More on alcohols

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  • More on alcohols
    • Alcohols are classified by counting how many other carbon atoms are bonded to the carbon that bears the -OH group
      • A primary alcohol is one in which the carbon atom joined to the OH group is also bonded to 1 other carbon atom
        • The general formula of a primary alcohol is RCH2OH
      • A secondary alcohol is one in which the carbon atom joined to the OH is also bonded to 2 other carbon atoms
        • The general formula of a secondary alcohol is R2CHOH
      • A tertiary alcohol is one in which the carbon atom joined to the OH is also bonded to three other carbon atoms
        • The general formula of a tertiary alcohol is R3COH
    • Oxidation and Reduction
      • Oxidation involves the gain of one or more oxygen atoms and/or the loss of one or more hydrogen atoms
      • Reduction involves the gain of one or more hydrogen atoms and/or the loss of one or more oxygen atoms
      • Oxidation of alcohols
        • The oxidising agent used to convert alcohols to other organic compounds is hot aqueous acidic potassium dichromate (K2Cr2O7)
        • The colour change when an alcohol is oxidised by hot aqueous acidic potassium dichromate is orange to green
        • The outcome of treatment with hot aqueous acidic potassium dichromate is different for primary, secondary and tertiary alcohols;
          • A primary alcohol is oxidised in two stages
            • First, it loses two hydrogen's to form a compound called an aldehyde
              • These reactions are performed by heating the alcohol briefly with aqueous acidic potassium dichromate
                • If the heating is carried out for a long time with excess oxidising agent, the aldehyde can be oxidised furthur to form an carboxylic acid
          • Secondary alcohols are oxidised to ketones, which are not oxidised furthur
            • Ketones are named with the root name indicating the structure of the carbon skeleton and the ending -one
          • Tertiary alcohols are not oxidised by hot, aqueous acidic potassium dichromate
    • Controlling the products of oxidation
      • Distillation
        • This method is used to make aldehydes from primary alcohols
          • It works by boiling the aldehyde away as soon as it is formed, and condensing it elsewhere, away from the oxidising agent so furthur oxidation does not occur
      • Refluxing
        • This method is used to make; carboxylic acids from primary alcohols and ketones from secondary alcohols
          • It involves a continuous cycle of boiling, condensing and return to the reaction flask
            • It ensures that the reaction can continue for a long time with the oxidising agent while avoiding losing material through evaporation
              • This helps ensure:
                • A good yield of product if the reaction is slow
                • Oxidation of a primary alcohol through both stages to the carboxylic acid
              • At the end of refluxing the product can be purified from the reaction mixture by distillation

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