Chapter 8 – Addition Reactions of Alkenes

Introduction to Addition Reactions

Addition reactions are fundamental transformations in organic chemistry, especially for alkenes. These reactions involve the addition of atoms or groups across the carbon–carbon double bond, converting unsaturated compounds into saturated ones. Understanding the mechanisms and outcomes of these reactions is essential for predicting product formation and for synthetic applications.

• Key Addition Reactions: Hydrogenation, Hydrohalogenation, Hydration, Halogenation, Hydroxylation, and Ozonolysis.

• Applications: Synthesis of alcohols, halides, and other functional groups; industrial processes such as fat hydrogenation.

Overview of Oxidation-Reduction (Redox) in Organic Chemistry

Definitions and Examples

Redox reactions in organic chemistry involve changes in the oxidation state of carbon atoms, typically through the gain or loss of oxygen or hydrogen.

• Reduction: Increasing hydrogen content or decreasing oxygen content of an organic molecule.

• Oxidation: Increasing oxygen content or decreasing hydrogen content.

Example:

Reduction of Alkenes: Hydrogenation

Mechanism and Catalysis

Hydrogenation is the addition of hydrogen (H2) to alkenes in the presence of a metal catalyst, converting double bonds to single bonds.

• Catalysts: Finely divided platinum, palladium, or nickel.

• Heterogeneous Catalysis: Catalyst is insoluble in the reaction medium.

• Syn Addition: Both hydrogens add to the same face of the double bond.

General Reaction:

Application: Partial hydrogenation of vegetable oils produces solid cooking fats; formation of trans fats is associated with health risks.

Electrophilic Addition to Alkenes

General Mechanism

Electrophilic addition involves the attack of an electrophile on the electron-rich double bond of an alkene, followed by nucleophilic addition.

• The π electrons of the double bond act as a nucleophile.

• The first step is electrophilic attack, forming a carbocation intermediate.

• The second step is nucleophilic attack on the carbocation.

General Scheme:

Markovnikov's Rule

Regioselectivity in Addition Reactions

Markovnikov's Rule predicts the major product in the addition of HX to an alkene: the hydrogen atom adds to the carbon with more hydrogens, and the halide adds to the more substituted carbon.

• Statement: In the addition of HX to an alkene, the hydrogen attaches to the carbon with the greater number of hydrogen atoms.

• Reason: The most stable carbocation intermediate forms, leading to the major product.

Example:

• (major product: 2-bromopropane)

Theoretical Explanation: The rate-determining step forms the most stable carbocation, which is stabilized by alkyl substituents.

Addition of Water to Alkenes: Acid-Catalyzed Hydration

Mechanism and Regioselectivity

Alkenes react with dilute aqueous acid to undergo Markovnikov addition of water, forming alcohols.

• The mechanism is the reverse of alcohol dehydration.

• The first step (carbocation formation) is rate-determining.

General Reaction:

Rearrangements in Addition Reactions

Carbocation Rearrangement

Carbocations formed during addition can rearrange to more stable carbocations via hydride or alkyl shifts, leading to unexpected products.

• 1,2-Hydride Shift: A hydrogen atom migrates to stabilize the carbocation.

• 1,2-Alkyl Shift: An alkyl group migrates to stabilize the carbocation.

Addition of Water: Oxymercuration-Demercuration

Mechanism and Importance

Oxymercuration-demercuration is a method for Markovnikov hydration of alkenes without carbocation rearrangement.

• Step 1: Alkene reacts with mercuric acetate to form a mercurinium ion.

• Step 2: Water attacks the more substituted carbon.

• Step 3: Sodium borohydride reduces the intermediate, yielding the alcohol.

General Reaction:

Addition of Water: Hydroboration-Oxidation

Anti-Markovnikov Hydration

Hydroboration-oxidation adds water across the double bond in an anti-Markovnikov fashion, with syn addition of hydrogen and hydroxyl groups.

• Step 1: Alkene reacts with borane (BH3).

• Step 2: Oxidation with hydrogen peroxide yields the alcohol.

General Reaction:

Addition of Halogens

Mechanism and Stereochemistry

Halogenation involves the addition of Cl2 or Br2 to alkenes, forming vicinal dihalides via anti addition.

• Step 1: Formation of a cyclic halonium ion intermediate.

• Step 2: Nucleophilic attack by halide ion opens the ring.

• Stereochemistry: Anti addition leads to trans products.

Example:

Halohydrin Formation

Mechanism and Regioselectivity

When halogenation is performed in aqueous solution, water acts as a nucleophile, forming halohydrins.

• Step 1: Formation of halonium ion.

• Step 2: Water attacks the more substituted carbon, opening the ring.

• Product: Halohydrin (contains both halogen and hydroxyl group).

Oxidation of Alkenes: Epoxidation and Hydroxylation

Epoxidation

Epoxides are three-membered cyclic ethers formed by reaction of alkenes with peroxy acids.

• General Reaction: (epoxide)

Hydroxylation

Alkenes can be converted to 1,2-diols (glycols) by reaction with osmium tetroxide or cold potassium permanganate.

• General Reaction:

Oxidative Cleavage of Alkenes

Hot KMnO4 and Ozonolysis

Strong oxidants cleave alkenes to carbonyl-containing compounds.

• Hot KMnO4: Cleaves double bonds to form carboxylic acids or ketones.

• Ozonolysis: Ozone cleaves double bonds to form aldehydes or ketones after reductive workup.

Example:

• (formaldehyde)

Carbenes and Cyclopropanation

Structure and Reactivity

Carbenes are neutral species with a divalent carbon atom and a lone pair of electrons. They are highly reactive and can add to alkenes to form cyclopropanes.

• Generation: Photolysis or thermolysis of diazomethane, or reaction of chloroform with base.

• General Reaction:

• Cyclopropanation: cyclopropane

Summary Table: Major Addition Reactions of Alkenes

Additional info: Mechanistic details, energy diagrams, and stereochemical outcomes are crucial for understanding the selectivity and product formation in alkene addition reactions. These reactions are foundational for organic synthesis and are frequently encountered in both laboratory and industrial settings.