CHM 004 Organic Chemistry Lecture 3: Isomerism Study Guide

Overview and Prerequisites of Isomerism

• Definition of Isomerism: The phenomenon where different compounds share the same molecular formula but have different properties and structures.
• Educational Objectives:     * Understand the basic concepts of isomerism and differentiate between various types.     * Identify which type of isomerism a specific compound exhibits.     * Draw and correctly name isomers derived from a provided molecular formula.
• Prior Knowledge Requirements: To fully grasp the material, students should be familiar with:     * Organic chemistry functional groups.     * The arrangement of chemical bonds around carbon atoms.     * Factors that influence the boiling points of organic molecules.

General Classification of Isomers

• Structural Isomerism: Molecules share the same molecular formula but have different structural formulae. These can be further divided into:     * Chain Isomerism     * Position Isomerism     * Functional Group Isomerism     * Metamerism     * Tautomerism
• Stereoisomerism: Molecules share the same molecular formula and the same structural formula, but their atoms occupy different positions in 3D space. These include:     * Geometrical Isomerism: Occurs due to restricted rotation of $C=C$ double bonds.     * Optical Isomerism: Occurs when molecules have a chiral center, resulting in two non-superimposable mirror images.

Structural Isomerism: Detailed Types

• Chain Isomerism:     * Definition: Caused by different arrangements of the carbon skeleton.     * Properties: Similar chemical properties but slightly different physical properties.     * Boiling Point Relationship: Increased branching results in a lower boiling point. Branching reduces the effectiveness of intermolecular forces; thus, less energy is required to separate molecules.     * Example: $C_4H_{10}$:         * Butane: A straight-chain molecule (Boiling point: $-0.5\,^{\circ}\text{C}$).         * 2-Methylpropane: A branched molecule (Boiling point: $-11.7\,^{\circ}\text{C}$).
• Position Isomerism:     * Definition: The carbon skeleton and functional group remain the same, but the functional group is located at a different position in the molecule.     * Properties: Similar chemical properties, but different physical properties.     * Example 1 (Double Bond): $C_5H_{10}$         * Pent-1-ene: Double bond between carbons 1 and 2.         * Pent-2-ene: Double bond between carbons 2 and 3.     * Example 2 (Halogen Positions): $C_4H_9Cl$         * 1-chlorobutane: Halogen on carbon 1.         * 2-chlorobutane: Halogen on carbon 2.         * Note: Position is measured from the end nearest the functional group.     * Example 3 (Benzene Ring): Relative positions on a ring.         * 1,2-dichlorobenzene: Ortho-dichlorobenzene.         * 1,3-dichlorobenzene: Meta-dichlorobenzene.         * 1,4-dichlorobenzene: Para-dichlorobenzene.
• Functional Group Isomerism:     * Definition: Compounds with the same molecular formula but different functional groups.     * Properties: Distinct chemical and physical properties.     * Common Pairs:         * Alcohols and Ethers: Ethanol ($CH_3CH_2OH$) vs. Methoxy-methane ($CH_3OCH_3$).         * Aldehydes and Ketones: Propanal ($CH_3CH_2CHO$) vs. Propanone ($CH_3COCH_3$).         * Carboxylic Acids and Esters: Propanoic acid ($CH_3CH_2COOH$) vs. Methyl ethanoate ($CH_3COOCH_3$).
• Metamerism:     * Definition: Same functional group, but a difference in the size of alkyl groups on either side of that functional group.     * Properties: Similar chemical properties, slightly different physical properties.     * Example: Ethoxyethane and Methoxypropane.
• Tautomerism:     * Definition: Isomers that differ in the location of a double bond and a hydrogen atom, existing in dynamic equilibrium.     * Properties: Different chemical and physical properties.     * Example: Enol and Ketone (e.g., 1-propen-2-ol and Propan-2-one).

