Reactions of Hydrocarbons - SS2 Chemistry Lesson Note
Hydrocarbons, organic compounds composed of carbon and hydrogen atoms, undergo a variety of reactions that play a crucial role in organic chemistry and related industries. Understanding the reactions of hydrocarbons is essential for predicting their behaviour, synthesising new compounds, and harnessing their potential in various applications.
Combustion:
Combustion is a common reaction of hydrocarbons with oxygen, resulting in the release of energy in the form of heat and light. The general equation for combustion is:
Hydrocarbon + Oxygen → Carbon Dioxide + Water + Energy
In this reaction, hydrocarbons act as fuels, and the carbon-hydrogen bonds are oxidised to form carbon dioxide (CO2) and water (H2O). Complete combustion occurs in the presence of sufficient oxygen, while incomplete combustion can lead to the formation of carbon monoxide (CO) or soot.
Halogenation:
Halogenation is a reaction where hydrocarbons react with halogens (chlorine, bromine, or iodine) to form halogenated derivatives. The reaction is typically initiated by light or heat. The halogen replaces one or more hydrogen atoms in the hydrocarbon, resulting in the substitution of hydrogen with a halogen atom. For example:
Methane + Chlorine → Chloromethane + Hydrogen Chloride
Halogenation reactions are often highly selective and occur predominantly in the presence of ultraviolet (UV) light or at elevated temperatures.
Hydrogenation:
Hydrogenation is a reaction in which unsaturated hydrocarbons (alkenes or alkynes) react with hydrogen gas (H2) to form saturated hydrocarbons (alkanes). The reaction is typically catalysed by metals such as platinum (Pt), palladium (Pd), or nickel (Ni). For example:
Ethene + Hydrogen → Ethane
Hydrogenation is an important industrial process used in the production of saturated fats, vegetable oils, and the synthesis of various organic compounds.
Hydration:
Hydration is a reaction in which an alkene reacts with water (H2O) to form an alcohol. The reaction is typically catalysed by acid, such as sulfuric acid (H2SO4) or phosphoric acid (H3PO4). For example:
Ethene + Water → Ethanol
Hydration reactions are essential in the production of alcohols and are also used in the production of various industrial chemicals.
Addition Reactions:
Additional reactions are characteristic of unsaturated hydrocarbons (alkenes and alkynes) and involve the breaking of multiple bonds and the addition of atoms or groups to the carbon atoms. Common addition reactions include:
● Hydrohalogenation: Addition of a hydrogen halide (HCl, HBr) to form a halogenoalkane.
● Hydroboration-Oxidation: Addition of borane (BH3) followed by oxidation to form alcohol.
● Halogenation: Addition of halogens (Cl2, Br2) to form dihaloalkanes.
● Ozonolysis: Cleavage of the double bond by ozone (O3) followed by reductive workup.
The reactions of hydrocarbons encompass a wide range of transformations, each with its significance and applications. Combustion provides energy release, halogenation introduces halogen atoms, hydrogenation converts unsaturated hydrocarbons to saturated forms, hydration forms alcohols, and addition reactions add various atoms or groups to the hydrocarbon backbone. Understanding these reactions allows chemists to predict and manipulate the behaviour of hydrocarbons, enabling the synthesis of new compounds and the development of diverse applications in fields such as energy, pharmaceuticals, and materials science.