Introduction to Organic Chemistry II: Isomerism

At the conclusion of the lecture on Introduction to Organic Chemistry II: Isomerism, students are expected to be able to:

Describe structural (constitutional) isomerism.
Describe cis-trans isomerism in alkenes and explain its origin in terms of restricted rotation due to the presence of $\pi$ bonds.
Explain the meaning of a chiral centre and how such a centre gives rise to optical isomerism.
Deduce possible isomers for an organic molecule given its molecular formula.
Identify chiral centres and cis-trans isomerism within a molecule provided in structural formula.

Isomers are defined as compounds that possess the SAME molecular formula but DIFFERENT structural formulae.
Isomerism is broadly categorized into two main types: Structural (constitutual) isomerism and Stereoisomerism.

Structural (constitutional) isomers have the same molecular formula but different structural formulae due to the different attachment or connectivity of atoms.
There are three primary types of structural isomerism:
- Chain isomerism.
- Positional isomerism.
- Functional group isomerism.

Definition: Chain isomers differ in their carbon skeleton or carbon chain (e.g., branched versus straight chains).
Characteristics: They possess the same functional group and belong to the same homologous series.
Example: $C_5H_{12}$
- Pentane: $CH_3CH_2CH_2CH_2CH_3$ (Linear chain).
- 2-Methylbutane: $CH_3CH(CH_3)CH_2CH_3$ (One branch).
- 2,2-dimethylpropane: $CH_3C(CH_3)_2CH_3$ (Two branches; more spherical).
Physical Properties and Intermolecular Forces:
- Pentane (the long straight chain) has the highest Van der Waals forces because a long chain provides a larger surface area.
- A larger surface area leads to a higher boiling point ( $bp$ ).
- 2,2-dimethylpropane is more spherical, resulting in a lower Total Surface Area ( $TSA$ ) and thus a lower boiling point ( $bp$ ).

Definition: Positional isomers differ in the position of the same functional group on the same carbon skeleton.
Example: $C_3H_7Cl$
- 1-chloropropane ( $CH_3CH_2CH_2Cl$ ).
- 2-chloropropane ( $CH_3CH(Cl)CH_3$ ).
Example: $C_4H_8$
- 1-butene ( $CH_2=CHCH_2CH_3$ ).
- 2-butene ( $CH_3CH=CHCH_3$ ).
Example: $C_8H_{10}$
- 1,2-dimethylbenzene.
- 1,3-dimethylbenzene.
- 1,4-dimethylbenzene.

Definition: Functional group isomers have different functional groups and therefore belong to different homologous series, despite sharing the same general formula.
Relationships based on General Formulae:
- $C_nH_{2n+2}O$ (where $n > 1$ ): Alcohols and Ethers. Alcohols can form hydrogen bonds resulting in high boiling points; ethers cannot. Alcohols are saturated.
- $C_nH_{2n}O$ (where $n \geq 3$ ): Aldehydes and Ketones. Both contain the carbonyl group ( $C=O$ ) but are unsaturated (lacking the $+2$ in the hydrogen count).
- $C_nH_{2n}O_2$ (where $n \geq 2$ ): Carboxylic acids and Esters. Carboxylic acids have a terminal hydroxyl group ( $OH$ ) attached to the carbonyl group ( $C=O$ ).
- $C_nH_{2n}$ (where $n \geq 3$ ): Alkenes and Cycloalkanes.
Specific Examples:
- For $C_2H_6O$ : Ethanol (alcohol) and Dimethyl ether (ether).
- For $C_6H_{12}$ : Hex-1-ene (alkene) and Cyclohexane (cycloalkane).
- For $C_3H_6O$ : Propanal (aldehyde) and Propanone (ketone).
- For $C_3H_6O_2$ : Propanoic acid (carboxylic acid) and Methyl ethanoate (ester).

Pair 1: $CH_3CH_2CH_2CH_2OCH_3$ and $CH_3CH_2CH_2CH_2CH_2OH$ . These are functional group isomers (Ether vs Alcohol).
Pair 2: Comparison of a straight chain structure to a branched structure. These are chain isomers.
Pair 3: Comparison of the same functional group located at different carbon positions. These are positional isomers.

Definition: Stereoisomers have the same structural formula but different spatial arrangements of atoms in 3D space.
Types of Stereoisomerism:
- Geometrical (cis-trans) isomerism (specifically focused on alkenes).
- Optical isomerism (enantiomers).

Origin: This isomerism is formed due to restricted rotation about a $C=C$ double bond in alkenes. The presence of the $\pi$ bond prevents free rotation, unlike the free rotation found in a single $C-C$ bond.
Terminology:
- Cis: From Latin meaning "on this side." Atoms or groups are locked on the same side of the double bond.
- Trans: From Latin meaning "across." Atoms or groups are locked on opposite sides of the double bond.
Example: 1,2-dichloroethene ( $CH(Cl)CH(Cl)$ ).
- $trans$ -1,2-dichloroethene: Chlorine atoms are on opposite sides.
- $cis$ -1,2-dichloroethene: Chlorine atoms are on the same side.
Requirement for Geometrical Isomerism: If one of the carbon atoms in the double bond is attached to two identical groups, geometrical isomerism is NOT possible. For example, 2-methylprop-1-ene cannot exhibit cis-trans isomerism.
Example Practice: For $CH_3CH=CHBr$ , the isomers are:
- Cis isomer: Both the methyl group ( $CH_3$ ) and Bromine atom ( $Br$ ) are on the same side (or both Hydrogens are on the same side).
- Trans isomer: The methyl group ( $CH_3$ ) and Bromine atom ( $Br$ ) are on opposite sides.

Definition: Optical isomers are optically active compounds that can rotate plane-polarized light.
- Dextrorotary: Rotates light to the right.
- Levorotary: Rotates light to the left.
- Rotation is measured using an instrument called a polarimeter.
Identifying Optically Active Compounds:
- Must contain a chiral carbon (also called a chirality centre or stereogenic centre), which is an $sp^3$ -hybridized carbon atom with 4 different atoms or groups attached to it.
- The molecule must not be superimposable on its mirror image.
Drawing Conventions: Optical isomers should be drawn in 3D to show the arrangement of the four bonds clearly. Mirror images (enantiomers) are drawn reflected across a vertical plane.
Example: For 2-butanol ( $CH_3CH(OH)CH_2CH_3$ ):
- 1. Find the chiral carbon: It is the second carbon, attached to $-H$ , $-OH$ , $-CH_3$ , and $-CH_2CH_3$ .
- 2. Draw the optical isomers in 3D.

Structural/Constitutional Isomerism (Same molecular formula, different connectivity):
- Chain: Differences in the carbon skeleton.
- Positional: Differences in the location of the functional group.
- Functional Group: Possession of different functional groups.
Stereoisomerism (Same structural formula, different spatial arrangement):
- Cis-Trans: Occurs due to no rotation on the $C=C$ bond.
- Enantiomer (Optical): Non-superimposable mirror images of a compound containing a chiral carbon.