2.10 Organic synthesis

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2.10 Organic synthesis Oxidation of alcohols
d. demonstrate an understanding of, and practise,
the preparation of an organic liquid (reflux and
distillation), eg oxidation of alcohols.

• Connector
• K2Cr2O7 is a mild oxidising agent oxidising
  agent.
• Balance the ionic half equation below and state
  whether the Cr2072- ion is being oxidised or
  reduced and justify your answer.
• Describe the colour change for the above
  reaction.
• Draw the skeletal structures of butan-1-ol,
  butan-2-ol and 2-methylpropan-2-ol,
  and their reaction products, if any, when they
  are heated separately with acidified potassium
  dichromate(VI) solution.
• The oxidation of propan-1-ol to propanal can be
  written as
• CH3CH2CH2OH O ? CH3CH2CHO H2O
• Write the corresponding equation for the complete
  oxidation of propan-1-ol. How would you carry out
  these two reactions?

Orange to green

• Crowe2013

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Acidified potassium dichromate(VI) and alcohols
O
O
K2Cr2O7/H
K2Cr2O7/H
1o
HEAT
HEAT
OH
OH
H
aldehyde
carboxylic acid
K2Cr2O7/H
2o
HEAT
O
OH
ketone
K2Cr2O7/H
3o
NO REACTION
HEAT
OH
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• CH3CH2CH2OH O ? CH3CH2CHO H2O
• Heat propan-1-ol with potassium dichromate (VI)
  and dilute H2SO4 and immediately distil off the
  product.
• CH3CH2CH2OH 2O ? CH3CH2COOH
  H2O
• Heat propan-1-ol with potassium dichromate (VI)
  and dilute H2SO4 under reflux. Then collect the
  propanoic acid by distillation.
• Draw labelled diagrams of the apparatus used for
  heating under reflux and distillation.
• Explain why each process is used, and describe
  what happens in each process.

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• The purpose is to thermally accelerate the
  reaction by conducting it at an elevated
  temperature (i.e. the solvent's boiling point.).
• The reaction will proceed at a constant
  temperature (i.e. the solvent's boiling point.).
• Any vapours given off are cooled back to liquid,
  and fall back into the reaction vessel
• Useful for performing chemical reactions under
  controlled conditions that require substantial
  time for completion.
• The advantage of this technique is that it can
  be left for a long period of time without the
  need to add more solvent or fear of the reaction
  vessel boiling dry as any vapour is immediately
  condensed in the condenser.

heat
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• Distillation is a physical separation process.
• Distillation is a method of separating mixtures
  based on differences in their volatilities in a
  boiling liquid mixture.

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• See exps 2.13 2.14 do reflux and dist parts

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Preparation of ethanal
3. Why is it necessary to close the tap of the
separating funnel when all of the sodium
dichromate(VI) and ethanol solution has been
added to the flask?
1. Explain why the collection container is
surrounded by a cooling mixture. 2. Explain why
it is important to add the sodium dichromate(VI)
and ethanol solution to the acid solution a few
drops at a time.
3. To prevent the ethanal vapour from escaping.
1. Prevents ethanal vapour escaping by condensing
it. 2. The exothermic reaction would be too
vigorous / hazardous / dangerous
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4. Write an equation to show the oxidation of
ethanol into ethanal using O to represent
the oxidizing agent.
CH3CH2OH O ? CH3CHO H2O
5. Write an equation to show the oxidation of
ethanol into ethanoic acid using O to
represent the oxidizing agent.
CH3CH2OH 2O ? CH3COOH H2O