Question

Given the parent compound, draw the mass spectrum fragment that is observed at m/z 71. Include any hydrogen atoms and the charge.

Given the parent compound, draw the mass spectrum fragment that is observed at m/z 71. Include any hydrogen atoms and the charge.

Answer 1

Concepts and reason

The concept used here is the fragmentation of heptane using mass spectrometry.

Mass spectrum is the graph of the molecule in which the mass to charge ratio is plotted on the X-axis and relative abundance is plotted on the Y-axis.

Fundamentals

In mass spectrometry organic molecules are bombarded with beam of high energy electrons which results in the breakdown of molecules. These electrons transfer the energy to the molecule which then becomes highly energy rich. These high energy species then ejects an electron to become a molecular ion. These molecular ions then breakdown into cations and many other fragments.

The highest peak is taken as , the remaining peaks are compared with the base peak . In Mass spectrum only the positively charged molecules are expressed.

Alkanes are fragmented extensively because weak bonds in alkanes break easily in comparison to stronger bonds in alkenes and alkynes.

Heptane has a molecular weight of . Therefore, the molecular ion peak is . The base peak observed in the case of pentane is at .

The mass spectrum of heptane is shown below.

‎The fragment observed at has molar mass of which corresponds to the alkane having carbon atoms. Therefore, the cation formed is pentyl cation.

Fragmentation of the molecular ion of heptane can be written as follows:

Ans:

The structure of mass spectrum fragment observed at is shown as follows: