Question

Question: Explain why triphenylene resembles benzene in that it does not undergo addition reactions with $\mathit{B}{\mathit{r}}_{\mathbf{2}}$, but phenanthrene reacts with $\mathit{B}{\mathit{r}}_{\mathbf{2}}$to yield the addition product drawn. (Hint: Draw resonance structures for both triphenylene and phenanthrene, and use them to determine how delocalized each $\mathbf{\pi }$bond is.)

Short answer

Answer

The more delocalized electron density in triphenylene than phenanthrene makes them not to undergo addition reactions with$B{r}_2$.

Step-by-step solution

Triphenylene

Tripheylene falls under the category of polycyclic aromatic compound. The molecule has a shape similar to a disc and possesses a planar structure.

Phenanthrene

Three benzene rings can be merged together in order to obtain the structure of phenanthrene. It is used in the creation of dyes, explosives, and drugs.

Resonance structures of triphenylene and phenanthrene

The resonance structures of triphenylene can be given as:

Resonance structures of triphenylene

In the structure, A-H, the three double bonds, and the three single bonds are located on the molecule's periphery.

This indicates that each ring acts as an isolated benzene ring subjected to substitution reactions rather than addition reactions as the $\mathrm{\pi }$electron density remains delocalized inside the six-membered ring. The resonance structure I does not exhibit this form.

The resonance structures of phenanthrene can be given as:

Resonance structures of phenanthrene

Four out of the five resonance structures contain a double bond at the labeled carbon shown above in phenanthrene. So, these carbons have more double bond character than the C-C bonds in phenanthrene.

Hence, phenanthrene is more susceptible to addition reactions than triphenylene.