Addition reaction

Addition reactions general overview

Addition reaction is a fundamental type of chemical reaction where two or more substances combine to form a single product. This reaction is prevalent in organic chemistry and plays a crucial role in the synthesis of many organic compounds, including polymers, alcohols, and haloalkanes. Addition reactions are characteristic of unsaturated compounds, such as alkenes and alkynes, where the reaction involves the addition of atoms or groups of atoms across the carbon-carbon double or triple bonds.

Mechanism

The mechanism of an addition reaction typically involves the breaking of a π (pi) bond and the formation of two new σ (sigma) bonds. This process can occur via various mechanisms, depending on the nature of the reactants and the conditions of the reaction. Some common mechanisms include:

• Electrophilic Addition: Common in the reactions of alkenes and alkynes, where an electrophile attacks the unsaturated carbon to form a carbocation intermediate, which then reacts with a nucleophile.
• Nucleophilic Addition: Typical for compounds with a polar double bond, such as carbonyl compounds, where a nucleophile attacks the carbon atom of the carbonyl group, followed by the addition of an electrophile.
• Free Radical Addition: Involves the addition of radicals to unsaturated compounds. This mechanism is often initiated by heat or light.

Types of Addition Reactions

Addition reactions can be classified into several types, based on the nature of the reacting substances:

• Hydrogenation: The addition of hydrogen to unsaturated compounds, typically in the presence of a catalyst.
• Halogenation: Involves the addition of halogens (e.g., chlorine, bromine) to unsaturated compounds.
• Hydrohalogenation: The addition of hydrogen halides (e.g., HCl, HBr) to alkenes and alkynes.
• Hydration: The addition of water, often catalyzed by acids, to unsaturated compounds to form alcohols.
• Oxymercuration-Demercuration: A two-step process where mercury acetate and water add across the double bond, followed by demercuration to give an alcohol.
• Hydroboration-Oxidation: Involves the addition of borane (BH3) to alkenes, followed by oxidation to produce alcohols.

Applications

Addition reactions are widely used in the synthesis of various organic compounds. They are essential in the production of polymers, where monomers undergo addition polymerization to form long chains. In the pharmaceutical industry, addition reactions are used to synthesize key intermediates and active pharmaceutical ingredients. Additionally, these reactions are crucial in the production of petrochemicals, agrochemicals, and fragrances.

See Also

• Organic Chemistry
• Chemical Reaction
• Polymerization
• Catalysis