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Chemistry: Unit F322: Chains, Energy and Resources Alcohols Ethanol Alcohols are a homologous series with the functional group CnH2n+1OH. Alcohols contain the atoms hydrogen and oxygen, and because of the electronegativity of these atoms, alcohols are able to form hydrogen bonds. Other molecules of water are then attracted to it; the δ+ hydrogen of one molecule is attracted to the δ- oxygen of another molecule. Oxygen has a greater electronegativity than hydrogen; the bonding pair of electrons in the covalent bond is therefore pulled slightly towards the oxygen. A Hydrogen Bond is a strong intermolecular force of attraction formed between the lone pair of electrons on an electronegative oxygen (nitrogen or fluorine) and an electron deficient hydrogen atom. Miscibility with Water Miscibility is a measure of the property of liquids to mix to form a homogenous solution. Alcohols are miscible because some of the hydrogen bonds between the ethanol molecules and some of the hydrogen bonds between the water molecules are replaced by hydrogen bonds between the water and the alcohol. As chain length increases, the alcohol becomes less miscible. Hydrogen bonds between Ethanol and water. Only the –OH group of the alcohol can form hydrogen bonds, so as the hydrocarbon gets longer, less of the hydrogen bonds broken are replaced, and all that is created are weak Van der Waals’ forces. Most of the chain in the longer alcohol molecules is made up from the non-polar hydrocarbon tail. Because this cannot make any hydrogen bonds, the alcohol is less miscible. Boiling Points of Alcohols The boiling point of an alcohol is always much higher than the alkane of the same carbon chain length The boiling point of the alcohols increases as the carbon chain length increases. The longer the carbon chain, the less volatile the alcohol is. Alkanes have a lower boiling point than alcohols because they can only form Van der Waals forces, and these are much weaker than the hydrogen bonds which alcohols can form. Alcohols have hydrogen bonding, Van der Waals forces and dipole-dipole interactions. As the length of the carbon chain increases and there are more electrons, the strength of the temporary dipoles that are set up increases, so more energy is needed to overcome the forces, thus giving a higher boiling point. The preparation of Alcohols 1. Fermentation of Sugars For alcoholic drinks Raw material: Starch Equation: C6H12O6 2C2H5OH + 2CO2 Reaction Conditions: yeast (zymase enzyme), absence of oxygen, 25-37 degrees temperature Purity of product: Not pure – reprocessing by distillation is required Advantages: Renewable; no high temperatures required; biological catalyst Disadvantages: Not 100% atom economy; processing required to obtain pure alcohol 2. Hydration of Ethene [mechanism of electrophillic addition] Raw material: Ethene Equation: C2H4 + H2O C2H5OH Reaction Conditions: Phosphoric acid catalyst, 330 degrees, high pressure Purity of product: Pure Advantages: 100% atom economy – addition reaction; fast, continuous process Disadvantages: Requires Ethene from crude oil, so not renewable; chemical catalyst required; lots of fuel needed to create high pressure and high temperature. For industrial alcohol Homologous Series Alkenes Functional Group C=C Example Name Alcohols C – OH Propan-1-ol Aldehydes CHO Propanal Ketones C=O Propan-2-one Carboxylic Acid COOH Propanoic acid Esters COOC Ethyl Methanoate Propene Alcohols can be classified into primary, secondary and tertiary; Primary alcohols have one carbon attached to the C-OH Carboxylic Acids have the highest boiling point; alcohols the next, and Propan-1-ol Aldehydes the lowest. This is because carboxylic acids have hydrogen bonds, dipole-dipole interactions and Secondary alcohols have two carbons bonded to the C-OH Van der Waals’ forces. Alcohols have hydrogen bonding and Van der Butan-2-ol Waals’ and Aldehydes have only dipole-dipole and Van der Waals’ interaction. This means that the most Tertiary alcohols have three carbons bonded to the C-OH. energy is needed to overcome the 2 methylbutan-2-ol forces in the carboxylic acid, so this creates the highest boiling point. Reactions of Alcohols 1. Combustion Alcohols can be used as fuels; they are burned in combustion to produce energy. Ethanol is often used as a petrol substitute in countries with limited oil reserves. C2H5OH + 3O2 2CO2 + 3H2O 2. Elimination: Dehydration Alcohols can be dehydrated by the elimination of a water molecule, forming an alkene. Under reflux and hot concentrated sulphuric acid or by passing alcohol vapour over hot aluminium oxide, this reaction occurs; Ethanol Ethene + Water 3. Esterification Esters are made when an alcohol reacts with a carboxylic acid under a hot concentrated sulphuric acid catalyst, forming an ester and water. Ethanol CH3CH2OH Propanoic Acid + CH3CH2COOH Ethyl Propanoate [+ Water] CH3CH2COOCH2CH3 + H2O 4. Oxidation (of primary and secondary alcohols) A primary alcohol can be oxidised to form an Aldehyde, and then further oxidised to form a carboxylic acid. The reagents used are acidified potassium dichromate (K2Cr2O7) and concentrated sulphuric acid H2SO4 (source of H+ ions). If an oxidisation reaction occurs, the acidified potassium dichromate turns from orange to green. 1. Alcohol to Aldehyde 2. Aldehyde to Carboxylic Acid (under reflux: continuous heating of the reactant mixture, followed by evaporation and condensation of the mixture back into the flask) Complete oxidation of a Primary Alcohol Combining the two steps above to give one procedure which turns alcohols into carboxylic acids (with a by-product of water). Primary Alcohol + 2 [O] Carboxylic Acid + Water This is done under reflux; a continuous process with high temperatures. Oxidation of a Secondary Alcohol Secondary Alcohol + [O] Ketone + Water Reagents: Acidified Potassium Dichromate (K2Cr2O7) H2SO4 (source of H+) Reflux Colour change from orange to green. Propan-2-ol + [O] Propan-2-one + Oxidation of a Tertiary Alcohol Tertiary alcohols cannot be oxidised. There are no hydrogens attached to the carbon with the –OH functional group. This therefore means that no hydrogens can be lost in oxidation, so a tertiary alcohol cannot be oxidised. Water