Functional Group Comparison Chart

• Alcohols vs. Ethers:     * Alcohol: Polar $O-H$ bond allows for hydrogen bonding; high boiling point; soluble in water; acts as a Lewis base; wide reaction range.     * Ether: No hydrogen bonding; low boiling point; insoluble in water; chemically inert.
• Aldehydes vs. Ketones:     * Aldehyde: Polar $C=O$ bond (dipole-dipole interaction); easily oxidized to acids; reduced to primary ($1^{\circ}$) alcohols.     * Ketone: Polar $C=O$ bond; oxidation occurs only under extreme conditions; reduced to secondary ($2^{\circ}$) alcohols.
• Carboxylic Acids vs. Esters:     * Carboxylic Acid: $O-H$ bond allows hydrogen bonding; high boiling point; water soluble; acidic properties.     * Ester: No hydrogen bonding; insoluble in water; fairly unreactive; can be hydrolyzed back to acids.

Stereoisomerism: Geometrical Isomerism

• Mechanical Cause: Restricted rotation about a carbon-carbon double bond ($C=C$). Unlike single bonds ($C-C$), which have free rotation, the groups on a double bond are "frozen" in space.
• Nomenclature Systems:     * E/Z System: Based on the Cahn, Ingold and Prelog (CIP) priority rules. The atomic number determines priority: $I > Br > Cl > F > C > H$.         * Z (zusammen): Higher priority groups are on the SAME side of the double bond.         * E (entgegen): Higher priority groups are on OPPOSITE sides of the double bond.     * Cis/Trans System: Used specifically when there are two Hydrogens and two non-hydrogen groups.         * Cis: Non-hydrogen groups on the SAME side.         * Trans: Non-hydrogen groups on OPPOSITE sides.
• Requirement for Geometrical Isomerism: Each carbon atom in the double bond must be attached to two different groups. If either carbon has two similar groups (e.g., two Hydrogens at one end), geometrical isomerism is impossible.
• Case Study: Butene ($C_4H_8$):     * Three structural isomers exist: But-1-ene, But-2-ene, and 2-methylpropene.     * Only But-2-ene exhibits geometrical isomerism (Cis/Trans or E/Z forms).

Stereoisomerism: Optical Isomerism

• Definitions:     * Enantiomers: Chiral molecules that are non-superimposable mirror images of one another.     * Chiral Centre: Contains an asymmetric carbon atom attached to four different atoms or groups arranged tetrahedrally.
• Identifying Chiral Centers: A carbon cannot be chiral if it has more than one of any atom/group attached (e.g., $CH_2$ or $CH_3$ groups).     * Example - 1-chlorobutane: Not chiral (all carbons have at least two identical hydrogens attached).     * Example - 2-chlorobutane: Chiral (the second carbon is attached to $H$, $CH_3$, $Cl$, and $C_2H_5$).
• Physical Differences: Enantiomers differ in their interaction with plane-polarized light.     * Dextrorotatory ($d$ or $+$): Rotates light to the right.     * Laevorotatory ($l$ or $-$): Rotates light to the left.     * Polarimeter: The instrument used to measure the rotation of polarized light.
• Racemate (Racemic Mixture): A 50-50 mixture of two enantiomers ($dl$ or $\pm$). The opposite rotations cancel each other out, resulting in no net optical activity.     * Applications: Critical in the pharmaceutical industry for drug development.

Activities and Exercises

• Activity 1: Draw all possible isomers for $C_4H_{10}O$ and categorize them by type.
• Activity 2: Determine which of the following exhibit cis-trans isomerism; draw and show geometry:     * (a) $CH_3CH=CH_2$     * (b) $(CH_3)_2C=CHCH_3$     * (c) $CH_3CH_2CH=CHCH_3$     * (d) $(CH_3)_2C=C(CH_3)CH_2CH_3$     * (e) $ClCH=CHCl$     * (f) $BrCH=CHCl$
• Activity 3: Define "chiral centre" and "enantiomers." Assess a provided structure for chirality and identify the specific chiral centers.