Properties of Alkenes

Properties of Alkenes

• General formula for alkenes: CnH2n.
• Alkenes are unsaturated molecules due to the presence of at least one double carbon bond (C=C).
• The double bond consists of one sigma bond and one pi bond:
  • Sigma bonds result from head-on overlap of s orbitals.
  • Pi bonds arise from sideways overlap of p orbitals, creating two regions of electron density.
• Alkenes exhibit trigonal planar geometry around the double bond with a bond angle of 120°.
• The spatial arrangement minimizes repulsion among the three bonding pairs.

Stereoisomerism

• Stereoisomers have the same structural formula but differ in atom arrangements in space.
• Alkenes can form cis/trans or E/Z isomers due to restricted rotation around the C=C bond.
• Cis isomers: Similar groups are on the same side of the double bond.
• Trans isomers: Similar groups are on opposite sides of the double bond.
• E/Z isomer labeling involves assigning priority based on molecular mass:
  • Higher molecular mass groups get higher priority.
  • Z isomers have high-priority groups on the same side; E isomers have them on opposite sides.
• Conditions for E/Z isomerism:
  • Presence of a double carbon bond (restricting rotation).
  • Each carbon of the C=C bond must be attached to two different groups.

Addition Reactions

• Alkenes are more reactive than alkanes due to the high electron density of the pi bond and weaker double bond strength.
• Addition reactions break the double bond, allowing other atoms to bond with carbon atoms, achieving 100% atom economy (one product formed).
• Hydrogenation: Hydrogen reacts with alkenes to form alkanes, requiring a nickel catalyst and a temperature of 150°C.
• Halogenation: Alkenes react with halogens at room temperature and pressure to produce halogenoalkanes via an electrophilic addition mechanism.

Mechanism of Halogenation

• The pi electrons in the C=C bond create a negative region that repels electrons in the halogen bond:
  • This forms an induced dipole in the halogen molecule.
• The pi electrons attack the positively charged halogen atom, resulting in bond formation.
• The halogen bond undergoes heterolytic fission, creating a negative halide ion.
• The carbon atom may become positively charged (carbocation intermediate) after bond formation.
• The negative halide ion then bonds covalently with the positive carbon to yield a dihalogenoalkane.
• Electrophiles (such as halogens) are species that accept a pair of electrons and play a key role in addition reactions.
• Reaction with bromine water serves as a test for unsaturation: unreacted bromine is orange, while formation of halogenoalkane turns it colorless.

This quiz covers the essential properties of alkenes, including their general formula CnH2n and the nature of the double bond. Understand the formation of sigma and pi bonds, as well as their implications in unsaturated molecules. Test your knowledge on the structure and bonding of alkenes.