Stereochemistry II

Welcome to class!

 Hello, my focused learner! I’m glad you are here again to continue our journey into Stereochemistry. In the last lesson, we laid the foundation — we learnt about isomerism, chirality, and how molecules can have the same atoms but different arrangements in space. Today, we are going deeper into understanding how to represent these arrangements, how to assign configurations to chiral centres, and why this knowledge is essential in real-life chemistry.

Stereochemistry II

Representation of Stereoisomers

Chemists use specific ways to represent 3D arrangements on a 2D surface like paper or a screen. The wedge-and-dash notation is common: a solid wedge shows a bond coming out towards the viewer, a dashed wedge shows a bond going away, and plain lines represent bonds in the plane of the paper. Fischer projections are another method, often used for carbohydrates and amino acids. In Fischer projections, horizontal lines point towards the viewer, and vertical lines go away from the viewer.

 

 

Assigning R and S Configurations

To describe the absolute configuration of a chiral centre, we use the Cahn–Ingold–Prelog (CIP) priority rules. First, assign priority numbers to the four groups attached to the chiral carbon based on atomic number — higher atomic number means higher priority. Next, orient the molecule so that the lowest-priority group is pointing away from you. If the sequence from highest to lowest priority (1 → 2 → 3) is clockwise, the configuration is R (rectus); if it is anticlockwise, it is S (sinister). This system allows chemists worldwide to describe stereochemistry precisely without confusion.

Meso Compounds

Some molecules have more than one chiral centre but are overall achiral because they have an internal plane of symmetry. These are called meso compounds. They do not rotate plane-polarised light despite having stereocentres. For example, meso-tartaric acid has two chiral centres but is optically inactive.

 

 

Optical Activity and Measurement

Optical activity refers to a molecule’s ability to rotate plane-polarised light. This is measured using a polarimeter. The observed rotation depends on the concentration of the solution, the length of the sample tube, and the specific rotation of the compound. A positive rotation indicates a dextrorotatory compound, while a negative rotation indicates a levorotatory compound. Importantly, the sign of rotation (+ or –) does not directly tell you whether a compound is R or S; it must be experimentally determined.

 

 

Importance of Absolute Configuration

In pharmaceuticals, absolute configuration can mean the difference between healing and harm. For example, one enantiomer of a drug might bind perfectly to a biological receptor, while the other might have little effect or cause side effects. In food chemistry, stereochemistry influences the sweetness, smell, and flavour of compounds. In materials science, it affects the mechanical and optical properties of plastics and fibres.

Summary

  • Wedge-and-dash notation and Fischer projections help represent stereoisomers in 3D.
  • The R/S system uses CIP priority rules to assign absolute configurations to chiral centres.
  • Meso compounds have chiral centres but are overall achiral due to internal symmetry.
  • Optical activity is the rotation of plane-polarised light by chiral molecules and is measured with a polarimeter.
  • Absolute configuration is crucial in medicine, food, and materials science.

Evaluation

  1. What is the purpose of the wedge-and-dash notation?
  2. Briefly explain the CIP priority rules for assigning R/S configuration.
  3. What is a meso compound? Give one example.
  4. How is optical activity measured, and what factors affect it?
  5. Why is absolute configuration important in pharmaceuticals?

You are mastering the finer details of stereochemistry — the kind of knowledge that separates an average chemist from an exceptional one. Afrilearn is proud to see you developing a sharp eye for molecular structure and the confidence to apply it in science and real life.

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