Planar Carbon: Alkenes (Leaving Cert Chemistry): Revision Notes

Planar Carbon: Alkenes

Unsaturated Organic Compounds

Unsaturated organic compounds contain double or triple bonds between carbon atoms.

In the case of alkenes, the presence of a carbon-carbon double bond (C=C) makes these compounds unsaturated. Unlike saturated compounds (like alkanes), alkenes can undergo addition reactions because their double bond can be broken to allow new atoms to bond with the carbon atoms.

Alkenes: Structure and Bonding

General Formula:

• Alkenes have the general formula $CₙH₂ₙ$ indicating that they have two fewer hydrogen atoms than their corresponding alkane. Double Bond:

• The $C=C$ double bond consists of a sigma ($σ$) bond and a pi ($π$) bond.

• The pi bond prevents free rotation around the double bond, making the molecule planar around the double-bonded carbons.

• The bond angle around these carbons is approximately 120°. Non-Polar Nature:

• Because the double bond in alkenes is between two carbon atoms, which have equal electronegativity, the bond is non-polar.

Structure and Nomenclature (Up to $C-4$)

• Ethene ($C₂H₄$): The simplest alkene with a single carbon-carbon double bond.
• Propene ($C₃H₆$): A three-carbon chain with one double bond between the first and second carbons.
• Butene ($C₄H₈$): A four-carbon chain. Isomers include:
  • But-1-ene (double bond between the first and second carbon).
  • But-2-ene (double bond between the second and third carbon).

Physical Properties of Alkenes

• State: The lower alkenes (e.g., ethene and propene) are gases at room temperature, while higher members are liquids.
• Solubility:
  • In water: Alkenes are insoluble in water due to their non-polar nature.
  • In non-polar solvents: Alkenes are soluble in non-polar solvents such as hexane and benzene, where van der Waals forces allow them to dissolve.

Use of Alkenes in Plastics

Alkenes are crucial raw materials for the polymer industry.

• Polyethylene and polypropylene, both common plastics, are made by the polymerisation of ethene and propene, respectively.
• In these processes, the double bonds of alkenes are broken, allowing the carbon atoms to link together to form long chains.

Carbonyl Compounds: Aldehydes

Aldehydes are organic compounds containing a carbonyl group ($C=O$) where the carbon is also bonded to at least one hydrogen atom.

The presence of the polar $C=O$ bond makes aldehydes more reactive than alkenes.

Structure and Nomenclature of Aldehydes (Up to $C-4$)

• Methanal (Formaldehyde, $CH₂O$): The simplest aldehyde, with one carbon atom.
• Ethanal (Acetaldehyde, $C₂H₄O$): A two-carbon aldehyde, with the carbonyl group attached to the first carbon.
• Propanal ($C₃H₆O$): A three-carbon aldehyde.
• Butanal ($C₄H₈O$): A four-carbon aldehyde.

Physical Properties of Aldehydes

• State: Lower aldehydes (methanal, ethanal) are liquids or gases at room temperature.
• Solubility:
  • In water: Aldehydes like ethanal are moderately soluble in water due to their ability to form hydrogen bonds with water molecules.
  • In cyclohexane: Aldehydes are generally soluble in non-polar solvents like cyclohexane, as their non-polar hydrocarbon chains can interact with non-polar solvent molecules.

Benzaldehyde

Benzaldehyde is an aromatic aldehyde found naturally in almond kernels.

• Though it's aromatic, benzaldehyde still possesses the reactivity of aldehydes due to its polar $C=O$ bond.