Unsaturated Hydrocarbon Reactions

This document details the reactions involving unsaturated hydrocarbons, with a particular emphasis on alkenes and alkynes. A thorough understanding of these reactions is vital for their application in industrial and laboratory contexts.

Definitions and Distinction

• Unsaturated Hydrocarbons: These are compounds characterised by the presence of double (C=C) or triple (C≡C) bonds, such as alkenes and alkynes. Such bonds confer higher reactivity compared to saturated hydrocarbons.
  • Example: Ethylene (C$_2$H$_4$) is an alkene, while acetylene (C$_2$H$_2$) is an alkyne.
• Influence on Physical Properties: The presence of unsaturation influences boiling and melting points.
  • Example: Due to its double bond, ethylene has a lower boiling point than ethane.

Structures of Alkenes and Alkynes

• General Formulas:
  • Alkenes: C$_n$H$_{2n}$.
  • Alkynes: C$_n$H$_{2n-2}$.
• Bonding and Geometry:
  • The double bonds in alkenes impart a planar structure, influencing polarity and reactivity.
  • Alkynes, with their triple bonds, have a linear geometry, impacting reactivity and bond angles.

Pi Bonds and Reactivity

• Pi Bonds:
  • These arise from the lateral overlap of p orbitals, forming reactive electron clouds.
  • Pi bonds are more reactive than sigma bonds, enabling addition reactions.

Addition Reactions Overview

• Addition Reactions: These reactions involve the addition of atoms or groups to the multiple bonds in alkenes or alkynes without any loss of atoms.

Detailed Mechanism of Addition Reactions

• Step-by-Step Breakdown:
  • Step 1: Interaction between pi bonds and another molecule.
  • Step 2: Formation of intermediates, which may involve charged species.
  • Step 3: The formation of stable sigma bonds concludes the reaction.

Hydrogenation Reactions

• Hydrogenation: This process involves adding hydrogen to unsaturated hydrocarbons, converting them into saturated hydrocarbons.

Mechanism of Hydrogenation

• Step 1: Hydrogen adsorption onto a metal catalyst.
• Step 2: Cleavage of H$_2$ into individual atoms.
• Step 3: Transition states form, converting pi bonds into sigma bonds.

Halogenation Reactions

• Halogenation: A reaction where halogens are added to unsaturated compounds.
• Common halogens used include chlorine (Cl$_2$) and bromine (Br$_2$).
• Electrophilic Addition:
  1. The halogen approaches the electron-rich pi bond.
  2. Formation of a halonium ion occurs.
  3. Halogen atoms add across the bond.

Hydrogen Halide Reactions

• Hydrogen Halides (HX): Compounds like HCl, HBr, and HI, consisting of hydrogen and halogens.
• They react with unsaturated hydrocarbons via electrophilic addition.

Role of Markovnikov's Rule

• Markovnikov's Rule: The halogen attaches to the more substituted carbon in the molecule.

Hydration Reactions

• Hydration: Involves the addition of water to unsaturated hydrocarbons using an acidic catalyst.

Detailed Mechanism of Hydration Reaction

• Initiation: An acid catalyst donates a proton, starting the reaction process.
• Carbocation Stabilisation: Essential for the progression of the reaction.
• Product Formation: A nucleophilic attack results in the formation of an alcohol.

Model Construction and Equation Writing

Building Molecular Models

• Tools: Utilise ball-and-stick kits to visualise molecular structures.
• Exercise: Construct models of ethylene (C$_2$H$_4$) and acetylene (C$_2$H$_2$).

Writing Reaction Equations

• Example:
  • Hydrogenation of ethylene: $\mathrm{C}_2\mathrm{H}_4 + \mathrm{H}_2 \rightarrow \mathrm{C}_2\mathrm{H}_6$

Stereochemistry and Geometry

• Visualising Stereoisomers: Crucial for real-world applications.

Worked Examples

Example 1: Hydrogenation of Propene

Propene undergoes hydrogenation to form propane:

$\mathrm{CH}_3\mathrm{CH}=\mathrm{CH}_2 + \mathrm{H}_2 \xrightarrow[\text{Pt/Pd/Ni}]{} \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_3$

Solution:

1. The double bond in propene (C=C) reacts with H₂ in the presence of a catalyst
2. Hydrogen atoms add across the double bond
3. The product is propane, a saturated hydrocarbon

Example 2: Hydration of Ethene

Ethene reacts with water in the presence of an acid catalyst to form ethanol:

$\mathrm{CH}_2=\mathrm{CH}_2 + \mathrm{H}_2\mathrm{O} \xrightarrow[\text{H}^+]{} \mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}$

Solution:

1. The acid catalyst protonates the double bond, forming a carbocation
2. Water attacks the carbocation to form an oxonium ion
3. Deprotonation yields ethanol as the final product

Example 3: Halogenation of Butene

When 2-butene reacts with bromine, 2,3-dibromobutane is formed:

$\mathrm{CH}_3\mathrm{CH}=\mathrm{CH}\mathrm{CH}_3 + \mathrm{Br}_2 \rightarrow \mathrm{CH}_3\mathrm{CHBr}\mathrm{CHBr}\mathrm{CH}_3$

Solution:

1. Bromine forms a cyclic bromonium ion intermediate with the alkene
2. The bromide ion attacks from the opposite side (anti addition)
3. This results in the trans configuration in the final product