# Question 16819

Nov 2, 2015

(a): $\left(S\right) , \left(S\right)$, (b): $\left(R\right) , \left(S\right)$ and (c): $\left(S\right) , \left(S\right)$

#### Explanation:

The configurations of each stereocentre is depicted in the scheme below:

(a):
1- Assign priorities by decreasing atomic number. For example, color(blue)(""_8O), color(red)(""_35Br and color(green)(""_1H, the priority is $\textcolor{b l u e}{2} , \textcolor{red}{1} \mathmr{and} \textcolor{g r e e n}{3}$ respectively.

2- For both stereocentres the rotation is counter clockwise; therefore, their configurations are $\textcolor{b l u e}{\left(S\right)} \mathmr{and} \textcolor{red}{\left(S\right)}$.

(b):
1- Assign priorities as explained in (a).

2- color(blue)("For the blue stereocentre" The hydrogen with priority 4 switch place with $C O O H$ with priority 2. The rotation will be counter clockwise which means $\left(S\right)$, but because of switching places between $2$ and $4$ the configuration becomes $\textcolor{b l u e}{\left(R\right)}$.

3- color(red)("For the red stereocentre" The rotation is counter clockwise with the lowest priority group facing backward, then the configuration is $\textcolor{red}{\left(S\right)}$.

(c):
1- Assign priorities as explained in (a).

2- color(blue)("For the blue stereocentre" The lowest priority $H$ is in the plane and the highest priority $B r$ is facing backward. These groups should switch positions and the rotation will be clockwise which means $\left(R\right)$, but because of switching places between $1$ and $4$ the configuration becomes $\textcolor{b l u e}{\left(S\right)}$.

3-color(red)("For the red stereocentre"# The lowest priority $H$ is in the plane and the highest priority $O H$ is facing backward. These groups should switch positions and the rotation will be clockwise which means $\left(R\right)$, but because of switching places between $1$ and $4$ the configuration becomes $\textcolor{red}{\left(S\right)}$.

Here are two videos that explain in details how to label stereogenic centres R or S: