Question

Molecular orbital calculations can be used to model the location of electron density from unpaired electrons in
a radical. Open the molecular models on the book’s website for the methyl, ethyl, and tert-butyl radicals. The
gray wire mesh surfaces in these models represent volumes enclosing electron density from unpaired electrons.
What do you notice about the distribution of unpaired electron density in the ethyl radical and tert-butyl radical,
as compared to the methyl radical? What bearing does this have on the relative stabilities of the radicals in
this series?

Answer 1

In methyl radical the electron density is located on the carbon atom while in ethyl and tertiary butyl radical the eelctron density due to lone pair of electron is distributed to other carbon atoms too.

This is due to the fact that ethyl and tertiarybutyl cation can undergo hyperconjugation. Hyperconjugation is a type of overlap between the sigma orbital on hydrogen (of alpha carbon) and pi orbital of carbon atom bearing the lone pair of electrons. Greater the number of hydrogen on the alpha carbon (carbon aton adjacent to the one bearing lone pair), more will be the number of hyperconjugative structure and as a result greater will be the stability.

In tertiary butyl radical 9 hyperconjugative structure is possible and in ethyl radical three hyperconjugative structure is possible. No hyperconjugative structures are there for methyl radical as no alpha carbon is present. the relative order of stability : tertiary butyl radical>ethyl radical> methyl radical