Q34E - Organic Chemistry (Mcmurry)

Question

Using resonance structures of the intermediates, explain why bromination of biphenyl occurs at ortho and para positions rather than at meta.

Step-by-Step Solution

Answer

The formation of the positive charge intermediate is stabilized by the ortho and para attack because of the resonance contribution from the other ring of the biphenyl which does not occur in meta attack.

Step 1: Bromination

It is a process of electrophilic substitution of the aromatic ring by replacing the hydrogen with a Br ion in the presence of ${\mathbf{Br}}_{\mathbf{2}}\mathbf{,}{\mathbf{FeBr}}_{\mathbf{3}}$or NBS which is an N-Bromo succinimide reagent.

Step 2: Bromination of biphenyl at ortho and para position

While bromination occurs at ortho and para positive, the positive charge intermediate is formed which is stabilized by another ring of the biphenyl. It leads to stabilization of both ortho and para position.

Step 3: Bromination of biphenyl at the meta position

While bromination occurs at ortho and para positive, the positive charge intermediate is formed which is not stabilized by another ring of the biphenyl. It leads to destabilization of meta-position due to steric hindrance of both the ring of the biphenyl rings. 

Step4: Resonating structure of ortho, para, and meta

The formation of the positive intermediate stabilization in ortho and para attack and destabilization meta-position  is shown below.

Stability of Biphenyl