Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a
σ -bond (usually
C−H ) with an adjacent empty or partially filled
p -orbital or a
π -orbital to give an extended molecular orbitalthatincreases the stability of the system. Consider the molecule propene, where
π bond is in conjugation with
3σC−H bond of adjacent methyl group. So, higher the number of adjacent
σC−H bond to
π-bond higher will be the hyperconjugation and higher will be the stability. 2.
(IV) The adjacent
σC−H bond to
π bond increases in the order of IV > III > II > I.
Therefore, stability follows the same order.