Alkenes

What Are Alkenes?

Alkenes: Unsaturated hydrocarbons characterised by at least one carbon-carbon double bond (C=C).

• Alkanes: Composed solely of single carbon bonds (C–C).
• Alkynes: Contain carbon-carbon triple bonds (C≡C).

The C=C double bond significantly influences the chemical behaviour and unique properties of alkenes.

Planar Structure and Geometry

Alkenes exhibit a planar structure:

• Bond Angles: Approximately $120^{\circ}$, due to sp2 hybridisation.
• Geometry: Results in a flat planar formation, fixing the molecular shape.

This geometric rigidity is essential for understanding alkene behaviour.

Chemical Reactivity

Alkenes are notably reactive due to the C=C double bond:

• Addition Reactions: Atoms are added across the double bond, increasing saturation.

Common Misconceptions

Alkenes cannot freely rotate around a C=C bond:

• This leads to mistakes in predicting molecular behaviour.
• Example: Misinterpreting the flexibility in structure affects conversion predictions.

Introduction to Homologous Series

Homologous Series: A collection of organic compounds with similar chemical properties, attributable to identical functional groups.

• Alkenes constitute a homologous series with a defining carbon-carbon double bond (C=C).
• Examples include smaller alkenes such as ethene ($\text{C}_2\text{H}_4$) and propene ($\text{C}_3\text{H}_6$).

Characteristics of Alkenes in the Homologous Series

• Functional Group:
  • Influential role: C=C double bond.
  • Determines reactivity through specific reactions.
• Typical Addition Reactions:
  • Hydrogenation: Incorporation of H$_2$ to saturate the molecule.
  • Halogenation: Involves the addition of halogens such as Cl$_2$ or Br$_2$.

Incremental Changes in Molecular Structure

• The inclusion of -CH$_2$- units results in variations in both molecular mass and physical properties.

Examples and Physical Properties

Alkene	Molecular Formula	Melting Point	Boiling Point
Ethene	$\text{C}_2\text{H}_4$	-169°C	-104°C
Propene	$\text{C}_3\text{H}_6$	-185°C	-48°C
Butene	$\text{C}_4\text{H}_8$	-185°C	-6°C
Pentene	$\text{C}_5\text{H}_{10}$	-138°C	30°C
Hexene	$\text{C}_6\text{H}_{12}$	-140°C	63°C
Heptene	$\text{C}_7\text{H}_{14}$	-91°C	94°C
Octene	$\text{C}_8\text{H}_{16}$	-101°C	122°C

• Trends:
  • As molecular weight increases, so do the melting and boiling points.
  • Van der Waals forces strengthen with increased molecular size, requiring more energy for phase transitions.

Introduction to Alkene Nomenclature

• IUPAC system: The global standard for systematic naming in chemistry.
• Applied in sectors such as pharmaceuticals, petrochemicals, and plastics.

Basic Rules for Naming Alkenes

• Base Name: Identify the longest carbon chain containing the double bond and append the suffix '-ene'.
• Numbering Steps:
  • Initiate numbering from the end nearest the double bond.

Examples and Practice

First Three Alkenes

Name	Formula	Structure
Ethene	$\text{C}_2\text{H}_4$	CH$_2$=CH$_2$
Propene	$\text{C}_3\text{H}_6$	CH$_3$-CH=CH$_2$
Butene	$\text{C}_4\text{H}_8$	CH$_3$-CH$_2$-CH=CH$_2$ or CH$_3$-CH=CH-CH$_3$

• Worked Example:
  • Structure: CH$_3$-CH=CH-CH$_2$-CH$_3$
  • Longest chain: 5 carbons = Pent
  • Double bond: Between 2nd and 3rd carbon = Pent-2-ene

Introduction to Alkene Isomerism

Isomerism: Existence of compounds sharing the same molecular formula yet possessing different structures.

• Structural (constitutional) isomerism
• Geometric (cis-trans) isomerism

Key Points:

• Identical formula, varying arrangements.

Structural Isomerism

Structural Isomerism: Occurs from different atom connectivities.

Example:

• 1-butene: Straight chain.
• 2-methylpropene: Branched configuration.

Geometric (Cis-Trans) Isomerism

Geometric Isomerism: Arises from restricted rotation around C=C bonds, leading to cis-trans isomers.

• Cis-Trans (E-Z) Isomerism:
  • Cis: High-priority groups are on the same side.
  • Trans: Positioned on opposing sides.

Example

• Reactivity: Varies by structure.
• Utilisation: Influenced by different reactivity profiles.

Significance:

Structurally similar compounds can exhibit vastly different industrial applications.

Differences in Physical and Chemical Properties

Boiling Points

• Longer chains usually exhibit higher boiling points due to increased surface area.

Melting Points

• Affected by molecular packing.

Solubility & Reactivity

• Structural variations impact differences in solubility and reactivity.

Introduction to Model Construction

Constructing alkene models aids in visualising molecular geometry and spatial configuration.

Techniques for Model Building

• Physical Models:
  • Employ organic model kits.
• Virtual Models:
  • Digital tools support the creation of virtual models.

Steps for Constructing Models

• Select Components:
  • Blue for carbon connectors, white for hydrogen.
• Create Bonds:
  • Use rods to represent hydrogen bonds, springs for C=C.

Common Mistakes and Tips

• Incorrect Bond Angles:
  • Solution: Use angle guides for precision.
• Improper Double Bond Representation:
  • Solution: Ensure double bonds are correctly represented and verified.