Benzoic Acid Experiment Flashcards
Draw the structure of benzoic acid.
In manual.
Describe how you carry out the benzoic acid experiment.
Let a measured volume of benzyl methanol react with excess concentrated MnO4- to ensure the full oxidation of benzyl methanol to benzoic acid.
Make sure it is under alkaline conditions by adding Na2CO3 to make the conditions less severe and make the reaction faster.
Place in a waterbacth at >60C for 20 minutes to give time to react.
There will be an almond smell of benzaldehyde showing the formation of an intermediate aldehyde.
The solution will turn from purple to brown precipitate as MnO2 forms.
Cool under a tap.
Add concentrated HCL to:
Neutralise the excess Na2CO3 to form CO2.
Turns sodium benzoate to benzoic acid by replacing Na+ with H.
Supplies acid conditions for the reaction with sodium sulphite to produce soluble Mn2+.
Add concentrated sodium sulphite in fume hood to reduce brown precipitate MnO2 to Mn2+ which is both soluble and colourless.
A precipitate of white crystals of benzoic acid is formed as it cools.
Place benzoic acid in ice bath to speed up and maximise precipitation.
Vacuum filter benzoic acid and rinse with ice cold deionised water.
Dry crystals overnight on top of a radiator.
What is the formula for the production of benzoic acid.
3C6H5CH2OH + 4KMnO4 = 3C6H5COOH + 4MnO2 + H2O + 4KOH.
Why was a saturated solution of potassium manganate (VII) used in this reaction?
To ensure the complete oxidation of phenyl methanol.
Give two reasons why the solution was acidified with conc. hydrochloric acid.
Convert sodium benzoate to benzoic acid.
Neutralise the sodium carbonate.
Provide acid conditions so Mn(IV) can be reduced to Mn2+ .
To neutralise KOH formed in reaction.
How would you test the solution to find out if enough acid had been added?
Dip the glass rod into the solution and touch against the blue litmus paper. Should turn red.
What reaction occurred when the sodium sulphite was added?
Sodium sulphite reduced Mn4+ to Mn2+.
Why were alkaline conditions / sodium carbonate solution used?
To speed up the reaction.
To prevent the breakdown of the phenol part of the molecule.
What caused the smell of almonds as the reaction proceeded?
Benzaldehyde.
Where else may you encounter the smell of benzaldehyde?
Almond milk.
Almond Kernels.
What are the safety precautions needed for this experiment?
Cl2 is a poisonous gas which may be produced by the reaction of conc. HCL and KMnO4.
Conc. HCL is corrosive.
SO2 is a poisonous has which may be produced by the reaction of conc. HCL with Na2SO3.
How do you purify benzoic acid?
Recrystallisation.
Describe the procedure for the purification of benzoic acid by recrystallisation.
Dissolve in minimum of hot water to maximise the yield.
Filter in hot apparatus to remove insoluble impurities and prevent premature crystallisation.
Cool filtrate to recrystallise.
Filter again to trap pure crystals and remove soluble impurities.
Wash crystals with ice cold water to remove last traces of impurities.
Dry crystals in a desiccator or on top of a radiator.
How could you check if the crystals were pure?
Check their melting point against that of benzoic acid.
Melts over several degrees= impure.
Melts over 1C= pure.
Describe the appearance of phenyl methanol at room temperature.
Colourless/ pale yellow liquid.
