Oxidation Reactions Primary alcohol Tertiary alcohols Secondary alcohol aldehydes carboxylic acid ketones do not oxidise Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols and aldehydes to oxidise. Key point: Aldehydes can be oxidised to carboxylic acids, but ketones cannot be oxidised. Reaction: aldehyde carboxylic acid Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: heat under reflux Oxidation of Aldehydes RCHO + [O] RCO2H C + [O] O C H H C H H H H C C O H O H C H H H H Full Equation for oxidation 3CH3CHO + Cr2O7 2- + 8H+ 3 CH3CO2H + 4H2O + 2Cr3+ Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Aldehydes can also be oxidised Tollen’s Reagent. This are used as a test for the presence of aldehyde groups Tollen’s Reagent CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+ 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 change.
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6.1.2 Carbonyl compounds
Reactions of carbonyl compounds (a) oxidation of aldehydes using Cr2O7 2–/H+ (i.e. K2Cr2O7/H2SO4) to form carboxylic acids In equations for organic redox reactions, [O] and [H] should be used.
6.1.2 carbonyls Page 1
Oxford Textbook Pages : 456 - 457
CGP Revision Guide Pages : 180