Alcohols
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Alcohols Alcohols are organic compounds that contain the hydroxyl functional group (-OH) attached to a carbon atom. The -OH group is what gives alcohols their characteristic properties. Alcohol An organic compound containing the hydroxyl functional group (-OH) attached to a carbon atom.
General Formula The general formula for alcohols is: C n H 2n+1 OH or R-OH where: n = number of carbon atoms R = alkyl group (hydrocarbon chain) -OH = hydroxyl group (functional group) Homologous Series Alcohols form a homologous series with: The same functional group (-OH) Similar chemical properties Gradual change in physical properties Each member differs from the next by CH₂ The First Six Alcohols Name Molecular Formula Structural Formula State at Room Temp Methanol CH₃OH CH₃-OH Liquid Ethanol C₂H₅OH CH₃-CH₂-OH Liquid Propan-1-ol C₃H₇OH CH₃-CH₂-CH₂-OH Liquid Butan-1-ol C₄H₉OH CH₃-CH₂-CH₂-CH₂-OH Liquid Pentan-1-ol C₅H₁₁OH CH₃-(CH₂)₃-CH₂-OH Liquid Hexan-1-ol C₆H₁₃OH CH₃-(CH₂)₄-CH₂-OH Liquid Naming Alcohols Alcohols are named by: Finding the longest carbon chain containing the -OH group Using the alkane name but changing the ending from -ane to -anol Adding a number to show the position of the -OH group if needed Examples: Meth-anol = 1 carbon + -OH Eth-anol = 2 carbons + -OH Propan-1-ol = 3 carbons with -OH on carbon 1 Propan-2-ol = 3 carbons with -OH on carbon 2 Physical Properties of Alcohols Boiling Points Alcohols have higher boiling points than alkanes with similar molecular mass Reason: -OH group allows hydrogen bonding between alcohol molecules Hydrogen bonds are stronger than van der Waals forces in alkanes More energy needed to break hydrogen bonds, so higher boiling point Boiling points increase as chain length increases Solubility in Water Short-chain alcohols (methanol, ethanol, propanol) are very soluble in water Reason: -OH group can form hydrogen bonds with water molecules Solubility decreases as chain length increases Reason: Longer hydrocarbon chain is non-polar and doesn't interact well with polar water Other Properties All the first few alcohols are liquids at room temperature Volatile (evaporate easily) - but less so than similar alkanes Pleasant smell (but methanol is toxic) Flammable - burn in air/oxygen Production of Ethanol Ethanol is the most important alcohol and can be made by two main methods: Method 1: Fermentation (Biological Method) Sugars are converted to ethanol by yeast enzymes in the absence of oxygen.
Conditions Sugar solution (from fruits, grains, or sugar cane) Yeast (contains enzymes that act as catalysts) Temperature: 25-35°C (optimum ~30°C) Anaerobic conditions (no oxygen/air) Time: several days Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (glucose → ethanol + carbon dioxide) Word Equation: Sugar (glucose) → Ethanol + Carbon dioxide Key Points: Uses renewable raw materials (plant sugars) Relatively slow process Maximum concentration ~15% ethanol (yeast dies above this) Needs distillation to concentrate the ethanol Used to make alcoholic drinks and biofuels Carbon neutral process (CO₂ released = CO₂ absorbed by plants) Method 2: Hydration of Ethene (Industrial/Synthetic Method) Ethene reacts with steam in an addition reaction to produce ethanol.
Conditions Ethene gas (from cracking crude oil) Steam (water vapor) Phosphoric acid (H₃PO₄) catalyst Temperature: 300°C Pressure: 60-70 atmospheres Equation: CH₂=CH₂ + H₂O → CH₃-CH₂OH (ethene + steam → ethanol) Key Points: Uses non-renewable raw material (ethene from crude oil) Fast, continuous process Produces pure, concentrated ethanol (100%) Used for industrial ethanol (not for drinks) High energy consumption Not carbon neutral (uses fossil fuels) Comparison of Methods Factor Fermentation Hydration of Ethene Raw materials Sugar (renewable) Ethene from crude oil (non-renewable) Rate of reaction Slow (days) Fast (continuous) Temperature Low (~30°C) High (300°C) Type of process Batch process Continuous process Purity of product Dilute (~15% max), needs distillation Pure (100%) Energy required Low High Use Alcoholic drinks, biofuel Industrial solvent, fuel Environmental Carbon neutral Not carbon neutral Chemical Properties of Alcohols 1.
Combustion (Burning) Alcohols burn in oxygen to produce carbon dioxide and water, releasing energy. Complete Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (ethanol + oxygen → carbon dioxide + water) Uses: Fuel for camping stoves (methylated spirits) Biofuel additive for petrol (gasohol) Spirit burners in laboratories Clean burning - produces no soot if enough oxygen 2.
Oxidation Alcohols can be oxidized by oxidizing agents to form carboxylic acids. Oxidizing Agents: Potassium dichromate(VI) (K₂Cr₂O₇) in dilute sulfuric acid Potassium manganate(VII) (KMnO₄) Oxygen in air (slow oxidation) Example - Ethanol to Ethanoic Acid: C₂H₅OH + 2[O] → CH₃COOH + H₂O (ethanol + oxygen → ethanoic acid + water) Observations Orange dichromate(VI) turns green (Cr³⁺) Purple manganate(VII) turns colorless Partial Oxidation (Controlled): Primary …
Interactive revision notes, videos and practice questions load below.