Overview
This lecture covers the properties, naming, and isomerism of alkenes, emphasizing naming conventions and geometric (cis/trans) isomerism due to double bonds.
Introduction to Alkenes
- Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond.
- Double bonds make alkenes highly reactive and give rise to diverse organic compounds.
- Common uses include detergents, rubbers, plastics, and gasoline.
- Double bonds in alkenes are sp² hybridized.
Naming Alkenes
- Alkenes are named similarly to alkanes but use the "-ene" suffix instead of "-ane".
- The position of the double bond must be indicated by the lowest possible number in the chain.
- Naming format: number-position-prefix-root-ene (e.g., 3-methyl-1-pentene).
- Always use dashes between numbers and letters in names.
- Choose the longest carbon chain containing the double bond as the parent chain.
Examples of Alkene Naming
- For 3-methyl-1-pentene: five-carbon chain, double bond starts at carbon 1, methyl group at position 3.
- For 6-methyl-3-heptene: seven-carbon chain, double bond starts at carbon 3, methyl group at position 6.
Isomerism in Alkenes
- Isomers share the same molecular formula but differ in structure or spatial arrangement.
- Geometric (cis/trans) isomerism arises due to restricted rotation around the double bond.
- Cis isomers: similar groups on the same side of the double bond.
- Trans isomers: similar groups on opposite sides of the double bond.
- Geometric isomerism requires each double-bonded carbon to have two different attached groups.
Geometric Isomerism Examples
- 3-methyl-1-pentene: does not exhibit cis/trans isomerism due to insufficient distinct groups.
- 6-methyl-3-heptene: exhibits cis isomerism since two R groups are on the same side of the double bond.
Key Terms & Definitions
- Alkene — hydrocarbon with at least one carbon-carbon double bond.
- Isomer — compounds with the same molecular formula but different structures.
- Geometric Isomers (cis/trans) — molecules with the same connectivity but different spatial arrangements due to double bonds.
- sp² Hybridization — type of orbital hybridization for double-bonded carbons.
- Parent Chain — the longest carbon chain containing the functional group.
Action Items / Next Steps
- Practice naming alkenes using the correct conventions and identifying geometric isomerism.
- Review homework assignments on alkene nomenclature and isomer identification.