Different types of alcohols C O H H H C H H C H H H C H H C H C H H H O H H C C H C H H H H H C H H O H H Propan-1-ol Primary Tertiary alcohols are alcohols where 3 carbon are attached to the carbon adjoining the oxygen Propan-2-ol Secondary methylpropan-2-ol Tertiary Primary alcohols are alcohols where 1 carbon is attached to the carbon adjoining the oxygen Secondary alcohols are alcohols where 2 carbon are attached to the carbon adjoining the oxygen. Oxidation reactions of the alcohols Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise. The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions. Partial Oxidation of Primary Alcohols Reaction: primary alcohol aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms: C O C H H H H An aldehyde’s name ends in –al It always has the C=O bond on the first carbon of the chain so it does not need Ethanal an extra number Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Write the oxidation equations in a simplified form using [O] which represents O from the oxidising agent When writing the formulae of aldehydes in a condensed way write CHO and not COH e.g.CH3CH2CHO N Goalby chemrevise.org 2 propan-1-ol propanal + [O] + H2O CH3CH2CH2OH + [O] CH3CH2CHO + H2O OH + [O] O + H2O C O C H H C H H H H C O H H H C H H C H H H Distillation In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid. Water in Water out Liebig condenser thermometer Heat Note the bulb of the thermometer should be at the T junction connecting to the condenser to measure the correct boiling point Note the water goes in the bottom of the condenser to go against gravity. This allows more efficient cooling and prevents back flow of water. It’s important to be able to draw and label this apparatus accurately. Don’t draw lines between flask, adaptor and condenser. Round bottomed flask N Goalby chemrevise.org 3 Full Oxidation of Primary Alcohols Reaction: primary alcohol carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished) C C O O H H C H H H H Propanoic acid propan-1-ol Propanoic acid + 2 [O] + H2O Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion CH3CH2CH2OH + 2[O] CH3CH2COOH + H2O C O H H H C H H C H H H C C O H O H C H H H H OH + 2[O] + H2O O OH Reflux Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated including the distillation set up Water in Water out Heat Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles It’s important to be able to draw and label this apparatus accurately. • Don’t draw lines between flask and condenser. • Don’t have top of condenser sealed • Condenser must have outer tube for water that is sealed at top and bottom • Condenser must have two openings for water in and out that are open Round bottomed flask condenser Oxidation of Secondary Alcohols Reaction: secondary alcohol ketone Reagent: potassium dichromate(VI) solution and dilute sulphuric acid. Conditions: heat under reflux C C C O H H H H H H Propanone Ketones end in -one When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one propan-2-ol Propanone C H H C H C H H H O H H C C C O H H H H H H + [O] + H2O There is no further oxidation of the ketone under these conditions. Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Tertiary alcohols cannot be oxidised at all by potassium dichromate: This is because there is no hydrogen atom bonded to the carbon with the OH group Distinguishing between Aldehydes and Ketones The fact that aldehydes can be further oxidised to carboxylic acids whereas ketones cannot be further oxidised is the chemical basis for two tests that are commoly used to distinguish between aldehydes and ketones Tollen’s Reagent Reagent: Tollen’s Reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3 )2 ]+ . Conditions: heat gently Reaction: aldehydes only are oxidised by Tollen’s reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no visible change CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+ Reagent: Fehling’s Solution containing blue Cu 2+ ions. Conditions: heat gently Reaction: aldehydes only are oxidised by Fehling’s solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide . Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react Fehling’s solution CH3CHO + 2Cu2+ + 2H2O CH3COOH + Cu2O + 4H+ The presence of a carboxylic acid can be tested by addition of sodium carbonate. It will fizz and produce carbon dioxide
22.214.171.124 Oxidation of alcohols
Alcohols are classified as primary, secondary and tertiary.
Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids.
Secondary alcohols can be oxidised to ketones. Tertiary alcohols are not easily oxidised.
Acidified potassium dichromate(VI) is a suitable oxidising agent.
Students should be able to:
• write equations for these oxidation reactions (equations showing [O] as oxidant are acceptable)
• explain how the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is obtained
• use chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.