Alkenes

Summary and Exam Tips for Alkenes

Alkenes is a subtopic of Organic Chemistry, which falls under the subject Chemistry in the Cambridge IGCSE curriculum. Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond (C=C), making them more reactive than alkanes. Their general formula is $C_nH_{2n}$. Alkenes can be produced through cracking, a process where larger alkanes are broken down into smaller alkanes and alkenes using high temperatures and a catalyst. This process is essential for producing more useful molecules like alkenes and hydrogen.

Alkenes undergo addition reactions, where the C=C bond breaks, allowing new atoms to attach, forming only one product. Common addition reactions include:

• Bromination: Alkene + bromine $\rightarrow$ dibromoalkane
• Hydration: Alkene + steam $\rightarrow$ alcohol
• Hydrogenation: Alkene + hydrogen $\rightarrow$ alkane

To distinguish between saturated and unsaturated hydrocarbons, a test with bromine water is used. Unsaturated hydrocarbons will decolorize bromine water from orange to colorless, while saturated hydrocarbons will not react, leaving the solution orange. Alkenes are crucial in manufacturing materials like plastics (e.g., polythene, PVC) and non-stick coatings like Teflon.

Exam Tips

• Structural Formulas: Remember that only double and triple bonds are shown in structural formulas.
• General Formulas: Familiarize yourself with the general formula for each homologous series, such as $C_nH_{2n}$ for alkenes.
• Bond Types: Saturated compounds have single bonds only, while unsaturated compounds have double bonds.
• Bromine Water Test: Use the term colorless instead of clear when describing the decolorization of bromine water in the presence of alkenes.
• Addition Reactions: Understand that addition reactions with alkenes result in only one product, highlighting the reactivity of the C=C bond.