Question

Which of the following alkenes will show the maximum number of hyperconjugation?
A.$C{{H}_{2}}=C{{H}_{2}}$
B.$C{{H}_{3}}-CH=C{{H}_{2}}$
C.$C{{H}_{3}}-C{{H}_{2}}-CH=C{{H}_{2}}$
D.$C{{H}_{3}}-C{{H}_{3}}-CH-CH=C{{H}_{2}}$

Answer 1

Hint: Alkenes, due to the presence of double bonds have the ability to possess hyperconjugation, which is the flow of electrons. This hyperconjugation gives rise to resonance structures of the molecule. The resonance structures are of low energy and high stability.

Complete answer:
Hyperconjugation is the ability of any unsaturated molecule to possess delocalization of electrons. The delocalization consists of sigma electrons. The delocalization is observed on the $s{{p}^{2}}$ hybridized carbon atom, and the hydrogen atoms attached adjacent to this carbon. The delocalization of sigma electrons occurs on the alkyl group which is directly attached adjacent to the carbon having double bonds (CH group) or to the atom having unshared p – orbital.
This means that the unsaturated group is $\beta $- carbon, and the electrons possessed by the $\alpha $- carbon has the delocalization. The delocalization will be maximum, if the $\alpha $- carbon has more sigma electrons. This means for an alkyl group, having more number of hydrogen atoms, or more number of $\alpha $- hydrogen in a molecule.
From the given molecules, we can see that,
$C{{H}_{2}}=C{{H}_{2}}$has 0 $\alpha $- hydrogen
$C{{H}_{3}}-CH=C{{H}_{2}}$has 3 $\alpha $- hydrogen
$C{{H}_{3}}-C{{H}_{2}}-CH=C{{H}_{2}}$has 2 $\alpha $- hydrogen
$C{{H}_{3}}-C{{H}_{3}}-CH-CH=C{{H}_{2}}$has 1 $\alpha $- hydrogen
From this it is clear that $C{{H}_{3}}-CH=C{{H}_{2}}$will show maximum number of hyperconjugation, as it has maximum alpha – hydrogen.

Hence, option B is correct.

Note:
Hyperconjugation is a permanent effect and is also called $\sigma ,\pi $conjugation. This is an extension for resonance. Also when a greater number of alkyl groups are attached with the positively charged carbon atom the hyper conjugation is maximum and has greater stability due to the presence of more alpha – hydrogen.