Alcohols

• What is the functional group for an alcohol?

  -OH
• General formula of an alcohol

   CnH2n+1OH
• Shape of an alcohol

  • two lone pairs on oxygen
  • lone pairs repel more than bonding pairs
  • this makes the bond angle approx 105 rather than 109.5
• Classification of alcohols


  • primary - 1 alkyl group attached to carbon with OH group on it
  • secondary- 2 alkyl groups attached to carbon with OH group on it
  • tertiary -3 alkyl groups attached to carbon with OH group on it

  
• What are some of the physical properties of alcohols?

  • hydrogen bonding between the lone pair on the oxygen and δ+ H atoms
  • means that they have relatively high melting and boiling points compared to alkanes of a similar molecular mass
  • soluble in water as the OH group of an alcohol can bond to water molecules with hydrogen bonding
• What are the two ways to produce ethanol?

  1. Ethanol produced from crude oil

  • ethene is produced when crude oil is cracked
  • ethene is hydrated, which adds water across the double bond and forms ethanol
  • phosphoric acid used as a catalyst

  
• What are the two ways to produce ethanol?

  2. Fermentation

  • breakdown of sugar in a process called anaerobic respiration
  • the rate of this reaction is affected by temperature- it is slow at low temperatures but the enzymes are denatured if the temperature gets too high
  • air is kept out of the fermentation vessels to prevent oxidation of ethanol to ethanoic acid
  • an aqueous solution is obtained and so it must then be distilled to get the water off and produce a pure sample of ethanol
  • yeast is used as an enzyme

  
• How can ethene be obtained?

  • obtained from cracking of crude oil
  • also can be obtained from dehydration ethanol made from sugar
• Reactions of alcohols


  • undergo combustion reactions as long as there is enough oxygen available
  • ethanol is often used as a fuel
• Elimination of alcohols


  • cases where a small molecule leaves the alcohol, always water in this case
  • known as dehydrations
  • aluminium oxide can be used as a catalyst, with temperatures of 600K
  • phosphoric acid an also be used as a dehydrating agent

  
• Mechanism of dehydration


  e.g.

  
• Oxidation of alcohols

  • combustion is usually complete oxidation
  • primary alcohols ---> aldehydes--->carboxylic acids
  • secondary alcohols ---> ketones
  • tertiary alcohols cannot be further oxidised (this is because oxidation needs a C-C bond to break, not a C-H bond)
• Experimental details of oxidation

  • acidified potassium dichromate as the reagent
  • turns orange to green when oxidation occurs

  
• Alcohols + oxidation 


  • To go from ethanol to ethanal, potassium dichromate, and heated
  • To go from ethanol to ethanoic acid, conc sulfuric acid and more potassium dichromate needed