Alkene Reactions

Introduction to Alkenes

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. They are prevalent in nature and industrial chemistry, serving as key intermediates in organic synthesis.

• Sources: Found in natural products (plants and animals) and petroleum (crude oil).

• Formation: Alkenes can be synthesized via elimination reactions (see Chapter 10), from alkynes (see Chapter 9), and through advanced methods such as the Wittig reaction and Olefin Metathesis (covered in Organic Chemistry II).

Reactions of Alkenes: Overview

Functional Group Interconversions and Mechanisms

Alkenes undergo a variety of reactions that convert them into different functional groups. These reactions can be categorized by the type of functional group formed or by their mechanism (e.g., radical, electrophilic addition).

• Functional group interconversions: Alcohols, alkyl halides, diols, epoxides, carbonyl compounds, cyclopropanes, etc.

• Mechanistic categories: Most alkene reactions proceed via electrophilic addition; radical mechanisms are discussed in later chapters.

Hydrohalogenation

Addition of HX to Alkenes

Hydrohalogenation is the addition of hydrogen halides (HCl, HBr, HI) to alkenes, resulting in alkyl halides. The reaction is regioselective and follows Markovnikov's rule.

• Mechanism: Two-step process involving a carbocation intermediate.

• Regioselectivity: For asymmetrical alkenes, the major product forms when the halide adds to the more substituted carbon (Markovnikov product).

General equation:

Example: Propene reacts with HBr to form 2-bromopropane (major) and 1-bromopropane (minor).

Halogenation

Addition of Cl2 or Br2 to Alkenes

Halogenation involves the addition of chlorine or bromine to alkenes, forming vicinal dihalides. The reaction is stereospecific and proceeds via anti addition.

• Halogen used: Cl2 or Br2 (not I2).

• Stereochemistry: Halogens add to opposite sides of the double bond (anti addition).

• Mechanism: Proceeds via a cyclic bromonium or chloronium ion intermediate, not a carbocation.

General equation:

Example: Cyclohexene reacts with Br2 to form trans-1,2-dibromocyclohexane.

Halohydrin Formation

Formation of Halohydrins in the Presence of Water

When halogenation is performed in aqueous solution, water acts as the nucleophile, resulting in halohydrin formation.

• Nucleophile: Water is a better nucleophile than halide ions.

• Regioselectivity: The OH group adds to the more substituted carbon (Markovnikov orientation).

• Stereochemistry: Anti addition is observed.

General equation:

Example: Propene reacts with Br2 and H2O to form 1-bromo-2-propanol.

Oxidation and Reduction: Overview

Definitions and Applications

Oxidation and reduction in organic chemistry refer to changes in the oxidation state of carbon atoms, often involving the formation or breaking of bonds to oxygen or hydrogen.

• Oxidation: Increase in oxidation number (loss of electrons, formation of C–O bonds).

• Reduction: Decrease in oxidation number (gain of electrons, breaking of C–O bonds).

Example: Conversion of an alcohol to a ketone (oxidation) or a ketone to an alcohol (reduction).

Equations:

(oxidation)

(reduction)

Acid-Catalyzed Hydration

Addition of Water to Alkenes

Acid-catalyzed hydration adds water across the double bond of an alkene, forming an alcohol. The reaction is regioselective and follows Markovnikov's rule.

• Reagents: Strong acid (H+, e.g., H2SO4) and water.

• Mechanism: Similar to hydrohalogenation, but water is the nucleophile.

• Regioselectivity: OH group adds to the more substituted carbon.

General equation:

Example: Propene reacts with water and acid to form 2-propanol (major) and 1-propanol (minor).

Additional info: Further reactions of alkenes, such as oxymercuration-demercuration, hydroboration-oxidation, hydrogenation, epoxidation, and oxidative cleavage, are covered in subsequent slides and chapters. These reactions expand the synthetic utility of alkenes in organic chemistry.