Organic Chemistry I: Hydrocarbons - SS2 Chemistry Past Questions and Answers - page 5

Reactions of Alkanes:

●     Combustion: Alkanes readily undergo combustion reactions in the presence of oxygen to produce carbon dioxide and water. This reaction releases a significant amount of energy and is used as a source of heat or power.

●     Halogenation: Alkanes can undergo halogenation reactions, where halogens (such as chlorine or bromine) replace hydrogen atoms in the alkane molecule. This reaction requires the presence of ultraviolet (UV) light or heat as an energy source.

●     Cracking: Cracking is a process in which larger alkanes are broken down into smaller hydrocarbons by the application of heat and/or catalysts. This reaction is important in the production of gasoline and other petroleum products.

Reactions of Alkenes:

●     Addition Reactions: Alkenes are highly reactive due to the presence of the carbon-carbon double bond. They undergo addition reactions, where atoms or groups are added across the double bond. Examples include hydrogenation (addition of hydrogen), halogenation (addition of halogens), and hydration (addition of water to form alcohols).

●     Polymerization: Alkenes can undergo polymerization reactions, where multiple monomer units are joined together to form a polymer. This reaction is essential in the production of plastics and synthetic materials.

Reactions of Alkynes:

●     Additional Reactions: Alkynes, like alkenes, undergo additional reactions across the carbon-carbon triple bond. The reactions are similar to those of alkenes, resulting in the addition of atoms or groups across the triple bond.

●     Acidity of Terminal Alkynes: Terminal alkynes (alkynes with a triple bond at the end of the carbon chain) are acidic and can undergo reactions with strong bases to form salts known as acetylides.

Reactions of Substitution:

●     Substitution reactions involve the replacement of one atom or group by another in a molecule. One example is the halogenation of alkanes, where a hydrogen atom is replaced by a halogen atom (e.g., chlorine or bromine). For instance, methane can undergo substitution with chlorine to produce chloromethane.

Reactions of Elimination:

●     Elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of a double bond or triple bond. One example is the dehydrohalogenation of alkyl halides, where a hydrogen halide is eliminated from the molecule to form an alkene. For example, 2-bromopropane can undergo elimination to produce propene.

Reactions of Oxidation:

Oxidation reactions involve the addition of oxygen or removal of hydrogen from a molecule. The products of oxidation are typically more oxidised (higher oxidation state) than the reactants. Alcohols can undergo oxidation to produce aldehydes, ketones, or carboxylic acids, depending on the conditions. For instance, primary alcohols can be oxidised to aldehydes or further to carboxylic acids.