3.5 Alcohols

• Making ethanol

  • ethanol can be made through fermentation of carbohydrates or reaction of ethene with steam
  • hydration of ethene is faster, gives purer ethanol and is a continuous process
  • fermentation uses renewable raw materials, takes place at lower temperatures and pressures and has lower labour costs
• Fermentation of carbohydrates

  • equation: C6H12O6 -> 2 C2H5OH + 2 CO2
  • conditions:
  • temperature - 35 degrees
  • pressure - normal
  • catalyst -enzymes from yeast
  • other - aqueous, anaerobic
  • materials: glucose, renewable
  • type of process: batch process
  • reaction rate : slow
  • purity : low
  • yield: 15%, fractionally distilled
• Hydration of ethene with steam

  • Equation: C2H4 + H2O <-> C2H5OH
  • conditions:
  • temperature - 300 degrees
  • pressure - 6500kPa
  • catalyst - concentrated H3PO4
  • raw materials: ethene, non renewable
  • process: continuous
  • reaction rate: fast
  • purity: high
  • yield: initially 5%, but recycling leads to 95% conversion
• Biofuels

  • biofuels are any kind of fuel derived or produced from a renewable biological source
  • most common are ethanol and biodiesel, which are made from crops including corn, sugarcane or rapeseed
  • bio-ethanol is usually mixed with petrol, biodiesel is used on its own or in a mixture
• Alcohol classification

  • similar to how carbocation intermediates are classified
  • depends on how many carbons surround the carbon that has the -OH functional group
  • $1^{\circ},2^{\circ},3^{\circ}$
• Oxidation of alcohols

  • oxidising agent: K2CR2O7, acidified
  • $Cr_2O_7^+$$\ \rightarrow\ Cr_2O_7$
  • colour change orange to green when oxidised
  • primary and secondary alcohols oxidise, but tertiary alcohols do not
• primary alcohols

  • primary alcohol +[O] -> (distill) aldehyde + water
  • aldehyde + [O] -> (reflux) carboxylic acid + water
  • heated under reflux with excess of [O]
  • allows an increase in rate and prevents the loss of volatile reactants and products
• secondary alcohols

  • only oxidised to ketones
  • alcohol + [O] -> ketone + water
  • there are no further oxidation’s as there are no more C-H bonds that an 0 can slot into, so no further oxidation
  • ketones, when being named, get the lowest number
• tertiary alcohols

  • can‘t be oxidised, because there is no H to be removed by oxygen on the carbon adjacent to the -OH functional group
• distinguishing

  • aldehydes and ketones are functional group isomers so need to use solutions to distinguish between the two
  • to distinguish, you can:
  • heat solution with oxidising agent
  • if solution is an aldehyde, a colour change will occur
  • with Fehling’s solution:
  • blue -> brick red
  • with acidified K2Cr2O7
  • orange -> green
  • Tollen’s reagent
  • silver mirror effect occurs
• Fehling’s solution

  • the method follows:
  • In a test tube, mix 1 pipette of Fehling’s A and 1 pipette of Fehling’s B
  • Add 7 drops of [aldehyde] and place in a beaker of hot water
  • Repeat for [ketone]
• Tollen’s Reagent

  • the method is as follows:
  • In a clean test tube, add 1 pipette of AgNO3 and 3 drops of NaOH
  • Add ammonia drop by drop until the precipitate dissolves
  • Add 2 drops of [aldehyde] and place in a beaker of hot water, leave for 5 minutes
  • Repeat for[ketone]
• Elimination

  • this is a dehydration reaction
  • alcohol -> alkene + water
  • reagent: hot, concentrated sulphuric acid
  • this can supply alkene monomers for polymerisation from a raw renewable material
  • e.g. sugar -> ethanol -> ethene -> poly(ethene)
  • the OH group and a hydrogen from the adjacent carbon are removed to create the double bond