Organic Chem : Oxidation of Phenylmethanol to Benzoic Acid

What is phenylmethanol also known as?

benzyl alcohol

What are the two stages involved in forming benzoic acid?

1. formation of sodium benzoate intermediate in alkaline conditions

2. formation of benzoic acid in acidic conditions

Give step 1

phenyl methanol added to conical flask

potassium manganate and sodium carbonate solutions added

reactants heated in water bath

conical flask cooled under cold running water

Give step 2

conc. hydrochloric acid added

sodium sulfite solution added

swirl until brown precipitate disappears

place in ice bath

white crystals observed

Give step 3

filtered using vacuum filtration using filter paper, Buchner flask and Buchner funnel.

conical flask rinsed and refiltered

crystals washed with cold water to remove soluble impurities

crystals air dried or dessicator

Describe the appearance of phenyl methanol at room temperature

colourless liquid

Name a suitable piece of apparatus to measure a precise small volume of the phenyl methanol

Graduated dropper

Why is a saturated solution of potassium permanganate used?

This ensures that this oxidising agent is in excess, there will be sufficient present to oxidise all of the phenyl methanol to benzoic acid

Why is sodium carbonate required?

To create alkaline conditions so hat the oxidation of the phenyl methanol works better and faster

What gives rise to a smell of almonds during the heating phase?

Formation of benzaldehyde

What is observed when the phenyl methanol, potassium manganate and sodium carbonate are heated?

Purple goes to brown precipitate

What gives rise to the brown precipitate observed during the heating stage?

Mn7+ ions in KMnO4 are reduced to Mn4+ ions in MnO2

What intermediate organic compound is formed?

sodium benzoate

Give 3 reasons why concentrated hydrochloric acid is added?

To convert the sodium benzoate intermediate to benzoic acid.

To neutralise any alkali still present.

To create an acidic environment for Mn4+ to be reduced to Mn2+

How can the solution be tested for that enough acid has been added?

Dip glass rod into solution, dab on blue litmus paper, Blue litmus turns red

Why is sodium sulfite added to the conical flask?

It's a reducing agent, reduced the insoluble Mn4+ to soluble Mn2+

What is observed upon the addition of sodium sulfite?

Brown precipitate dissolves to colourless/ decolourises

White crystals of benzoic acid become visible

Give the changes in oxidation number of manganese during the experiment.

Mn7+ >>> Mn4+ >>> Mn2+

+7 +4. +2

Why is the conical flask placed in an ice bath?

Benzoic acid is only slightly soluble in cold water.

This helps to ensure the crystallisation of benzoic acids complete and maximises the yield of crystals

How are the crystals isolated?

Vacuum filtration using filter paper, Buchner funnel and Buchner flask.

Conical flask rinsed with the filtrate from the Buchner flask and is re-filtered to ensure maximum yield of benzoic acid

Why is vacuum filtration preferred over gravity filtration in this experiment?

Speeds up filtration process

To help dry the crystals

Why are the crystals washed with cold water?

To remove any soluble impurities

How are the benzoic acid crystals further dried?

Air dry/ desiccator

Give the overall colour change in this experiment

colourless >>> purple >>> brown >>> colourless