Chemistry (9701) | Isomerism: structural isomerism and stereoisomerism

Welcome to the World of Isomerism!

Hello! This chapter dives into one of the most fascinating aspects of organic chemistry: Isomerism. Don't worry if it sounds complicated—it just means that sometimes, molecules that have exactly the same collection of atoms (the same molecular formula) can be put together in completely different shapes.

Understanding isomers is crucial because the way atoms are arranged determines everything about a compound: its melting point, its boiling point, how it reacts, and even how it works as a medicine!

Key Learning Outcomes (Syllabus 13.4 and 29.4)

• Describe Structural Isomerism (Chain, Positional, Functional Group).
• Describe Stereoisomerism (Geometrical/cis/trans and Optical).
• Explain the origin of geometrical isomerism due to restricted rotation.
• Explain what a chiral centre is and how it leads to enantiomers.
• Understand optical activity, racemic mixtures, and the biological importance of chirality.

Section 1: Structural Isomerism

Structural Isomerism (sometimes called constitutional isomerism) occurs when compounds have the same molecular formula but their atoms are connected together in a different order. It's like rearranging the plumbing in your house—the components are the same, but the layout is completely different, leading to different properties.

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1.1 Chain Isomerism

This type of isomerism involves a difference in the arrangement of the carbon skeleton (the main carbon chain).

• Straight-chain compounds vs. Branched-chain compounds.
• Example: C₄H₁₀
  
  We can have n-butane (a straight chain of four carbons) or 2-methylpropane (a branched chain of three carbons with one methyl group attached).

Did you know?

Branching typically leads to lower boiling points because the molecules are more spherical, reducing the surface area available for strong Van der Waals' forces to act.

1.2 Positional Isomerism

Here, the carbon skeleton remains the same, but the position of a functional group (or a substituent, like a halogen) changes along the chain.

• Example 1: C₃H₈O (an alcohol)
  Propan-1-ol (OH group on the first carbon)
  Propan-2-ol (OH group on the second carbon)
• Example 2: C₄H₉Cl (a halogenoalkane)
  1-chlorobutane vs. 2-chlorobutane.

1.3 Functional Group Isomerism

This is the most dramatic difference in structural isomerism. The isomers possess different functional groups entirely. Since the functional group dictates chemical reactivity, these isomers have drastically different chemical and physical properties.

• Example 1: C₂H₆O
  Ethanol (an alcohol: R-OH) reacts with sodium metal.
  Dimethyl ether (an ether: R-O-R') is inert towards sodium.
• Example 2: Carboxylic acids (R-COOH) and Esters (R-COO-R') often share the same molecular formula.

Section 2: Stereoisomerism

Stereoisomerism is where things get truly 3D! Stereoisomers have the same molecular formula and the same sequence of bonded atoms (they are connected in the same order), but they differ in the arrangement of their atoms in space.

We focus on two main types: Geometrical Isomerism and Optical Isomerism.

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2.1 Geometrical (cis/trans) Isomerism

Geometrical isomerism (often called cis/trans isomerism, though E/Z nomenclature is also accepted) is a specific type of stereoisomerism that arises due to restricted rotation.

Origin: Restricted Rotation of the π bond (Syllabus 13.4.3)

In molecules with single C-C bonds (like alkanes), rotation around the bond is free. However, a double bond (C=C) is made up of a strong σ bond and a weaker π bond (formed by the sideways overlap of p orbitals).

The presence of the π bond locks the atoms in place, preventing rotation without breaking the bond. This rigidity means that the groups attached to the double-bonded carbons are fixed in their positions relative to each other.

Conditions for Geometrical Isomerism

Geometrical isomers only exist if each carbon atom of the double bond is attached to two different groups.

• Bad example (no isomers): Propene (CH₃-CH=CH₂). The second carbon atom is attached to two identical hydrogen atoms.
• Good example (isomers exist): But-2-ene (CH₃-CH=CH-CH₃). Both carbons are attached to an H and a CH₃ group.

Naming Conventions (cis and trans)

We compare the positions of the two identical groups (or two specific high-priority groups, if using E/Z):

• cis isomer: The identical groups are on the same side of the double bond (think of them as "sisters" on the same side).
• trans isomer: The identical groups are on opposite sides (diagonal) of the double bond (think of them as "trans-porting" across the bond).

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2.2 Optical Isomerism (Chirality)

Optical isomerism is a type of stereoisomerism where the isomers are non-superimposable mirror images of each other. They are called enantiomers.

The Chiral Centre (Syllabus 13.4.4)

Optical isomerism arises when a molecule contains a chiral centre (or chiral carbon).

• A chiral centre is a carbon atom bonded to four different groups.
• Example: Butan-2-ol. The second carbon is bonded to H, OH, CH₃, and CH₂CH₃ (ethyl). Since all four groups are different, this carbon is a chiral centre.

Optical Activity and Enantiomers (Syllabus 29.4.1, 29.4.3)

Enantiomers have identical physical and chemical properties, except in one key area: their interaction with plane-polarised light.

• Plane-polarised light is light where all waves vibrate in a single plane.
• Optically Active: A substance is optically active if it can rotate the plane of plane-polarised light. Enantiomers are optically active.
• One Enantiomer: Rotates the light clockwise (designated as + or Dextrorotatory).
• The Other Enantiomer: Rotates the light counter-clockwise (designated as - or Laevorotatory) by an equal magnitude.

Racemic Mixtures (Racemates) (Syllabus 29.4.2)

A racemic mixture (or racemate) is a mixture containing equal amounts (50:50) of both enantiomers.

• Since one enantiomer rotates light clockwise and the other rotates it counter-clockwise by the same amount, their effects cancel out.
• A racemic mixture is therefore optically inactive.

Chirality in Drug Synthesis (Syllabus 29.4.4)

This is where isomerism becomes critically important in the real world. Many biological receptors (like enzymes in the body) are themselves chiral, meaning they are designed to fit only one specific 3D structure—like a lock fitting only a right-handed key.

1. Different Biological Activity: In a drug containing a chiral centre, one enantiomer (say, the right-handed molecule) might provide the therapeutic effect, while the other enantiomer (the left-handed molecule) might be inactive, or worse, cause harmful side effects.
2. The Need for Separation: When chemists synthesize a drug in the lab, if they start from non-chiral materials, they often produce a racemic mixture (50:50 mix). To ensure safety and effectiveness, the useful enantiomer must be separated from the useless or harmful one, or...
3. Chiral Catalysts: Modern pharmaceutical synthesis often uses chiral catalysts. These special catalysts ensure that the reaction produces almost exclusively one single, pure optical isomer, avoiding the need for costly and complex separation steps later on.