Chemistry Exp. To Oxidise Phenylmethanol

How is Benzoic acid prepared in the lab?
Oxidising phenylmethanol using potassium permanganate solution in the *presence of sodium carbonate*
Equation for this reaction
And explaining of colour change observed
3C₆H₅CH₂OH + 4KMnO₄ (purple) → 3C₆H₅COOH + 4MnO₂(brown) + H₂O + 4KOH
In what apparatus is the phenylmethanol reacted with potassium manganate
Phenylmethanol = known mass/vol.
Potassium permanganate = saturated
Sodium carbonate = solution
In conical flask, in boiling water bath
Why is the potassium permanganate solution saturated?
To ensure sufficient is present to oxidise all of the phenylmethanol
What is the function of the sodium carbonate?
Ensures pH stays at an alkaline level, as this oxidation reaction works best in alkaline conditions
What further steps are taking in isolating the benzoic acid?
Conical flask taken out of water bath when brown, run under tap to cool.
Placed in fume cupboard, HCl added
Dip glass rod in solution, then on litmus paper to make sure environment is acidic
Sodium sulphite solution added until brown precipitate disappears
Placed in ice
Crystals formed filtered in Buchner flask
Conical flask rinsed out with solution in Buchner
Crystals washed with ice-cold water
Crystals left dry overnight
Weight taken
Why is HCl added?
To convert the intermediate compound sodium benzoate to benzoic acid
Why is the flask cooled?
Benzoic acid is only slightly soluble in cold water, causes benzoic acid to be precipitated out
Use of HCl during cooling process
Neutralises any excess sodium carbonate added and potassium hydroxide produced in reaction
Provides an acidic medium for Mn⁴⁺ ions to be reduced
Mn⁴⁺ + 2e⁻ → Mn²⁺ (in the presence of H⁺)
Chemical formula for sodium sulfite
Why is sodium sulphite added to the conical flask once crystals are formed?
Reducing agent; Reduces Mn⁴⁺ + 2e⁻ → Mn²⁺
Mn⁴⁺ is insoluble, and would contaminate the benzoic acid, so reduced to Mn²⁺ which is soluble
Half reaction for conversion of sulphite to sulfate
SO₃²⁻ + H₂O → SO₄²⁻ + 2H⁺ + 2e⁻
(provides electrons and protons, I think)
Why is the conical flask placed in ice?
Benzoic acid low solubility in water => maximise yield of crstals
2 benefits of suction filtration? (using a Buchner flask)
Dries crystals
Speeds up filtration process
Why is the conical flask rinsed with the filtrate from the Buchner flask?
To get rid of any remaining crystals
Why are the crystals washed in ice-cold water?
Remove any soluble impurities

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