Question

The red \(\mathbf{C}-\mathrm{H}\) bond has a low bond dissociation energy because it is doubly allylic.

Short answer

Answer: The red C-H bond has low bond dissociation energy due to being doubly allylic because extensive resonance stabilization occurs between the adjacent double bonds. This delocalization of the unpaired electron over a larger portion of the molecule results in a more significant stabilization effect, leading to a lower bond dissociation energy and making the bond easier to break compared to a regular C-H bond or an allylic C-H bond.

Step-by-step solution

1. Define bond dissociation energy

Bond dissociation energy is the amount of energy required to break one mole of a specific type of chemical bond in a molecule or a compound, separating its atoms into individual free radicals. It is an indicator of the bond's stability. A lower bond dissociation energy indicates weaker bonding and a greater increase in the bond's potential energy.

2. Define allylic

An allylic position on a molecule refers to the carbon atom adjacent to a carbon-carbon double bond. This position is chemically distinct because the π-electrons (the electrons in the double bond) can contribute to the stability of any potential free radicals formed at that location due to resonance structures. Resonance allows the unpaired electron of the radical to delocalize across multiple carbon atoms in the molecule.

3. Define doubly allylic

A doubly allylic position is one that is two carbon atoms away from two non-consecutive double bonds. In other words, it is positioned between two allylic carbons. This proximity allows for more resonance structures to stabilize the radical formed when the bond dissociates, resulting in a lower bond dissociation energy.

4. Explain the low bond dissociation energy of the doubly allylic C-H bond

When the red C-H bond dissociates and forms a radical, the unpaired electron on the carbon atom can engage in resonance not with just one double bond (as in a single allylic system), but with both double bonds that are adjacent to the carbon atoms on either side. This extensive resonance stabilization leads to a lower bond dissociation energy for the doubly allylic C-H bond, making it easier to break compared to a regular C-H bond or an allylic C-H bond. The presence of both double bonds allows for the delocalization of the unpaired electron over a larger portion of the molecule, resulting in a more significant stabilization effect.