SSC CHSL 2023 PYQ — Question 27

The correct answer is option 1 i.e. 1848.

In 1848, Louis Pasteur discovered molecular chirality and observed that crystals of some compounds, like tartaric acid, exist as mirror images, laying the foundation for modern stereochemistry. 

• The Problem: Pasteur was studying the crystals of sodium ammonium tartrate, a salt of tartaric acid, and noticed that some crystals were right-handed and some were left-handed, forming mirror images of each other. 
• The Discovery: He observed that solutions of the right-handed crystals rotated plane-polarized light clockwise (dextrorotatory), while solutions of the left-handed crystals rotated it counterclockwise (levorotatory). 
• Racemic Mixture: Pasteur also observed that a mixture of both types of crystals, a racemic mixture, did not rotate light because the optical activity of one molecule canceled the effects of the other molecule. 
• Significance: This was the first time anyone had demonstrated molecular chirality, and it also provided the first explanation of isomerism, paving the way for modern stereochemistry, the study of how bonds are oriented in three-dimensional space. 
• Further Research: Pasteur's work was later supported by the experimentation of Jean-Baptiste Biot, who first observed optical activity. 
• Modern Stereochemistry: Today, we recognize that Pasteur had achieved the first resolution of a racemic mixture, and laid the foundation of what we now call stereochemistry.