Question

Which among the following functional groups has been given the highest priority while assigning $R$-$S$ configuration?

• A. $-C_6H_5$
• B. $-CN$
• C. $-C_2H_5$
• D. $-CH_3$

Answer 1

Answer: (B)

(b)

Answer 2

For CIP priority rules, the atomic numbers of the atoms directly attached to the stereocenter are compared. Here, all groups attach via a carbon. Then, compare the atoms bonded to that carbon:

• –C₆H₅ (Phenyl): The attached carbon is bonded to other carbons.
• –CN (Cyano): The attached carbon is triple-bonded to nitrogen. In CIP terms, this is considered as bonded to three nitrogen atoms (atomic number 7), which is higher than carbon.
• –C₂H₅ (Ethyl) and –CH₃ (Methyl): Their attached carbon is bonded to hydrogen and carbon atoms.

Thus, –CN has the highest priority.