Lecture 9: Organic Reactions

Functional Groups and Their Reactions

Introduction

  • This document discusses the following functional groups and their reactions:
    • Alkenes
    • Haloalkanes (Halogenoalkanes)
    • Alcohols
  • Reaction Mechanisms:
    • Illustrate interactions between molecules to form products.
    • Breakdown of how reactions proceed, showing electron transfer between species.
    • Curly arrows indicate electron movement:
    • Full arrows: movement of electron pairs.
    • Half arrows: movement of single electrons.

Alkenes

  • Definition: Unsaturated hydrocarbons.
  • Key Reactions: All reactions occur at the double bond due to high electron density. Common reactions include:
    • Electrophilic Addition
    • Halogenation
    • Hydration
    • Addition Polymerisation

Electrophilic Addition

  • General Mechanism:
    • Step 1: Alkene's double bond creates a region of high electron density.
    • Molecule (X-Y) approaches, with Y being more electronegative.
    • Electrons move from the double bond to the least electronegative atom.
    • Step 2: New bond forms with X, creating a positive charge on the carbon previously involved in the double bond.
    • Resulting X- becomes a negative ion attracted to the positive carbon.

Halogenation

  • Two types of halogenation reactions:
    1. Alkene + X$_2$ → Di-halogenoalkane
    • Example: Propene + Bromine → 1,2-dibromopropane
    1. Alkene + HX → Halogenoalkane
    • Example: Propene + HBr → a mixture of 1-bromopropane and 2-bromopropane

Mechanism for Halogenation:

  1. Reaction of Propene with Bromine produces 1,2-dibromopropane.
  2. Reaction of Propene with HBr results in two products from carbocation intermediates, one being more stable than the other, resulting in a higher concentration of that product.

Hydration

  • Definition: Addition of H$_2$O across the double bond to form alcohol.
  • Example 1: Ethene + H$2$SO$4$ + H$_2$O → Ethanol.
  • Example 2: But-2-ene + H$2$SO$4$ + H$_2$O → Butanol.
  • Mechanism:
    • Steps include adding sulfuric acid followed by warming with H$_2$O.

Addition Polymerisation

  • Definition: Formation of long chain molecules from monomers through alkenes.
  • Examples:
    • Ethene ⇒ Polyethene (polythene)
    • Propene ⇒ Polypropene (polypropylene)
    • Other examples include polymerization of phenylethene and tetrafluoroethene.

Halogenoalkanes

  • Definition: Alkanes with one or more halogen atoms.
  • Formation: Halogenoalkanes can be synthesized from alkenes.
    • Example: Ethene + Bromine → Dibromoethane.

Reactions of Halogenoalkanes

  • Undergo Nucleophilic Substitution Reactions:
    1. Formation of Alcohols
    • Example: Bromoethane with NaOH → Ethanol.
    1. Formation of Nitriles
    • Example: Bromoethane with KCN → Nitrile.
    1. Formation of Amines
    • Example: Bromoethane with NH$_3$ → Amine.

Alcohols

  • Classification: Primary (1$^o$), Secondary (2$^o$), Tertiary (3$^o$).
    • Examples:
    • Ethanol (1$^o$)
    • Propan-2-ol (2$^o$)
    • 2-Methylpropan-2-ol (3$^o$)

Formation of Alcohols

  • Ethanol Production: From fermentation of sugar with yeast at ~35$^{ ext{o}}$C produces ~12-14% yield.
  • Hydration of Alkenes:
    • Example Reactions:
    1. Ethene + H$_2$O → Ethanol.
    2. Butene + H$_2$O → Butanol.

Reactions of Alcohols

  • Dehydration: Alcohols can be converted to alkenes by removing H$_2$O.

    • Example: Propanol → Propene + H$_2$O.

  • Oxidation: Using acidified potassium dichromate (H$2$SO$4$/K$2$Cr$2$O$_7$):

    • Primary alcohols oxidize to aldehydes and further to carboxylic acids.
    • Secondary alcohols oxidize to ketones.

Oxidation Examples:

  • Primary: Ethanol → Ethanal.
  • Secondary: Propan-2-ol → Propanone.

Combustion of Alcohols

  • Alcohols combust in oxygen to produce water and carbon dioxide.
    • Example: Ethanol + O$2$ → H$2$O + CO$_2$.