Q25E - Organic Chemistry (Mcmurry)

Question

The following molecular model of a dimethyl-substituted biphenyl represents the lowest-energy conformation of the molecule. Why are the two benzene rings tilted at a 63° angle to each other rather than being in the same plane so that their p orbitals overlap? Why doesn’t complete rotation around the single bond joining the two rings occur?

Step-by-Step Solution

Answer

The two benzene rings are tilted at a 63° angle to each other due to steric hindrance.

Rotation doesn’t occur around a single bond due to methyl group repulsion.

Step-by-Step Solution Step1: Two benzene ring tilted at 63°

In biphenyl, two benzene rings are joined by a single bond and the ring is present in tilted form rather than flat form because:

If the two rings are in a plane, it leads to high steric hindrance. Even the ethyl group repulsion does not let the molecule be in plane which makes one of the rings tilted and leads to a decrease in steric hindrance and makes the molecule stable.

Step 2: Two benzene rings do not show rotation around a single bond

Two benzene rings do not show rotation around a single bond because of the presence of methyl group as the substituent on both rings. Due to the repulsion between the methyl group, rotation is restricted and they do not allow the complete rotation of the molecule.