Alcohols

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  • Alcohols
    • An alcohol is a compound which contains the functional group -OH, which is called an alcohol group
    • Physical Properties
      • Solubility in water: Alcohols with short carbon chains are very soluble in water. This is because the -OH group is able to form hydrogen bonds to water
        • However, alcohols with longer carbon chains are less soluble because the carbon chain part is non-polar and cannot form hydrogen bonds
      • Relatively low volatility. Alcohols are harder to vaporise than hydrocarbons with molecules of a similar size
        • To boil these compounds, intermolecular forces must be broken. Alcohols with shorter carbon chains can also form hydrogen bonds, which require more energy to break
    • Preparation of Alcohols
      • Hydration of Alkenes
        • Alkenes will react with steam in the presence of a phosphoric acid catalyst to form alcohols
          • This is an electrophilic addition. Because it is H2O that is added, it is often referred to as hydration
    • Uses and production of alcohol
      • Ethanol is an important chemical. Uses are;
        • In the form of alcoholic drinks
        • As a solvent, most commonly in an impure form called methylated spirit
        • As a biofuel
      • 1. Hydration of Ethene
        • This is the most important method of making industrial ethanol in the U.K as ethene can be obtained easily by cracking of petroleum
          • The reaction is: H2C=CH2 + H2O -> C2H5OH
            • Conditions: Ethene and steam are passed over a catalyst of phosphoric acid
              • Advantages
                • Ethene can be obtained easily by cracking of petroleum - so it is convenient in countries with a petrochemical industry
                • It can be operated as a continuous process, so it can be fuel and time efficient
                • Pure ethanol is easily obtained, rather than a dilute solution
              • Disadvantages
                • The petroleum needed is non renewable
                • If the ethanol produced is used as fuel, it will add new CO2 to the atmosphere, contributing to climate change
      • 2. Fermentation from sugars e.g. Glucose
        • Plant materials containing starch or sugars can be fermented into ethanol. This is how drinks are made, but it is also increasingly being used to produce industrial ethanol
          • The reaction is: C6H12O6 -> 2C2H5OH + 2CO2
            • The necessary conditions are
              • Yeast (which uses the sugars in anaerobic respiration)
              • Absence of air (in the presence of oxygen, the yeast respires aerobically, so little ethanol is produced)
              • Temperature of about 30-35 degrees centigrade (warm enough for yeast to grow)
              • Water (solvent, needed for yeast growth)
          • The fermentation process can continue until the solution contains about 15% of ethanol, but no more, because ethanol of this concentration kills the yeast
            • More concentrated ethanol can be obtained by distillation
          • Advantages
            • The sugars needed as a feedstock are a renewable resource as we can grow new plants
            • Ethanol produced this way is potentially a carbon-neutral fuel as the CO2 released when it burned was taken in from the atmosphere when the plant grew
          • Disadvantages
            • It produces a dilute aqueous solution, from which ethanol has to be distilled
            • It is a batch process: it has to be stopped when the ethanol concentration becomes too high for yeast, This is less efficielt
    • Uses of methanol
      • Used as an additive to petrol. It improved combustion, allowing the hydrocarbons to burn more smoothly
      • Used as a feedstock for the production of other industrially important chemicals e.g. Methanal, which is used in the production of plastics
    • Reactions of Alcohols
      • Combustion
        • Ethanol and other alcohols readily burn in air, forming carbon dioxide and water
      • Elimination of water to form an alkene
        • An elimination is a reaction in which atoms are lost from the starting molecule without being replaced by new ones
          • In these cases, since the atoms eliminated form H2O, the reaction is also sometimes called a dehydration
            • Elimination of water from an alcohol can be achieved by being heated with a concentrated acid, usually sulphuric acid or phosphoric acid
    • Esterification
      • An alcohol reacts with a carboxylic acid in the presence of concentrated sulphuric acid catalyst to form an ester
      • Carboxylic acids are compounds which contain the group -COOH
      • Esters: When carboxylic acids react with alcohols, the hydrogen of the -OH group of the carboxylic acid is replaced by the alkyl group from the alcohol
      • Properties and uses of esters
        • Low molecular weight esters are volatile, with a fruity smell.
          • They occur in fruits and other natural products and esters are used as artificial fruit flavourings and in perfumes. Some esters are also important as esters

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