Question

Which among the following groups has lowest priority in assigning R,S convention?

• A. $-COOH$
• B. $-CONH_2$
• C. $-CH_2OH$
• D. $-CN$

Hint:

While assigning R,S configuration, always compare atoms step-by-step using CIP rules and consider multiple bonds as equivalent multiple single bonds.

Answer 1

The Correct Option is D

Step 1: Understanding the CIP priority rules.
According to Cahn–Ingold–Prelog (CIP) rules, priority is assigned based on the atomic number of the atom directly attached to the chiral carbon. Higher atomic number means higher priority.
Step 2: Compare the directly attached atoms.
All groups are attached through a carbon atom. Hence, we compare the atoms attached to that carbon.
Step 3: Expanding each group.
(A) $-COOH$: Carbon is attached to O, O, and O atoms.
(B) $-CONH_2$: Carbon is attached to O, N, and N atoms.
(C) $-CH_2OH$: Carbon is attached to O, H, and H atoms.
(D) $-CN$: Carbon is attached to N, N, and N atoms via triple bond.
Step 4: Assigning priority.
Among these, the $-CN$ group has the lowest effective priority as per CIP comparison rules.
Step 5: Conclusion.
The group with the lowest priority in assigning R,S configuration is $-CN$.