Alkenes and Cycloalkenes

Introduction to Alkenes and Cycloalkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond, while cycloalkenes are cyclic compounds with one or more double bonds within a ring structure. The presence of double bonds imparts unique chemical and physical properties to these molecules.

• Alkene Examples: Ethene, propene, 1-butene, trans-2-butene, cis-2-butene

• Cycloalkene Examples: Cyclopropene, cyclobutene, cyclopentene, cyclohexene, ethylenecyclohex-1-ene

• Dienes: Penta-1,4-diene (unconjugated), buta-1,3-diene (conjugated)

• Bond Properties: Double bonds are shorter and stronger than single bonds due to increased electron density between the carbons. Note: stronger does not mean less reactive!

Nomenclature of Alkenes

How to Name Alkenes

Alkene nomenclature follows IUPAC rules, similar to alkanes, but with the suffix -ene replacing -ane. The longest carbon chain containing the double bond is chosen as the parent structure.

• Parent Chain: Select the longest chain containing the double bond.

• Suffix: Use -ene instead of -ane.

• Substituents: Name and number substituents (e.g., ethyl, methyl, propyl) as in alkanes.

• Special Groups: Vinyl and allyl substituents are named when present.

• Example: A nine-carbon chain with a double bond and substituents: ethyl, two methyl, and one propyl group. Parent name: nonene.

Defining Location of Substituents

Number the parent chain so the double bond gets the lowest possible number. If the double bond is in the middle, use standard substituent numbering rules.

• Numbering: Assign numbers to the chain to give the double bond the lowest locant.

• Substituent Positions: Indicate the position of each substituent and the double bond.

• Example: 7-ethyl, 6-methyl, 5-methyl, 5-propyl, 3-alkene → 7-ethyl-5,6-dimethyl-5-propyl-3-nonene or 7-ethyl-5,6-dimethyl-5-propylnon-3-ene.

Alkene Configuration

Cis and Trans Isomerism

Alkenes cannot freely rotate about the double bond, leading to stereoisomerism. The cis isomer has substituents on the same side, while the trans isomer has them on opposite sides.

• Examples: trans-2-butene vs. cis-2-butene; trans-1,2-dichloroethene vs. cis-1,2-dichloroethene

• Usage: Cis/trans naming is only applicable when comparing substituents with respect to hydrogen.

E and Z Isomerism

When substituents are not hydrogen, the E/Z system is used, based on the Cahn-Ingold-Prelog priority rules.

• Priority Assignment: Higher atomic number = higher priority.

• E (Entgegen): High priority groups on opposite sides.

• Z (Zusammen): High priority groups on the same side.

• Example: (E)-2-chlorobut-2-ene (Cl and C on opposite sides), (Z)-2-chlorobut-2-ene (Cl and C on same side).

• Further Priority: If directly bonded atoms are the same, move outward and compare next atoms. If atomic number is equal, compare the number of such atoms.

• Example: (E)-3-ethyl-4-methylpent-2-ene vs. (Z)-3-ethyl-4-methylpent-2-ene (1 C vs. 2 C attached).

Putting It All Together

When naming complex alkenes, include all substituents and configuration (cis/trans or E/Z). For molecules with multiple double bonds, indicate the position for each configuration label (e.g., (3E, 5Z)).

• Example: trans-7-ethyl-5,6-dimethyl-5-propyl-3-nonene or (E)-7-ethyl-5,6-dimethyl-5-propylnon-3-ene.

Naming Cycloalkenes

Cycloalkene Nomenclature

Naming cycloalkenes follows similar rules to cycloalkanes, with the double bond assigned to C1-C2 and substituents numbered for lowest possible locants. For monoalkenes, the double bond position can be omitted. Hydroxyl groups take priority at C1. Small rings require cis/trans labels.

• Examples:

  • 1-methylcyclopentene

  • 1,3-dimethylcyclohexa-1,4-diene

  • 4-chloro-3,5-dimethylcyclohexene

  • 6-isopropyl-2-methylcyclohept-4-enol

Bonding in Alkenes

Hybridization and Bonding

Alkene carbons are sp2 hybridized, resulting from mixing one s orbital and two p orbitals to form three sp2 hybrid orbitals and one unhybridized p orbital.

• Electron Configuration: Carbon: 1s2 2s2 2p2

• sp2 Hybridization: Three sp2 orbitals form sigma (σ) bonds; one p orbital remains unhybridized for pi (π) bonding.

• Bond Formation: sp2 orbitals overlap to form C–C σ-bond and C–H σ-bonds; unhybridized p orbitals overlap to form the π-bond.

• Geometry: Trigonal planar geometry around each sp2 carbon.

Diagram: (as shown in the notes) Carbon atomic orbitals and their hybridization.

Summary Table: Alkene and Cycloalkene Examples

Key Equations

• General Alkene Formula:

• Bonding: Sigma () and pi () bonds in double bond

Additional info: The notes provide foundational knowledge for further study of alkene reactivity, stereochemistry, and synthesis, which are essential for understanding organic reaction mechanisms and molecular structure.