Question

1. Look up a three dimensional structure of hexaphenylbenzene. a) What are the orientation of the outside benzene rings with respect to the central benzene ring (dihedral angle)?

b) Hexaphenylbenzene appears to have an extended conjugation system, and might be expected to be colored, however, the compound is white. Briefly explain this observation

Answer 1

1 a) The structure has arrived from the benzene ring where 6 phenyl groups have substituted the small Hydrogens.

So The large Phenyl groups creat a steric overcrowding in the plane .So the 6 phenyl rings rotates out of plane of the central benzene ring.They are 65° rotated out of plane(dihedral angel=Angle between the plane of central ring and the plane of substituted phenyl= 65°).They make propeller type structure.

b).The molecule have 21 alternate double bonds .But for extended conjugation the whole molecule should be planar so that the perpendicular p orbitals can overlap.Here the substituted phenyl rings are at 65° of central ring making it non planer.So there is no extenbed conjugation.So the energy gap between MO s does not fall in visible region and the colour is seen white.