Question

Identify the ions responsible for the peaks in the mass spectrum shown in Figure 20-22b.

Short answer

The ions responsible for the peaks in the mass spectrum shown in Figure 20-22b are :

$\mathrm{m}/\mathrm{z}=131$is due to localid="1645697862568" ${}^{35}\mathrm{Cl}_3{\mathrm{CCH}}^{2{+}_{}}$

$\mathrm{m}/\mathrm{z}=133$is due to localid="1645697936629" ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}_2{\mathrm{CCH}}^{2+}$

$\mathrm{m}/\mathrm{z}=135$is due to localid="1645697950886" role="math" ${}^{37}\mathrm{Cl}_{235}{\mathrm{ClCCH}}^{2+}$

$\mathrm{m}/\mathrm{z}=117$is due to ${}^{35}\mathrm{Cl}_3{\mathrm{C}}^{+}$

$\mathrm{m}/\mathrm{z}=119$is due to ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}_2{\mathrm{C}}^{+}$

$\mathrm{m}/\mathrm{z}=121$is due to localid="1645697971176" ${}^{37}\mathrm{Cl}_{235}{\mathrm{ClC}}^{+}$

Step-by-step solution

Concept Introduction

Mass spectrometry is a powerful analytical technique which is used to elucidate structure of compounds, quantify materials, identify unknown compounds etc. In mass spectrophotometer, the molecule is ionized by giving it a positive charge by removing an electron or by adding a proton. The molecule may break into several positively charged fragments according to the ionization method used. Then, in a mass analyzer, these cationic molecular fragments are separated based on their mass and detected by the detector. Only positively charged molecular fragments are detected by the detector.

Observing the figure

The figure 20-22b is as follows -

The molecular ion of $1,1,1,2$-tetrachloroethane can be fragmented in such a way given below:

Finding m/z values

Let us find the $m/z$values of ions :

These cation fragments could have either of two isotopes of chlorine which are ${}^{35}\mathrm{Cl}$and ${}^{37}\mathrm{Cl}$. The radical formed in the second step is not detected in mass spectrometer.

$\mathrm{m}/\mathrm{z}=131$is due to localid="1645697758887" ${}^{35}\mathrm{Cl}_3{\mathrm{CCH}}^{2+}$

$\mathrm{m}/\mathrm{z}=133$is due to localid="1645697804970" ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}_2{\mathrm{CCH}}^{2+}$

$\mathrm{m}/\mathrm{z}=135$is due to localid="1645697816685" role="math" ${}^{37}\mathrm{Cl}_{235}{\mathrm{ClCCH}}^{2+}$

$\mathrm{m}/\mathrm{z}=117$is due to ${}^{35}\mathrm{Cl}_3{\mathrm{C}}^{+}$

$\mathrm{m}/\mathrm{z}=119$is due to ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}_2{\mathrm{C}}^{+}$

$\mathrm{m}/\mathrm{z}=121$is due to localid="1645697837011" ${}^{37}\mathrm{Cl}_{235}{\mathrm{ClC}}^{+}$