Question

Allegra, a common prescription drug with the structure shown below, is given for treatment of seasonal allergies. How many stereo-genic carbons does Allegra possess?

A.1
B.2
C.3
D.4

Answer 1

Hint: Stereo-genic carbon is associated with chiral carbons present in the molecule. Chiral molecules usually contain at least one carbon atom with four non-identical substituents.

Complete answer:
- Allegra is the brand name for a pharmaceutical drug, Fexofenadine. It is a known antihistamine drug used in the treatment of allergy symptoms, such as hay fever and urticaria.
- To find out the presence of stereo-genic carbons in Allegra molecules, it is important for us to know the basis of stereo-genic carbon atoms.
- A stereo-genic carbon atom is known as the “centre of chirality”. Thus, it is associated with chiral carbon atoms.
- A chiral carbon is an asymmetric carbon atom consisting of 4 different atoms or groups of atoms attached to it. For each chiral carbon in a molecule there are 2 optical isomers.
- In the given molecular structure, it is clearly visible that the extreme right Carbon atom is not attached to 4 different groups. Instead, there are two methyl groups present.
- Therefore, there is 1 stereo-genic carbon present in the structure.

So, the answer to the question is (A) 1.

Note:
A stereo-centre is an atom, usually carbon, has four attachments that are different from each other. Since each attachment is unique, if any two attachments are moved into another position, a new molecule can be formed.