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Magazine Chapter 12: Q.8P (page 629)
Question: Show the fragmentation that accounts for the cation at m/z 57 in the mass spectrum of 2-methylpentane. Explain why this ion is less abundant than those at m/z 71 and 43.
Short Answer
Answer
Abundance of ion depends upon the stability of ion as the stability order of ions (Carbonations) M/z=43>M/z=71>M/z=57
as M/z =43 ion 2o carbocation with 6 伪-hydrogens so, the most stable followed by M/z=71 carbocation M/z=71 1o carbocation with 2 伪-hydrogens and least stable M/z=57 ion 1o carbocation having only 1-伪 hydrogen.
Step by step solution
Peat at m/z 57 for the given molecule
Fragmentation of 2-methylpentane
Formation of m/z 57 ion:
Primary carbocation
This is a primary carbocation.
Peat at m/z 43 and m/z 71 for the given molecule.
C-C bond breaks from the parent molecular ion, leading to loss of an ethyl radical from the parent molecular ion (mass change = 86 - 29 = 57)
Formation of M/z 43 ion:
M/z=43 2O Carbocation:
Formation of M/z 71 ion:
Abundance of ion depends upon the stability of ion as the stability order of ions (Carbonations) M/z=43>M/z=71>M/z=57
as M/z =43 ion 2o carbocation with 6 伪-hydrogens so most stable followed by M/z=71 carbocation M/z=71 1o carbocation with 2 伪-hydrogens and least stable M/z=57 ion 1o carbocation having only 1-伪 hydrogen.
So M/z=57 ion is less abundant than M/z=43 and M/z=71
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