Question

In assigning R-S configuration which among the following groups has highest priority?
A. $- {\text{S}}{{\text{O}}_3}{\text{H}}$
B. $- {\text{COOH}}$
C. $- {\text{CHO}}$
D. $- {{\text{C}}_6}{{\text{H}}_5}$

Answer 1

We know that while determining the R-S configuration of a molecule, we number all the substituents attached to the carbon. The substituents are numbered according to their priority. The atomic number decides the priority. In R-S configuration, R stands for rector and S stands for sinister.

Complete step by step solution:
We know that higher the atomic number of the substituent atom, higher is the priority.
We are given four substituents $- {\text{S}}{{\text{O}}_3}{\text{H}}$, $- {\text{COOH}}$, $- {\text{CHO}}$ and $- {{\text{C}}_6}{{\text{H}}_5}$.
The $- {\text{S}}{{\text{O}}_3}{\text{H}}$ group is attached to the carbon through the sulphur atom. The atomic number of sulphur is 16.
The $- {\text{COOH}}$ group is attached to the carbon through the carbon atom. The atomic number of carbon is 6.
The $- {\text{CHO}}$ group is attached to the carbon through the carbon atom. The atomic number of carbon is 6.
The $- {{\text{C}}_6}{{\text{H}}_5}$ group is attached to the carbon through the carbon atom. The atomic number of carbon is 6.
Among the given groups, the atomic number of sulphur is highest. i.e. 16. Thus, the first number of highest priority is given to sulphur.
Thus, in assigning R-S configuration the group having highest priority is $- {\text{S}}{{\text{O}}_3}{\text{H}}$.

**Thus, the correct option is (A) $- {\text{S}}{{\text{O}}_3}{\text{H}}$.

Note:**
The R-S configuration is also known as CIP rule because it was given by Cahn, Ingold and Prelog. When four groups or substituents are attached to the chiral carbon atom, they are ranked in the order of priority. The highest priority is given to the atom (through which the group or substituent is attached) having the highest atomic number. The highest priority group has the lowest number.