Question

In the conversion of optically active $\left(R\right)$ form of $2$-chlorobutane to $\left(R\right)$ form of $2$-bromobutane, the following sequence of reactions is carried out.
Direct conversion is not feasible since $Cl$ cannot be replaced by $Br$ in one step.
In which of the following steps does the inversion occur?

• A. Step $1$
• B. Step $2$
• C. Step $3$
• D. None

Answer 1

Answer: (A), (C)

Step $1$
Step $3$

If a bond at the asymmetric C atom is broken ${SN}^2$ reaction occurs and inversion takes place.

In Step 1, $(C-Cl)$ bond is broken to $(C-OH)$, hence inversion occurs.

In step 2, $(C-O)$ bond is not broken but $(O-H)$ bond is broken to give $(-O-Ts)$.

Hence, no inversion occurs.

In step 3, $(C-O)$ bond is broken, i.e., $C-OTs\to C-Br$ and hence inversion occurs.

Options A and C are correct.