Chemistry (9701) | Isomerism: optical

🔬 Comprehensive Study Notes: Optical Isomerism (Chirality)

Welcome to the World of 3D Chemistry!

Hello! This chapter explores a fascinating area of organic chemistry: Optical Isomerism. Don't worry if it sounds complicated—it’s just about molecules that are mirror images of each other, like your hands!

Understanding optical isomerism is critical because the 3D shape of a molecule dictates how it behaves, especially in living systems. For example, one mirror image of a drug might cure an illness, while the other might be useless or even harmful!

1. Defining Stereoisomerism and Optical Isomers

Before diving into optical isomerism, remember that it falls under the main category of Stereoisomerism.

Stereoisomers are compounds with the same molecular formula and the same structural formula, but the atoms are arranged differently in 3D space.

• Stereoisomerism is divided into two main types:

(i) Geometrical (cis/trans) Isomerism: Found mainly in alkenes due to restricted rotation around a C=C double bond. (We cover this elsewhere, but it's good to remember the division!)
(ii) Optical Isomerism (Chirality): Arises when a molecule cannot be perfectly superimposed on its mirror image.

The "Handedness" Analogy: Enantiomers

Imagine your left hand and your right hand. They are mirror images of each other, but you cannot place one perfectly on top of the other (superimpose them) so that every part aligns. They are non-superimposable mirror images.

Molecules that exhibit optical isomerism are called Enantiomers (the optical isomers themselves) and they possess this same "handedness."

Quick Key Takeaway: Optical isomers (enantiomers) are non-superimposable mirror images, just like your left and right hands.

2. The Chiral Centre: The Source of Optical Activity

What makes a molecule optically active?

A molecule is only capable of optical isomerism if it contains a Chiral Centre (also called an asymmetric carbon atom).

Definition of a Chiral Centre (C*):

A chiral centre is a carbon atom that is bonded to four different groups.

Because carbon atoms usually form a tetrahedral shape (bond angle \(109.5^\circ\)), having four different groups attached creates a unique 3D structure that lacks a plane of symmetry. This lack of symmetry (chirality) is what allows the molecule to exist as non-superimposable mirror images.

How to identify a Chiral Centre: Step-by-Step

To find a chiral centre (C*) in a molecule like butan-2-ol, look at each carbon atom:

1. Check C1: Attached to three H atoms (not four different groups).
2. Check C2 (The potential chiral centre):
   It is attached to:
   • Group 1: \(\text{H}\)
   • Group 2: \(\text{OH}\)
   • Group 3: \(\text{CH}_3\) (Methyl group)
   • Group 4: \(\text{CH}_2\text{CH}_3\) (Ethyl group)
   Since all four groups are different, C2 is a chiral centre.
3. Check C3 & C4: Both are attached to multiple identical H atoms (not chiral).

Example: Butan-2-ol exists as two enantiomers because it contains one chiral centre.

Quick Key Takeaway: Chirality is caused by an asymmetric carbon atom (chiral centre) bonded to four distinct groups.

3. Optical Activity and Plane-Polarised Light

Enantiomers have identical physical and chemical properties (melting point, boiling point, density, reactivity with non-chiral reagents) except for one crucial difference: their interaction with light.

What is Plane-Polarised Light?

Normal light waves vibrate in all directions perpendicular to the direction of travel. When passed through a special filter called a polariser, the light is filtered so that the waves vibrate in only one plane. This is plane-polarised light.

The Effect of Optical Isomers

A substance that can rotate the plane of polarised light is said to be optically active.

The two enantiomers of a compound (like butan-2-ol) affect plane-polarised light in opposite ways:

1. One enantiomer rotates the plane of light to the right (clockwise). This is called the dextrorotatory (+) isomer.
2. The other enantiomer rotates the plane of light to the left (anticlockwise). This is called the laevorotatory (-) isomer.

Crucially, the two isomers rotate the light by exactly the same amount, just in opposite directions.

The Racemic Mixture

When preparing many organic compounds synthetically in the lab, we often end up with a 50:50 mixture of the two enantiomers. This equal mix is called a Racemic Mixture (or racemate).

• Since one isomer rotates the light right (+) and the other rotates it left (-) by the same magnitude, the rotations cancel out.
• A racemic mixture is therefore optically inactive.

Quick Key Takeaway: Optical isomers rotate plane-polarised light by equal amounts in opposite directions. A 50:50 mixture (racemic mixture) is optically inactive.

4. Relevance of Chirality in Drug Synthesis (A-Level Extension)

This is where optical isomerism becomes really important in the real world, especially in pharmacology (drug science).

4.1 The Problem: Different Biological Activity

Biological systems—like enzymes, receptors, and antibodies in the body—are themselves chiral (they have specific 3D shapes).

Think of a biological receptor site as a glove.

• If the drug is the hand, the receptor is the glove.
• The right-handed enantiomer (the "right hand") will fit perfectly into the glove. It can bind strongly and trigger the desired biological effect.
• The left-handed enantiomer (the "left hand") is the mirror image. It cannot fit correctly into the glove. It might be biologically inactive, or worse, it might bind to a different receptor and cause harmful side effects!

Therefore, it is often necessary to use only one pure enantiomer in medicine.

4.2 Challenges in Drug Synthesis

Standard laboratory synthesis often involves reagents that are not chiral. When these reagents react to create a chiral centre, the chance of forming the (+) isomer is equal to the chance of forming the (-) isomer. Result: a racemic mixture (50:50).

The need to obtain a single, pure optical isomer presents two major challenges in the pharmaceutical industry (Syllabus 29.4.4):

1. The Need for Separation (Resolution): The synthetic racemic mixture must be separated (or 'resolved') into its two pure enantiomers, which is a difficult and expensive process since they have identical physical properties.
2. Minimising Waste/Toxicity: You must ensure the patient only receives the biologically active isomer, avoiding the inactive or toxic mirror image.

4.3 The Use of Chiral Catalysts

To overcome the difficulties of separating a racemic mixture, modern chemistry often uses chiral catalysts (Syllabus 29.4.4(c)).

• A chiral catalyst is itself a complex molecule with specific handedness.
• When the non-chiral starting materials bind to the catalyst, the 3D environment of the catalyst forces the reaction to proceed in a way that generates only the desired single enantiomer (or a very high proportion of it).
• This method is highly efficient as it produces the pure optical isomer directly, avoiding the costly separation step.

Quick Key Takeaway: Chirality is vital in biology because receptors are specific (lock and key). Synthetic production of chiral drugs requires either separating the racemic mixture (resolution) or using chiral catalysts to produce only the desired enantiomer.

Quick Review of Optical Isomerism

Identify the key features of an optically active molecule:

• Must contain a chiral centre (carbon attached to four different groups).
• Exists as two enantiomers (non-superimposable mirror images).
• Enantiomers rotate plane-polarised light by equal amounts in opposite directions.
• A 50:50 mix is a racemic mixture and is optically inactive.

Keep practising identifying those chiral centres in various molecules—it is the most common examination skill for this topic!