Question

The compounds $A,B,C$ have $R$ or $S$ configurations respectively:

• A. $R,R,S$
• B. $R,S,R$
• C. $R,R,R$
• D. $S,R,S$

Answer 1

The correct option is C $R,R,R$

Rules:

• Give preference to group which starts with high atomic number element. If starting element is same. then apply it to next element.
• In case of double and triple bonds, write it as (Refer to Image)

In this case give preference to $-C\equiv N$ because here $3^{rd}$ element $N$ is higher atomic number than $H$.

• Assign $1,2,3$ as order of preference. If we go from $1⟶3$, if it is clockwise $⟶R$ ; Anti clockwise $⟶S$
  
• If $4^{th}$ preference group is in horizontal position, reverse the configuration.

Clockwise $⟶S$

Anti Clockwise $⟶R$

(A) (Refer to Image 2)

Anti clockwise but $4^{th}$ preference group is in horizontal position. So, its configuration is $R$ .

(B) (Refer to Image 3)

Anti clockwise but $4^{th}$ preference group is in horizontal position. So, its configuration is $R$ .

(C) (Refer to Image 4)

Anti clockwise but $4^{th}$ preference group is in horizontal position. So, its configuration is $R$ .