Chemistry: Molecular Orbital Theory and Conjugation

[r4mbini]

I am working my way through Ian Fleming's 'Molecular Orbtials and Organic Chemical Reactions' and I am having trouble with one of the exercise questions. (Ch2 Q1)

It reads: Given that two pi bonds conjugated together have a lower energy than two separate pi bonds and a C-H bond conjugated with a pi bond also lowers the energy, explain why two C-H bonds conjugated to each other are not stabilised.

Is this a question of symmetry not allowing mixing?

This is my attempt at some sort of MO diagram I can't really see why one of the situations (left or right) would be favoured over the other but those are the lines I'm thinking along.

any help is appreciated, thank you

 

[Asim Riaz]

!Yes, symmetry plays a role in why two C-H bonds conjugated to each other are not stabilised. In the MO diagram shown, the two C-H bonds (represented by the red lines) would be considered as two separate pi bonds, rather than one larger conjugated system. This is because there is a symmetry issue that prevents the two separate pi bonds from mixing and forming one large conjugated system. Furthermore, the energy of two separate pi bonds is higher than that of a single conjugated pi bond, thus the energy of two C-H bonds conjugated to each other is not stabilised.