Alcohols

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions
• Alcohols are a homologous series of compounds that have the general formula CnH2n+1OH

  • differ by one -CH2 in the molecular formulae from one member to the next

  
• Ethanol (C2H5OH) is one of the most important alcohols

  • Ethanol can also be represented by its structural formula CH3CH2OH
• Ethanol is the type of alcohol found in alcoholic drinks such as wine and beer

  • It is also used as fuel for cars and as a solvent
• Alcohols burn in excess oxygen and produce CO2 and H2O
• There are two methods used to manufacture ethanol:

  • The hydration of ethene with steam
  • The fermentation of glucose
• Hydration of ethene

  • A mixture of ethene and steam is passed over a hot catalyst of phosphoric acid at a temperature of approximately 300 °C
  • The pressure used is 60 atmospheres (6000kPa)The gaseous ethanol is then condensed into a liquid for use
  • Yeast uses aqueous glucose for energy for breaking down to alcohol and carbon dioxide
• Ethene + steam → ethanol
• Fermentation of glucose (1)

  • Sugar or starch is dissolved in water and yeast is added
  • The mixture is then fermented between 25 and 35 °C with the absence of oxygen for a few days
  • Yeast contains enzymes that catalyse the break down of starch or sugar to glucoseI
  • If the temperature is too low the reaction rate will be too slow and if it is too high, the enzymes will become denatured
• Fermentation of glucose (2)

  • The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide
  • The yeast are killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again
  • This is the reason that ethanol production by fermentation is a batch process
• Yeast uses aqueous glucose for energy for breaking down to alcohol and carbon dioxide

  • requires 30C for the reaction
• Glucose (aq) + yeast → carbon dioxide + alcohol

  • C6H12O6 → 2CH3CH2OH + 2CO2
• In fermentation, air needs to be kept out otheriwse it could oxidize into ethanoic acid
• Fermentation is a batch process and takes time because of the enzyme reaction
• Pure ethanol is extracted via fractional distillation
• The type of sugar determines what type of drink it is

  • grapes → wine
  • hops -> beer
• • Comparing methods of ethanol production

  A) complex

  B) simple

  C) non-renewable

  D) renewable

  E) continuous

  F) constantly

  G) efficient

  H) catalyst

  I) batch

  J) mixed

  K) reaction vessel

  L) several days

  M) inefficient

  N) fast

  O) slow

  P) pure ethanol

  Q) dilute

  R) processing

  S) greenhouse gas

  T) burning

  U) fossil

  V) fuels

  W) high

  X) temperature

  Y) carbon dioxide

  Z) greenhouse gas

  [) high

  \) energy input and cost

  ]) low

  
• Uses of ethanol:

  • fuel for vehicles
  • solvent for paints, varnishes, perfumes
• Ethanol’s properties:

  • colourless liquid
  • neutral
  • volatile
  • conducts electricity
  • burns with a blue flame in air
• Complete combustion with ethanol:

  • Ethanol + oxygen → carbon dioxide + water
  • CH3CH2OH + 3O2 → 2CO2 + 3H2O
• Alcohol Comparision Practical

  • Set up a stand with a can of water dangling off it and a lit burner of each alcohol (ethanol, propanol etc.) underneath
  • Measure the starting mass of the alcohol burner
  • Measure the starting temperature of the water
  • Set a timer for around 3 minutes to allow the water to heat up
  • Compare the changes in mass and temperature to see which alcohol is a better fuel source
  • change in mass = mass of fuel burned
  • 100ml = 100g
• Fermentation uses normal pressure compared to hydration using high pressures
• Fermentation needs little energy while hydration of ethene needs a lot of energy
• Hydration of ethene uses ethene and steam to make ethanol
• Fermentation uses sugar from plant material to make ethanol
• What are the products of complete combustion?

  The products of complete combustion are carbon dioxide (CO2) and water (H2O).
• How to identify isomers 


  To identify isomers you should look at moving the functional group to another carbon, e.g. butan-2-ol, or seeing if you break and branch the chain, e.g. 2-methylpropan-2-ol
• Why is the rate of fermentation fast at the start and eventually decreases?


  The rate increases initially as there is more yeast present and there's an increase in temperature. The rate eventually decreases because it runs out of glucose so there is not enough energy for the process. The rate also decreases as it produces enough alcohol to kill the yeast, hindering the rate. 
• What are the products of fermentation?

  1. solution of ethanol
  2. carbon dioxide
• What happens when alcohols and water add together?

  When alcohols are added to water, they dissolve to give neutral solutions