Overview
This lecture covers the rules for naming alkenes, including location of double bonds, dealing with substituents, naming rings, and the cis/trans (E/Z) isomerism systems.
Basic Alkene Nomenclature
- Alkenes are named based on the longest carbon chain containing the double bond, with the suffix "-ene."
- Number the chain to give the double bond the lowest possible number.
- For multiple double bonds, use "diene" (two), "triene" (three), or "tetraene" (four), placing locants before the suffix (e.g., 1,3-butadiene).
- Name substituents and their positions using standard alkane prefixes and position numbers.
Numbering and Substituents
- Always number from the end nearest a double bond to minimize the locant.
- Substituents (e.g., methyl, ethyl) are named and numbered as prefixes in alphabetical order.
- Double bonds take priority over other substituents for lowest numbering.
Naming Cyclic Alkenes
- Rings with a double bond are named with "cyclo" prefix and "-ene" suffix (e.g., cyclopentene).
- The carbon atoms of the double bond are assigned positions 1 and 2 by default.
- Substituents are numbered for lowest set of numbers, starting from the double bond.
Cis/Trans and E/Z Isomerism
- Cis isomer: similar groups on the same side of the double bond; trans isomer: on opposite sides.
- The E/Z system is used for alkenes with four different groups attached to the double bond.
- Assign priority to substituents based on atomic number; higher atomic number = higher priority.
- Z ("zusammen") = highest priority groups on same side; E ("entgegen") = on opposite sides.
- For multiple double bonds, specify the E/Z configuration and the position of each double bond.
Challenging Cases and Examples
- For multi-double-bond compounds, use locants and E/Z designations for each bond (e.g., (2Z,5E)-3-chloro-6-methylocta-2,5-diene).
- Always choose the chain containing the most double bonds and the greatest length.
Key Terms & Definitions
- Alkene — a hydrocarbon with at least one carbon-carbon double bond.
- Isomer — molecules with the same formula but different structures.
- Cis/Trans Isomerism — stereoisomerism in alkenes due to restricted rotation around the double bond.
- E/Z Isomerism — nomenclature based on priority rules for groups on each double-bonded carbon.
- Locant — the number indicating the position of a group or bond on the chain.
Action Items / Next Steps
- Practice naming alkenes using both cis/trans and E/Z systems.
- Complete assigned textbook exercises on alkene nomenclature.
- Review alkane parent chain names for 1–10 carbons.