Question

What is present in a racemic mixture?

• A. Only dextrorotatory (D) form is present.
• B. Only laevorotatory (L) form is present.
• C. D and L forms are present in a 1:1 ratio, making the mixture optically inactive.
• D. It is a meso compound.

Hint:

Do not confuse these terms: \[ \boxed{ \begin{aligned} \text{Racemic mixture} &\rightarrow \text{External compensation}\\ \text{Meso compound} &\rightarrow \text{Internal compensation} \end{aligned} } \] A racemic mixture contains \[ \boxed{50\% \text{ D-form } + 50\% \text{ L-form}} \] and is therefore optically inactive.

Answer 1

The Correct Option is C

Concept: A racemic mixture is an equimolar mixture of two enantiomers. The two enantiomers are:

• Dextrorotatory (\(+\) or D) form

• Laevorotatory (\(-\) or L) form
Each rotates plane-polarised light by the same magnitude but in opposite directions. As a result, their optical rotations cancel each other, making the racemic mixture optically inactive. This phenomenon is known as external compensation.

Step 1: Understand the composition of a racemic mixture.
A racemic mixture contains \[ \boxed{50\% \text{ D-form } + 50\% \text{ L-form}.} \] Thus, both enantiomers are present in equal amounts.

Step 2: Determine its optical activity.
The D-form rotates plane-polarised light to the right. The L-form rotates it by the same amount to the left. Hence, \[ (+\alpha)+(-\alpha)=0. \] Therefore, \[ \boxed{\text{The mixture is optically inactive}.} \]

Step 3: Choose the correct option.
A racemic mixture contains \[ \boxed{\text{D and L forms in a }1:1\text{ ratio}.} \] Therefore, \[ \boxed{\textbf{Option (C)}} \] is the correct answer.