Question

Question:Match each structure to its mass spectrum.

Short answer

Answer

Compound A :

Compound B :

Compound C :

Step-by-step solution

Mass spectroscopy

Mass spectroscopy is a method employed for the determination of the molecular weight and molecular formula of an organic compound. In the case of mass spectrometry, the individual mass of each element is taken into consideration, and the spectra are the plot of relative abundance against the m/z ratio.

In a mass spectrum, there is a base peak or the M peak, which is the tallest. M+1, M+2 peaks occur due to fragmentation. The peak with an m/z value equal to the compound's molecular weight is known as the molecular ion peak

Matching each structure to its mass spectrum

Compound A:

The compound contains a halogen: Br

The two stable isotopes of bromine are: ${}^{\text{79}}\text{Br\hspace{0.17em}}{,}^{\text{81}}\text{Br}$

The molecular weight for the compound containing${}^{\text{79}}\text{Br\hspace{0.17em}}$isotope is 122 amu.

The molecular weight for the compound containing${}^{\text{81}}\text{Br}$isotope is 124amu.

The relative abundance ratio of the two isotopes of bromine is almost 1:1. Thus, in the spectra, the two peaks for the two molecular weights should have equal heights.

Hence, the spectra for compound A are:

Mass spectra of compound A

Compound B:

The compound contains a halogen: Cl

The two stable isotopes of chlorine are: ${}^{\text{35}}{\text{Cl\hspace{0.17em},}}^{\text{37}}\text{Cl}$

The molecular weight for the compound containing${}^{\text{35}}\text{Cl\hspace{0.17em}}$ isotope is 122 amu.

The molecular weight for the compound containing ${}^{\text{37}}\text{Cl}$isotope is 124amu.

The relative abundance of ${}^{\text{35}}\text{Cl\hspace{0.17em}}$is 75%, and that of ${}^{\text{37}}\text{Cl}$is 25%. Thus, the molecular ion peak of ${}^{\text{35}}\text{Cl\hspace{0.17em}}$will be taller than that of role="math" localid="1649161008005" ${}^{\text{37}}\text{Cl\hspace{0.17em}}$, and the molecular ion ratio will be 3:1.

Hence, the spectra for compound B are:

Mass spectra of compound B

Compound C

The given compound contains only C, H, and O. There are no isotopes present, and thus, the compound will have a single molecular ion peak.

The molecular weight of the compound is 122 amu.

Thus, the spectrum of the compound is:

Mass spectrum of compound C