Question

Fragmentation of Alkanes loss of e C6H14 radical cation = (M)+.

5. If the exact mass of C6H14 is 86.11 g/mol, then what is the mass of the radical cation?

Answer 1

5. If the exact mass of C₆H₁₄ is 86.11 g/mol, then the mass of radical cation is also 86.11 g/mol.

The given radical cation [C₆H₁₄]^.+ is generated from the hexane molecule. When hexane molecules are bombarded with high energy electron beam in mass spectrometer, then the hexane molecules loose one electron each to generated a molecular radical cation. This is radical cation is nothing but the hexane molecule which lost one of its electrons. Loss of single electron make the molecule a radical and the positive charge indicates it is electron deficient.

In the radical cation, there is the difference of one electron only with hexane molecules. The mass in an atom is contributed by protons and neutrons. The electrons are negligible with respect to their masses. A change in mass of a molecule can be observed when there is a change in proton or neutron number and not electron number.

As in this case, there is no change in the number of protons or neutrons in radical cation, the mass of radical cation would be the same as that of neutral molecules.

6. ¹²C and ¹³C are the isotopes of carbon. ¹²C has a mass of 12 amu or 12 g/mol. ¹³C has a mass of 13 amu or 13 g/mol. We generally made calculations by taking ¹²C as its isotopic abundance is comparatively very high than ¹³C.

In the first given structure all carbons are ¹²C and hence the mass for the hexane molecule with all ¹²C carbons follows the normal calculations.

The mass of hexane is already given in problem 5 which is calculated with all ¹²C is 86.11 g/mol (Structure-I)

The difference between the first and second structures is, in the place of one of the ¹²C in structure - I there exist a ¹³C in structure - II. Thus the difference between the masses of two structures is only one unit. IInd structure structure has 1 unit more mass than structure - I.

Thus the mass of hexane molecule containing one ¹³C atom is 86.11 + 1 g/mol = 87.11 g/mol (Structure-II)