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The scheme below shows redox reactions of some organic compounds - Leaving Cert Chemistry - Question 8 - 2017
Question 8
The scheme below shows redox reactions of some organic compounds. 1. KmNO₄ / OH⁻ 2. HCl 3. Na₂SO₄ When phenylmethanol is converted to benzoic acid under the co... show full transcript
Worked Solution & Example Answer:The scheme below shows redox reactions of some organic compounds - Leaving Cert Chemistry - Question 8 - 2017
Step 1
What is the overall colour change that occurs in the reaction?
Answer
The overall colour change in the reaction when phenylmethanol is converted to benzoic acid involves a transition from a colourless state to a purple state, and ultimately to a brown state as the reaction proceeds. Initially, the colour of the potassium permanganate (KMnO₄) in an alkaline medium changes from purple (MnO₄⁻) to colourless/dark brown (MnO₂) during the oxidation of phenylmethanol.
Step 2
Explain this colour change by reference to the transition metal reagent used.
Answer
The transition metal reagent used in this reaction is potassium permanganate (KMnO₄). As KMnO₄ is reduced, the Mn(VII) in the permanganate ion is converted to Mn(II) or Mn(IV) in manganese dioxide (MnO₂), resulting in the characteristic colour change. The purple colour of MnO₄⁻ transitions to a colourless state as it gets reduced and ultimately leads to the brown precipitate of MnO₂.
Step 3
Copy the structures of phenylmethanol and benzoic acid into your answerbook and mark clearly the bond(s) involving carbon in this oxidation reaction.
Answer
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Structure of phenylmethanol:
- C₆H₅-CH₂OH
- The hydroxyl group (-OH) is attached to the phenyl ring. The carbon atom connected to the -OH indicates the carbon that is oxidized in the reaction.
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Structure of benzoic acid:
- C₆H₅-COOH
- The carbon atom of the carboxylic acid group (-COOH) indicates where the oxidation leads, this is the carbon oxidized from phenylmethanol.
Step 4
Step 5
Step 6
Suggest a weak oxidation reagent – a transition metal compound – that could be used to distinguish between a sample of A and a sample of B.
Answer
A suitable weak oxidation reagent for this purpose is Tollen's reagent, which will oxidize A (propanal) to form carboxylic acid while having little to no effect on B (propan-2-ol).
Step 7
Name an ester that is a structural isomer of propanoic acid.
Answer
An ester that is a structural isomer of propanoic acid is methyl ethanoate.
Identify the alcohol and the carboxylic acid used in the synthesis of this ester:
- Alcohol: Methanol (CH₃OH)
- Carboxylic acid: Ethanoic acid (CH₃COOH)