The oxidation of phenylmethanol to benzoic acid

Describe the appearance of phenylmethanol.

Phenylmethanol is a colourless, oily liquid.

Draw the structure of phenylmethanol.

What type of reactant is the phenylmethanol?

It is a limiting reactant. It determines how much product (benzoic acid) is formed.

What 3 substances are mixed in the conical flask in step 1?

Phenylmethanol, KMnO4 (oxidising agent), Na2CO3 (base-sodium carbonate).

Describe what happens after the substances are mixed. (describe step 2)

The conical flask is placed in a water bath and heated on a hot plate.

The mixture turns from purple to brown.

In step 2, why does the mixture turn purple from brown.

The Mn ions in the KMnO4.

They go from MN+7 ions (purple) to MN+4 ions (brown)

What happens to the phenylmethanol on heating at stage 2?

The phenylmethanol becomes partially oxidised to form sodium benzoate salt.

Draw the structure of sodium benzoate salt.

How do you cool the mixture down?

By running the conical flask under the cold tap.

What is added to the mixture in the fume cupboard?

HCL

Why is HCL added to the mixture? (3 reasons)

1. Provide H+ ions for the complete reduction of Mn+4 ions to Mn+2 ions.

2. To neutralise the excess sodium carbonate base.

3. To supply H+ ions for the conversion of the intermediate sodium benzoate into benzoic acid.

How do we test to ensure that enough acid has been added?

Use blue litmus paper. It should turn red.

In step 4, what reducing agent do we add?

sodium sulphite (Na2 SO3)

What does the reducing agent sodium sulphite do?

It reduces the manganese ions from Mn+4 to Mn+2

When the manganese ions are reduced from Mn+4 to Mn+2, what colour does the mixture become?

Brown to colourless

Describe benzoic acid’s solubility in water.

Yes, benzoic acid is soluble in water. However, its solubility decreases as the water temperature decreases.

Given that benzoic acid is less soluble in water at low temperatures, what will we do with our conical flask?

We place the conical flask in an ice bath to precipitate the benzoic acid out of solution. The benzoic acid appears as white crystals in the solution.

What two things do we with the filter paper before carrying out the filtration?

Take the mass of a dry piece of filter paper.

Fold the filter paper so that it is fluted.

What is the advantage of fluted filter paper?

It increases the surface area across which solution can pass and so speeds up the rate of filtration.

How can we dry the crystals?

Place the filter paper in a dessicator to remove all of the water from the crystals and the filter paper/ let crystals air dry overnight in the lab.

How to calculate mass?

Take mass of the filter paper and crystals. Subtract this mass from the mass of the dry filter paper.

Draw the structure of benzoic acid.

Another very important aspect of this experiment.

KNOW HOW TO DO CALCULATIONS.

What is the potassium permanganate solution saturated?

to ensure that sufficient is present to oxidise all the phenylmethanol completely to benzoic acid

What is the function of the base sodium carbonate?

To ensure that the solution is alkaline as this oxidation works best at an alkaline pH.

Why might you notice the smell of almonds?

Due to the formation of benzaldehyde.