6.3.8 - Chemical Properties of Alcohols

• reactivity of alcohols vs alkanes

  alcohols are more reactive than alkanes because the C-O and O-H bonds in alcohol molecules are more polar
• combustion of alcohols

  oxidation reaction

  produces carbon dioxide and water

  burns with a pale blue flame
• combustion of ethanol

  fuel by it's own right or in mixtures with petrol

  gasohol has 10-20% ethanol
• production of ethanol

  by fermentation which is a useful way for countries without an oil industry to reduce imports of petrol
• forming chloroalkanes

  substitution reaction where -OH is replaced by chloride

  done by reacting alcohol with phosphorous (V) chloride or with concentrated hydrochloric acid
• making chloroalkane using phosphorous (V) chloride

  ROH + PCl5 -> RCl + HCl + POCl3

  misty fumes of hydrogen chloride

  mainly done with primary alcohols, maybe secondary
• making chloroalkanes with concentrated hydrochloric acid

  ROH + HCl -> RCl + H2O

  done at room temperature

  only done with tertiary alcohols because reaction is fastest with them

  slowest reaction with primary alcohols so not suitable for synthesis of primary chloroalkanes
• making bromoalkanes

  substitution reaction where -OH is replaced by bromide

  conditions: acid catalyst - 50% concentrates sulfuric acid - room temperature
• stages in synthesis of bromoalkane

  1. 2KBr + H2SO4 -> HBr + K2SO4
  2. C2H9OH + HBr -> C4H9Br +H2O separate from water by distilling it
• making iodoalkanes

  substitution reaction where -OH is replaced by iodide

  made using phosphorous triiodide

  3ROH + PI3 -> 3RI + H3PO3

  HI isn't used because it's too unstable
• making phosphorous (III) iodide

  red phosphorous + iodine -> phosphorous (III) iodide

  made in situ (within reaction mixture) by refluxing alcohol with red phosphorus and iodine
• dehydration

  elimination reaction where -OH- and H+ are removed

  catalyst: phosphoric acid

  forms an alkene which may be 2 products due to stereoisomers
• why isn't sulfuric acid used as a catalyst in the dehydration of alcohols

  strong oxidising agent so oxidises some of the alcohol to CO2 and is reduced to SO2

  both would have to be removed
• what's the oxidising agent in the oxidation of alcohols

  potassium dichromate (VI) in dilute sulfuric acid
• oxidation of primary alcohols

  oxidised to aldehydes then to carboxylic acid
• how to get just the aldehyde in the oxidation of primary alcohols

  gently hear excess alcohol with oxidising agent in distillation apparatus so aldehyde is distilled off immediately because it has a lower boiling point than the alcohol
• how to get just the carboxylic acid in the oxidation of primary alcohols

  alcohol is mixed with excess oxidising agent and heated under reflux to ensure any volatile aldehyde condenses and flows back into the flask to be oxidised
• oxidation of secondary alcohols

  oxidised to a ketone

  not further oxidised because ketones aren't easily oxidised
• oxidation of tertiary alcohols

  not oxidised by potassium (VI) dichromate so solution stays orange

  oxidised by burning
• distinguishing between primary/secondary and tertiary alcohols

  react each alcohol with potassium (VI) dichromate in dilute sulfuric acid

  primary/secondary alcohols are oxidised so chromium ions are reduced from +6 to +3 so solution goes from orange to green

  tertiary alcohols aren't oxidised so no colour change as there's no reaction
• distinguishing between primary and secondary alcohols

  use benedict's solution: blue solution of complexed copper (II) ions dissolved in sodium carbonate

  aldehyde: blue copper (II) ions are reduced to a brick red precipitate of copper (I) oxide

  ketone: nothing happens as ketones aren't easily oxidised so solution stays blue
• general formula for aldehydes and ketones

  C(n)H(2n)O