What is an Addition Reaction?
An addition reaction is a type of organic chemical reaction where two or more molecules combine to form a larger, single molecule. This typically occurs across a multiple bond (like a double or triple bond) in an unsaturated compound, where the multiple bond breaks and new single bonds are formed with the added atoms or groups.
How Addition Reactions Occur
The defining feature of an addition reaction is the saturation of an unsaturated compound. For example, in alkenes (containing C=C double bonds) or alkynes (containing C≡C triple bonds), the pi (π) bond within the multiple bond is broken, allowing two new atoms or groups to attach to the carbon atoms that were part of the multiple bond. The carbon chain becomes 'saturated' with more atoms.
A Practical Example: Hydrogenation of Alkenes
A common example is the hydrogenation of ethene (C₂H₄), an alkene. In this reaction, hydrogen gas (H₂) is added across the double bond of ethene in the presence of a catalyst (like nickel, palladium, or platinum). The double bond breaks, and a hydrogen atom attaches to each carbon, forming ethane (C₂H₆), a saturated alkane: Ethene + H₂ → Ethane.
Importance and Applications
Addition reactions are crucial in various fields. Industrially, they are used to produce polymers (e.g., polyethylene from ethene monomers), convert unsaturated oils into saturated fats (hydrogenation in margarine production), and synthesize diverse organic chemicals. Biologically, similar reactions occur in metabolic pathways, such as the addition of water to unsaturated fatty acids.