Lecture 8: Isomerism

Representing Molecules

  • General Formula: Algebraic formula describing any member of a compound family.
    • Example: Alkanes = CnH{2n+2}
  • Structural Formula: Represents the order of carbon atoms and connected hydrogens and functional groups.
    • Example: Butane: CH3CH2CH2CH3; Butan-2-ol: CH3CHOHCH2CH_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: C4H{10} - Butane and 2-Methylpropane.
    • Example 2: C7H{16} - Heptane and various methyl derivatives (e.g., 2-Methylhexane, 3-Methylhexane).

  • Positional Isomers:

    • Example 1: C4H9Cl - Possible chlorobutane structures: 1-Bromohexane, 2-Bromohexane, etc.

  • Functional Group Isomers:

    • Example 1: C6H{12} with possible structures: Hex-1-ene, Cyclohexane, etc.
    • Example 2: C2H6O - Ethanol vs. Dimethyl ether.
    • Example 3: C3H6O - 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
      ightarrow Di-halogenoalkane
    • Type 2: Alkene + HX
      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.