Organic Chemistry

The One Thing

The structure of an organic compound, particularly its functional groups and carbon–carbon bonding, determines its characteristic reactions and physical properties. These relationships explain the behaviour and uses of alkanes, alkenes, alcohols, carboxylic acids, polymers and fuels.

Reactions, Processes and Experiments

Ethanol production by fermentation is renewable but relatively slow and produces a dilute solution. Hydration of ethene is faster and produces purer ethanol, but it uses a non-renewable raw material.

Key Terms

• Organic compound: A compound based mainly on carbon atoms, usually containing hydrogen and sometimes oxygen or other elements.
• Hydrocarbon: A compound made only of carbon and hydrogen.
• Homologous series: A family of organic compounds with the same functional group and general formula, showing similar chemical properties.
• Alkane: A saturated hydrocarbon containing only single carbon–carbon bonds.
• Alkene: An unsaturated hydrocarbon containing at least one carbon–carbon double bond.
• Saturated: Containing only single bonds between carbon atoms.
• Unsaturated: Containing one or more carbon–carbon multiple bonds.
• Functional group: An atom or group of atoms responsible for the characteristic reactions of an organic compound.
• Alcohol: An organic compound containing the hydroxyl functional group, –OH, joined to a carbon atom.
• Carboxylic acid: An organic compound containing the carboxyl functional group, –COOH.
• Ethanol: An alcohol with the formula C2H5OH, used as a solvent and fuel and produced by fermentation or hydration of ethene.
• Ethanoic acid: A carboxylic acid with the formula CH3COOH, found in vinegar and formed when ethanol is oxidised.
• Polymer: A very large molecule made from many repeating monomer units joined together.
• Monomer: A small molecule that joins with many others to form a polymer.
• Addition polymerisation: A reaction in which alkene monomers join to form a polymer without producing another small molecule.
• Cracking: The thermal decomposition of large hydrocarbon molecules into smaller, more useful hydrocarbons, including alkenes.
• Fractional distillation: A method used to separate crude oil into fractions with similar boiling points.
• Combustion: Reaction with oxygen that releases energy; complete combustion of a hydrocarbon produces carbon dioxide and water.
• Repeating unit: The section of a polymer chain that repeats throughout its structure.

Easily Confused

• Alkanes and alkenes: Alkanes are saturated and contain only single carbon–carbon bonds; alkenes are unsaturated and contain at least one carbon–carbon double bond.
• Complete and incomplete combustion: Complete combustion produces carbon dioxide and water; incomplete combustion may produce carbon monoxide and carbon when oxygen is limited.
• Fermentation and hydration of ethene: Fermentation uses yeast at about 25–35 °C without oxygen and is renewable but slow, whereas hydration uses steam and a catalyst and is faster but relies on a non-renewable raw material.
• Cracking and fractional distillation: Cracking chemically decomposes long-chain hydrocarbons into smaller molecules; fractional distillation physically separates crude oil into fractions according to boiling point.
• Monomer and polymer: A monomer is a small molecule that joins with others; a polymer is the large molecule formed from many repeating monomer units.
• Saturated and unsaturated: Saturated compounds contain only single carbon–carbon bonds; unsaturated compounds contain one or more carbon–carbon multiple bonds.
• Ethanol and ethanoic acid: Ethanol is an alcohol with the formula C2H5OH; ethanoic acid is a carboxylic acid with the formula CH3COOH and can form when ethanol is oxidised.
• Viscosity and volatility with chain length: Increasing hydrocarbon chain length generally increases viscosity and boiling point but decreases volatility and ease of ignition.

What Gets Asked

• State and apply the general formulae: alkanes are CnH2n+2, while alkenes with one carbon–carbon double bond are CnH2n. Marks are lost by confusing the two formulae.
• Identify members of the first four alkane and alkene series: methane, CH4; ethane, C2H6; propane, C3H8; butane, C4H10; ethene, C2H4; propene, C3H6; butene, C4H8; and pentene, C5H10.
• Use bromine water to test for unsaturation. The required observation is that orange bromine water becomes colourless in the presence of an alkene.
• Complete or interpret combustion equations, including CH4 + 2O2 → CO2 + 2H2O and C2H5OH + 3O2 → 2CO2 + 3H2O. Limited oxygen must not be described as complete combustion.
• Compare ethanol production by fermentation and hydration of ethene, including temperature, oxygen conditions, speed, purity, renewability and raw-material limitations.
• Write or identify reactions of carboxylic acids, including 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2, and distinguish the products of reactions with metals, bases and carbonates.
• Explain fractional distillation, cracking and addition polymerisation, including nCH2=CH2 → (–CH2–CH2–)n. Marks are lost by treating fractional distillation as a chemical reaction or omitting that cracking produces shorter alkanes and alkenes.
• Link hydrocarbon chain length to properties and environmental effects: longer chains generally have higher viscosity and boiling point but lower volatility and ease of ignition; fossil-fuel combustion releases carbon dioxide and may release carbon monoxide, sulfur dioxide and nitrogen oxides, while most addition polymers do not readily biodegrade.