Reaction of Dihydroxybenzene Isomers with Ferric Chloride

Demonstration Experiment on Video

Reaction of Dihydroxybenzene Isomers with Ferric Chloride

Objective: Distinction between the three Dihydroxbenzene Isomers

Peter Keusch

German version

Chemicals:
catechol
resorcinol
hydroquinone
FeCl₃ · 6 H₂0

Solution of the dihydroxybenzenes: 0.625 g of catechol, resorcinol and hydroquinone, respectively, are dissolved in 20 mL of dist. water. The solutions should be colorless; if needed a "spatula-tip" full of charcoal is added. After shaking the suspension is filtered.
Ferric chloride solution: 1 g FeCl₃ · 6 H₂0 are dissolved in 150 ml of dist. water.

Glass wares:
3 conical measures, graduated, 500 mL
3 glass stirring rods
beaker 200 mL
3 beakers 40 mL
3 snap-cap vials 20 mL
volumetric pipet 4 mL
volumetric pipet 10 mL
2 volumetric pipets 20 mL
1 pipette bulb
measuring cylinder 100 mL

Hazards and safety precautions:

Hydroquinone is probable carcinogen. Toxic. Severe skin irritant. Harmful by inhalation and ingestion. May cause sensitization. Eye and respiratory irritant.
Benzoquinone is highly toxic. May be fatal if swallowed, inhaled or absorbed through skin. Severe irritant.

Catechol and resorcinol are harmful if swallowed or inhaled and in contact with skin. Skin and respiratory irritant.

Safety glasses, protective gloves, suitable ventilation! The solutions of the dihydroxybenzenes should be prepared inside a fumehood!

Experimental procedure:

Three conical measures are set up as described in the following table.

conical measure 1	20 mL of catechol solution	380 mL of dist. water
conical measure 2	20 mL of resorcinol solution	380 mL of dist. water
conical measure 3	20 mL of hydroquinone solution	300 mL of dist water + 80 g of ice

The individual dihydroxybenzene solutions are mixed with aqueous FeCl₃. 4 mL of FeCl₃ solution are poured into the first conical measure, 10 mL into the second measure and 20 mL into the third measure.

Results:

When treated with aqueous FeCl₃, the aqueous dihydroxybenzene solutions will show the characteristic color change.

		Color of solution
conical measure 1	catechol	violet®smaragd green
conical measure 2	resorcinol	pale violet
conical measure 3	hydroquinone	deep blue®yellowish

Only for a couple of seconds a deep blue color can be seen in conical measure 3.

isomere Dihydroxybenzole

Video clip (Download RealPlayer .rm file)

Discussion:

Like phenol, catechol and resorcinol ban also form a colored complex with FeCl₃.

Hydroquinone is oxidized rapidly to p-benzoquinone, which does not generate a colored complex with FeCl₃.

equation

Redox equilibrium between hydroquinone and p-benzoquinone:

Resorcinol shows no redox reaction with Fe³⁺. The two hydroxy groups in the meta position can not form a quinoid system. Thus a redox reaction between resorcinol and Fe³⁺ is impeded.

Catechol is only partially oxidized to o-benzoquinone.

equation

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