Alkenes & Alkynes: Structure, Isomerism, and Elimination Reactions

Alkenes & Alkynes

Introduction to Alkenes and Alkynes

Alkenes and alkynes are unsaturated hydrocarbons characterized by the presence of multiple bonds between carbon atoms. Their unique bonding leads to distinct chemical properties and reactivity.

• Alkenes: Contain at least one carbon-carbon double bond. Example: Ethylene ($CH_2=CH_2$).

• Alkynes: Contain at least one carbon-carbon triple bond. Example: Ethyne ($HC ext{≡} CH$).

Cis & Trans Isomers

Geometric Isomerism in Alkenes

Alkenes with two different substituents on each carbon of the double bond can exhibit cis and trans isomerism, which affects their physical and chemical properties.

• Cis isomer: Substituents are on the same side of the double bond.

• Trans isomer: Substituents are on opposite sides of the double bond.

• Example: 2-butene Cis-2-butene: $CH_3CH=CHCH_3$ (CH3 groups on same side) Trans-2-butene: $CH_3CH=CHCH_3$ (CH3 groups on opposite sides)

E/Z Naming

Advanced Stereochemical Nomenclature

The E/Z system is used for alkenes with more than two substituents to specify the relative positions of the highest priority groups.

• Z (zusammen): Higher priority groups are on the same side.

• E (entgegen): Higher priority groups are on opposite sides.

• Priority Rules: Highest priority is assigned to the atom with the largest atomic number directly attached to the double bond. If a decision cannot be made, move to the next atom until a difference is found.

• Multiple Bonds: Atoms involved in multiple bonds are treated as if they are bonded to an equivalent number of single-bonded atoms.

• Example: (E)-1-bromo-2-chloro-1-propene

Stability of Alkenes

Heat of Hydrogenation and Alkene Stability

The stability of alkenes can be measured by their heat of hydrogenation. More substituted alkenes are generally more stable.

• Heat of Hydrogenation: The energy released when an alkene is hydrogenated to an alkane. Lower heat indicates greater stability.

• Order of Stability: Most substituted > cis > trans > terminal

• Equation: $C=C + H_2 \xrightarrow{Pt} C-C$

Cycloalkenes

Ring Size and Isomerism

Cycloalkenes with five or fewer carbons exist only in the cis form due to ring strain. Larger rings can accommodate both cis and trans isomers.

• cis form only: Cyclopropene, cyclobutene, cyclopentene

• cis & trans possible: Cyclohexene (trans is too strained at room temperature), cyclooctene, cyclononene (trans is stable at room temperature)

Eliminations

Dehydrohalogenation and Base Preparation

Elimination reactions are key for synthesizing alkenes and alkynes. Dehydrohalogenation involves the removal of a hydrogen and a halogen from adjacent carbons.

• Dehydrohalogenation: $CH_3CH_2CHBrCH_3 + NaOEt \rightarrow CH_3CH=CHCH_3 + EtOH + NaBr$

• Preparation of Bases:

  Reaction	Product
  $2ROH + 2Na \rightarrow 2NaOR + H_2$	sodium alkoxide
  $2H_2O + 2Na \rightarrow 2NaOH + H_2$	sodium hydroxide
  $2EtOH + 2Na \rightarrow 2NaOEt + H_2$	sodium ethoxide
  $2(CH_3)_3COH + 2K \rightarrow 2KOC(CH_3)_3 + H_2$	potassium t-butoxide

Elimination: E2

Bimolecular Elimination Mechanism

The E2 reaction is a single-step process where the base removes a proton and the leaving group departs simultaneously, forming an alkene. The major product is determined by Zaitsev's rule.

• Zaitsev's Rule: The most substituted alkene is the major product.

• Example: $CH_3CHBrCH(CH_3)_2 + CH_3CH_2O^- \rightarrow CH_3CH=CHCH_3$ (major, 70%) $CH_3CH_2CH=CHCH_3$ (minor, 30%)

Additional info:

• Halogens (fluorine, chlorine, bromine, iodine) are often involved as leaving groups in elimination reactions.

• Anti-coplanar geometry is required for E2 reactions, meaning the base and leaving group must be on opposite sides of the molecule.