Question

Question: Why is the bond dissociation energy for the C-C bond in ethane much higher than the bond dissociation energy for the labeled C-C in but-1-ene?

Short answer

Answer

There is a driving required for the but-1-ene to break to form the resonance stabilized allylic radical, as the bond dissociation energy of but-1-ene is low.

But in case of ethane, the bond dissociation energy is high since the bond breaks to form two unstable methane radicals and hence higher energy is required. There is a driving force required for the but-1-ene to break to form the resonance stabilized allylic radical; hence the bond dissociation energy of but-1-ene is low.

Step-by-step solution

Bond Dissociation Energy

The cleavage of any bond requires energy. Bond dissociation energy is the measure of the energy required to cleave a bond to give the constituent radicals species.

It gives an idea about the stability of the bond. Greater the bond dissociation energy, greater is the stability of the bond, making it difficult to cleave.

Comparison of bond dissociation energy

The two compounds given are:

When the labeled bonds are cleaved, the constituent radicals are shown below:

In case of ethane

The cleavage of ethane into constituent radicals

In case of ethane, the radicals are not stabilized by resonance as there is no delocalization or pi-bond. Thus, the radicals formed are unstable.

In case of but-1-ene

The cleavage of but-1-ene into constituent radicals

The constituent radicals of but-1-ene are a methyl radical and an allyl radical. The allyl radical can participate in resonance as shown below:

Resonance of the allylic radical

Since there is a driving for the but-1-ene to break to form the resonance stabilized allylic radical, the bond dissociation energy of but-1-ene is low.

But in case of ethane it is high since the bond breaks to form two unstable methane radicals.