Oxidation of Alcohols - Primary Alcohols
Primary alcohols are easily oxsidised to aldehydes
Oxidation of Alcohols - Secondary Alcohols
Secondary alcohols are oxidised to ketones and no further
Tertiary alcohols are not easily oxidised due to C-C bond instead of C-H being broken
Ethanol ---> Ethanal or Ethanoic acid?
Use dilute acid and less potassium dicrhromate (IV) than is needed for oxidation to carboxylic acid
- Use excess alcohol so there isn't enough oxidising agent for further oxidation
- Ethanol vapourises and distils of as soon as it is formed to prevent further oxidation.
Use concentrated acid and excess dichromate ions to ensure full oxidation
- Heated under reflux
Alcohols can be used as intermediates in the chemical industry
Ethanol via crude oil
Ethanol is made by reacting ethene (from crude oil - naphtha fraction) with steam using phosphoric acid (H3PO4) as a catalyst
Ethanol via fermentation of sugars
Glucose ---> 2C2H5OH + 2CO2
- Compromise of 35 degrees is made
- Air kept out of fermentation vessels to prevent ethanol --> ethanoic acid
- Ethanol via fermentation is carbon-neutral: 6CO2 in via respiration, 2CO2 out via fermentation and 4CO2 out via combustion
Tests for aldehydes and ketones
- Aldehydes and ketones both have a C=O group which is called a carbonyl
- Aldehydes: RCHO
- Ketones: RCOR'
Tollens' (silver mirror) test
- Tollens' reagent (silver nitrate in aqueous ammonia) oxides aldehydes but not ketones. Silver ions are reduced from colourless to metallic if aldehyde
- Benedict's reagent contains blue Cu(II) ions which will oxidise aldehydes but not ketones. Blue solution changes to red if aldehyde
Dehydration of Alcohols
- OH group and a hydrogen from the next carbon is removed from the alcohol.
Elimination in the context of alcohols is always dehydration
- Alcohol vapour is passed over excess hot sulfuric acid/aluminium oxide
- Alkene is formed
- Dehydration of longer-chained alcohols may produce a mixture of alkenes including E-Z isomers