organic chemistry - Is the transformation of pi bonds to sigma bonds always energetically favorable?

Is the move from pi bonds to sigma bonds always favorable?

My professor claims so, except in the case of conjugated pi bonds. I can see how pi bonds going to sigma bonds might be favorable; I know that double bonds are actually not twice as strong as single bonds because of poorer overlap ... but are there any exceptions to this guideline that the reduction of pi bonds to sigma bonds is favorable except in the case of conjugated systems?

EDIT: what about in the case of organic chemistry? It certainly seems that hydrogenation (a way of transforming pi bonds to sigma bonds) is generally a favorable move.

Answer

The answer to your question is no.

In terms of organic chemistry, consider the two hydrocarbons, $\ce{H_3C-CH_3}$ and $\ce{HC\equiv CH}$. The $\ce{C-C}$ bond dissociation energy is 368 kj/mol while the $\ce{C\equiv C}$ triple bond has a bond dissociation energy of 682 kj/mol, nearly twice that of the single bond.

The transformation of this triple to single bond may be thermodynamically favorable via hydrogenation but may not always be the case. How does temperature affect hydrogenation and can $\Delta H$ be positive under other conditions? Furthermore, if we want to talk about the 'strength' of the CC bond, then clearly the triple bond is stronger.

Phillip brought up a good point in the comments that merits attention. Hydrogenation contains an appreciable barrier for this process to occur. When we take into account the formation of the additional single bonds by hydrogen then the energy is lower. However, if we consider strictly a pi to sigma transformation (between two carbons in this example) and not the formation of other bonds, based on the energetics of the bond we are transforming, the energy will increase.

Reference for bond energies: http://web.chem.ucsb.edu/~zakariangroup/11---bonddissociationenergy.pdf