Organic reactions and useful compounds

## Big idea: organic reactions and useful compounds - **Key concept: Change.** Alkenes contain a carbon-carbon double bond and undergo addition reactions. Alcohols and carboxylic acids have different functional groups and reaction patterns. - **Related concepts: Models and evidence.** Use a scientific explanation to make predictions, then test it against observations. - **Global context: Scientific and technical innovation.** Organic reactions produce fuels, solvents and materials. ## Fossil Fuels - A **fuel** is a substance that releases **heat energy** when burned. - Common fossil fuels: **coal**, **natural gas** (mainly methane, CH₄), and **petroleum** (crude oil). - **Hydrocarbons** are compounds containing only hydrogen and carbon atoms. - Petroleum is a mixture of hydrocarbons separated by **fractional distillation** into fractions with similar boiling points. - Fractional distillation uses a column with a temperature gradient: hot at bottom, cool at top. - Properties of fractions: as chain length increases, **viscosity** increases, **volatility** decreases, and **boiling point** increases. - Uses: refinery gas (heating/cooking), gasoline (car fuel), naphtha (chemicals), kerosene (jet fuel), diesel (engines), fuel oil (ships), lubricating oil, bitumen (roads). ## Alkanes - Alkanes are **saturated hydrocarbons** with only single C–C bonds; general formula **CₙH₂ₙ₊₂**. - First five: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂). - They undergo **complete combustion**: alkane + O₂ → CO₂ + H₂O. - They can be **cracked** into smaller molecules (alkanes, alkenes, H₂). - **Substitution reaction** with halogens (e.g., Cl₂) in UV light: H atom replaced by halogen (e.g., CH₄ + Cl₂ → CH₃Cl + HCl). ## Alkenes - Alkenes are **unsaturated hydrocarbons** containing at least one C=C double bond; general formula **CₙH₂ₙ**. - First four: ethene (C₂H₄), propene (C₃H₆), but-1-ene (C₄H₈), pent-1-ene (C₅H₁₀). - **Catalytic cracking** of long-chain alkanes (600–700°C, alumina/silica catalyst) produces shorter alkanes, alkenes, and H₂. - Alkenes **decolourise bromine water** (orange → colourless) – test for unsaturation; alkanes do not. - More reactive than alkanes due to the C=C bond. ## Addition Reactions - In **addition reactions**, atoms add across the C=C bond; only one product formed. - With **bromine**: ethene + Br₂ → 1,2-dibromoethane (saturated). - With **hydrogen** (Ni catalyst): ethene + H₂ → ethane. - With **steam** (hydration, acid catalyst): ethene + H₂O → ethanol. - These reactions produce saturated compounds from alkenes. ## Alcohols - Alcohols contain the **hydroxyl (-OH) functional group**; general formula **CₙH₂ₙ₊₁OH**. - First three: methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH). - Ethanol is used in alcoholic drinks, as a fuel, and as a solvent; burns to CO₂ and H₂O. - Manufacture: **hydration of ethene** (300°C, 60 atm, H₃PO₄ catalyst) – continuous, pure product, uses non-renewable crude oil. - Manufacture: **fermentation** of glucose (yeast, 25–35°C, anaerobic) – batch process, renewable, dilute product. - Fermentation equation: C₆H₁₂O₆ → 2CO₂ + 2C₂H₅OH. ## Carboxylic Acids - Carboxylic acids contain the **-COOH group**; they behave as typical acids. - React with **metals**: e.g., 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. - React with **bases** (neutralisation): CH₃COOH + KOH → CH₃COOK + H₂O. - React with **carbonates**: 2CH₃COOH + K₂CO₃ → 2CH₃COOK + H₂O + CO₂. - Salts end in **-anoate** (e.g., ethanoate from ethanoic acid). ## Ethanoic Acid & Esterification - Ethanoic acid (CH₃COOH) is made by **oxidation of ethanol**: via fermentation (air, bacteria) or using acidified KMnO₄ (purple → colourless). - **Esterification**: alcohol + carboxylic acid → ester + water, with concentrated H₂SO₄ catalyst. - Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ (ethyl ethanoate) + H₂O. - Esters are sweet-smelling oils used in flavourings and perfumes. - Naming: first part from alcohol (ends -yl), second from acid (ends -oate); e.g., pentanol + butanoic acid → pentyl butanoate. ## Think like a scientist - Use teacher-provided results to compare the reactions of an alkane and an alkene with bromine water; do not repeat this test at home. - **Comparison:** whether the compound is an alkane or an alkene. **Outcome:** the change in bromine-water colour. - **Control:** keep reagent amount and observation time constant. Explain why this makes the comparison fairer. - **Evidence:** Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause. - **Safety:** Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it. - **Inquiry task:** State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction. ## Evaluate the science - Organic reactions produce fuels, solvents and materials. - A positive bromine-water test supports unsaturation but does not establish a complete molecular structure. - **Evaluation task:** Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.