)$ loses methyl radical $\left(\text{M}-15\right)$ to give a fragment ion at $m\text{/}z=127$.

The molecular ion $\left(m\text{/}z=142\right)$ loses ethyl radical $\left(\text{M}-29\right)$ to give a fragment ion at $m\text{/}z=113$.

The molecular ion $\left(m\text{/}z=142\right)$ loses butyl radical $\left(\text{M}-57\right)$ to give a fragment ion at $m\text{/}z=85$.

Hence, the fragments of 3-ethyl, 3-methylheptane at $m\text{/}z=127$, $m\text{/}z=113$ and $m\text{/}z=85$ are shown below.

Figure 3

Conclusion

The fragments of 3-ethyl, 3-methylheptane at $m\text{/}z=127$, $m\text{/}z=113$ and $m\text{/}z=85$ are shown in Figure 3.