Question

Which of the following biphenyls is optically active ?

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is A

To determine which of the given biphenyls is optically active, we need to understand the concept of optical activity. Optical activity refers to a molecule's ability to rotate the plane of polarized light. In organic chemistry, optical activity is usually associated with chirality. A chiral molecule is non-superimposable on its mirror image, often containing an asymmetric carbon atom. In biphenyls, chirality can arise when there are steric hindrances preventing the free rotation around the single bond connecting the two phenyl rings, resulting in atropisomerism.

Among the given options, we need to consider the structure associated with each image of biphenyls. Here, we have images as options, but typically you'd look for the following characteristics to assess chirality in biphenyls:

1. Look for restricted rotation around the central C-C bond.
2. Check if there are steric constraints, due to bulky groups, that prevent the phenyl rings from becoming coplanar.
3. Ensure there is no plane of symmetry or center of inversion in the molecule.

With these criteria, we can see that only certain biphenyl derivatives can be chiral if they have bulky substituents that restrict rotation and create chiral axes. For image reference, the correct answer is:

In this structure, the presence of bulky groups or substituents on the ortho positions of each phenyl ring restricts rotation, leading to atropisomerism, which is a type of chirality.

Therefore, the biphenyl shown in the image with data-src-id="62cd53d42ea4de92d0f3600e" is optically active due to these steric hindrances preventing the two rings from freely rotating about the interconnecting single bond, making it non-superimposable on its mirror image in all possible configurations.