3.3.5 Alcohols

• The general formula of alcohols is CnH2n+1OH
• Alcohols are polar molecules due to the presence of hydrogen bonding as the oxygen is more electronegative than hydrogen in the hydroxyl group.
• The hydrogen bonding in alcohols makes them polar and therefore soluble in water.
• The strength of polarity in alcohols reduces as the alkyl chain gets longer.
• Alcohols can combust in the presence of oxygen to release carbon dioxide and water. Ethanol can be used as car fuel to reduce the use of fossil fuels.
• Primary alcohols have 1 R-group and 2 hydrogen atoms bonded to the carbon atom bonded to the -OH group e.g. propan-1-ol.

  
• Secondary alcohols have 2 R-groups and 1 hydrogen atom bonded to the carbon atom bonded to the -OH group e.g. butan-2-ol.

  
• Tertiary alcohols have 3 R-groups and 0 hydrogen atoms bonded to the carbon atom bonded to the -OH group e.g. 2-methylpropan-2-ol

  
• Biofuels are fuels made from renewable, biological sources and they are said to be carbon-neutral because it assumes there is no net carbon dioxide emissions.
• Ethanol can be produced using either hydration or fermentation.
• The conditions for hydration include high temperatures, high pressure and a phosphoric acid catalyst.
• The advantages of hydration to produce ethanol is that it produces ethanol quickly, it is a continuous process and produces a virtually pure product.
• The disadvantages of hydration to produce ethanol is that the reactant ethene is non-renewable (comes from cracking of crude oil fractions) and it is expensive due to high energy costs.
• The advantages of fermentation to produce ethanol is that sugar used is renewable and it is carbon neutral in theory.
• The disadvantages of using fermentation to produce ethanol is that it is a batch process so there a high production costs, ethanol produced isn’t pure and land used to grow crops for fermentation could be used to grow food crops which could lead to food shortages.
• The equation for fermentation is C6H12O6 → 2C2H6O + 2CO2
• The hydration of ethene mechanism is via an electrophilic addition.

  • The acid catalyst acts first to add to the alkene.
  • The lone pair on the oxygen then attacks the carbocation intermediate, forming a coordinate bond.
  • Finally, a proton (H+) is lost to give an alcohol and to reform the acid catalyst.

  
• Alcohols can undergo dehydration (elimination) reactions to form an alkene and a water molecule.

  
• Alcohols can be dehydrated using a hot concentrated acid catalyst.

  
• Alkenes produced by dehydration reactions of alcohols are produced from renewable resources. They can be used to produced polymers without using monomers from crude oil
• If the OH group is part of an asymmetric secondary alcohol, then during the dehydration two potential isomers can be formed depending on which side of the OH group the Hydrogen is removed from. 
• When you oxidise a secondary alcohol, the molecule you get is called a ketone. 

  • Ketones have a carbon double bonded to an oxygen atom, and two other single carbon-carbon bonds.

  
• The conditions of oxidation of an alcohol include heat and acidified potassium dichromate as an oxidsing agent. It is represented in equations as [O]. Each [O] is equal to one oxygen atom.
• You can oxidise a primary alcohol to an aldehyde.

  • An aldehyde has a carbon-oxygen double bond and at least one carbon-hydrogen bond.

  
• Aldehydes can be oxidised to form carboxylic acids but ketones cannot.

  
• Aldehydes and ketones both contain the carbonyl group C=O. 

  • In aldehydes the carbonyl group is at the end of the carbon chain. 
  • In ketones the carbonyl group is in the middle of the carbon chain. 
• Tertiary alcohols cannot be oxidised at all because this would have to involve the breaking of a C-C bond which does not happen under these conditions.
• In distillation, the condenser is positioned horizontally while in reflux the condenser is positioned vertically.
• A water bath is used to heat alcohols not bunsen burners (open flames) because alcohols are flammable.
• Anti-bumping granules are added when we are heating alcohols to prevent the formation of bubbles and help it heat evenly.
• The conditions for fermentation are temperature of around 35 °C, addition of yeast and anaerobic conditions.
• The ethanol produced by fermentation of glucose may be regarded as a carbon-neutral fuel.

  Justify this statement.

  Include the relevant chemical equations in your answer.

  • 6CO2 + 6H2O → C6H12O6 + 6O2
  • C6H12O6 → 2C2H5OH + 2CO2
  • 2C2H5OH + 6O2 → 4CO2 + 6H2O

  No net emission of carbon dioxide as formation of C6H12O6 takes in 6CO2 and fermentation and combustion of ethanol together give out 6CO2
• Describe what happens when a reaction mixture is refluxed and why it is necessary for complete oxidation from an alcohol (ethanol) to a carboxylic acid (ethanoic acid) (3)


  • A mixture of liquids is heated to boiling point for a prolonged time
  • Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture
  • Any ethanal and ethanol that initially evaporates can then be oxidised