Lecture 8: Isomerism

Representing Molecules

• General Formula: Algebraic formula describing any member of a compound family.
  • Example: Alkanes = $C<em>nH</em>{2n+2}$
• Structural Formula: Represents the order of carbon atoms and connected hydrogens and functional groups.
  • Example: Butane: $CH<em>3CH</em>2CH<em>2CH</em>3$; Butan-2-ol: $CH<em>3CHOHCH</em>2CH_3$
• Displayed Formula: Shows the arrangement of all atoms and the bonds between them.
• Skeletal Formula: Shows carbon atoms and their hydrogen connections as zig-zag skeletal representations, illustrating functional groups only.
  • Example: Butane and Butan-2-ol.

Isomerism - Types of Isomers

• Isomers: Molecules with the same molecular formula but different arrangements of atoms in their structural/skeletal formulas.
  • Types of Isomerism:
  • Structural Isomers
  • Stereoisomers
  • Optical Isomers (Enantiomers)

Structural Isomers

• Atoms are connected differently; three types:
  • Chain Isomers: Different arrangements of carbon chains.
  • Positional Isomers: Same carbon skeleton but different positions of functional groups.
  • Functional Group Isomers: Same formula but different functional groups.

Examples of Structural Isomers

• Chain Isomers:

  • Example 1: $C<em>4H</em>{10}$ - Butane and 2-Methylpropane.
  • Example 2: $C<em>7H</em>{16}$ - Heptane and various methyl derivatives (e.g., 2-Methylhexane, 3-Methylhexane).

• Positional Isomers:

  • Example 1: $C<em>4H</em>9Cl$ - Possible chlorobutane structures: 1-Bromohexane, 2-Bromohexane, etc.

• Functional Group Isomers:

  • Example 1: $C<em>6H</em>{12}$ with possible structures: Hex-1-ene, Cyclohexane, etc.
  • Example 2: $C<em>2H</em>6O$ - Ethanol vs. Dimethyl ether.
  • Example 3: $C<em>3H</em>6O$ - Propanone vs. Propanal.

Stereoisomers

• Same structural formula, but arrangements differ spatially.
  • Example: (E)-But-2-ene vs. (Z)-But-2-ene:
  • E Isomers: Groups on opposite sides (Trans).
  • Z Isomers: Groups on the same side (Cis).

Functional Groups and Their Reactions

• Key functional groups:
  • Alkenes
  • Haloalkanes
  • Alcohols

Reaction Mechanisms

• Illustrate interactions between molecules steps to produce a product.
• Show electron transfer with 'curly' arrows (movement of electron pairs) and 'half' arrows (movement of single electrons).

Alkenes

• Unsaturated hydrocarbons; reactions center around the double bond due to high electron density.
• Types of Reactions:
  • Electrophilic Addition
  • Halogenation
  • Hydration
  • Addition Polymerisation

Electrophilic Addition Reaction Mechanism

• Step 1: Alkene with high electron density attracts a molecule (X, Y).
• Electrons move from the double bond to the least electronegative atom, creating a positive charge on one carbon atom in the double bond.
• Resultant ion attracts the negative ion formed.

Halogenation of Alkenes

• Two types:
  • Type 1: Alkene + $X_2 <br /> ightarrow$ Di-halogenoalkane
  • Type 2: Alkene + $HX <br /> ightarrow$ Halogenoalkane
  • Example: Propene with Bromine to produce 1,2-dibromopropane or a mixture of 1-bromopropane and 2-bromopropane.

Halogenation Mechanism

• First Reaction: Propene and Bromine leading to 1,2-dibromopropane.
• Second Reaction: Propene with Bromine generating a mixture of products based on carbocation stability (more stable carbocation yields more product).
  • Products depend on intermediate reaction pathways and stability of carbocations.