aldehydes and ketones

• Preparation of a/k(oxidation of primary/secondary alcohol)

  Reagents: Potassium dichromate (VI), dilute H2SO4

  Conditions: [a, heat gently to 60degrees, distil off][k, heat under reflux, distil off]

  Observation: Orange --> green

  ROH + [O] --> RCHO/RCOR' + H2O

  Cr2O72- reduced to Cr3+
• HCN

  Reagents: 2NaCN + H2SO4 --> 2HCN(toxic gas made in situ)+NA2SO4

  Conditions: heat under reflux at pH 8 [HCN<->H+ + CN-, some base is added, if pH is too low there isn't enough CN- ions to form a bond with the delta+ carbon atom, equilibrium shifts to the left//if pH is too high not enough HCN molecules to regenerate the catalyst]

  Reaction type: nucleophilic addition

  
• 2,4-dinitrophenylhydrazine 


  Reaction type: Addition-Elimination

  {method}

  yellow(simple)/orange(aromatic) precipitate

  
• Oxidation of aldehyde

  Reagent: Heat in warm water bath with acidified Potassium dichromate (VI)solution

  Observation: Orange to green

  Cr2O72- + 14H+ + 6e- --> 2Cr3+ + 7H2O

  
• Tollens' reagent

  Oxidises aldehydes only

  Conditions: warm in water bath (clean test tube)

  Observations: colourless solution --> silver mirror

  Equation: Ag+ + e- --> Ag+
• Fehling's solution


  Only oxidises aldehydes

  Conditions: warm in water bath

  Observations: blue solution --> red precipitate

  Equation: Cu2+ + e- --> Cu+
• Reduction (lithal LiAlH4)


  Conditions: Heat under reflux, in dry ether, add dilute HCl

  Equation: A RCHO +2[H] --> PRIMARY ROH

  K RCOR' + 2[H] --> SECONDARY ROH

  reduces polar C=O, pi bonds