Verification of phenoles with iron(III) chloride

Chemicals needed

catechol Xn    R 21/22-36/38    S (2)-22-26-37

distilled water

iron(III) chloride solution Xn    R 22-38-41    S 26-39

hydroquinone Xn    N    R 22-40-41-43-50     S (2)-26-36/37/39-61

1-naphthol Xn    R 21/22-37/38-41    S (2)-22-26-37/39

2-naphthol Xn    N    R 20/22-50     S (2)-24/25-61

phenol C T     R 24/25-34    S (1/2)-28-45

resorcin Xn    N    R 22-36/38-50     S (2)-26-61

Stehen nicht alle der oben genannten Phenole zur Verfügung, so kann man den Versuch entsprechend einschränken. Man kann es natürlich auch mit anderen Phenolen probieren (interessante Färbungen kann man aber im Allgemeinen nur von hinlänglich wasserlöslichen erhoffen, z.B. Salicylsäure oder Pyrogallol). Phenol ist besonders giftig, evtl. weglassen.

Equipment needed

test tube rack
pro analyte a test tube

Test procedure

• Safety goggles!
• One adds from each of the phenols which are to be analyzed a small spatula-tipful (from the napthols a very small spatula-tipful) into the test tubes.
• Dissolve each of the phenols with approx. 2 ml water.
• Add a couple of drops of iron(III) chloride solution.
• Phenol acquires immediately a red-violet color, catechol a deep violet, resorcin a deep green, hydroquinone becomes slowly yellow then brown and the naphtols produce voluminous deposits.

Disposal

• The solutions can be disposed of as halogene-free organic waste (the little bit of chloride doesn't count).

Elucidation

Phenol, catechol and resorcin form with iron(III) ions intensely colored charge-transfer complexes. Hydroquinone is oxidized slowly to brown benzoquinone. The naphthols form insoluble iron(III) salts.

Photos

From left to right: phenol, catechol, resorcin, hydroquinone, 1-naphthol, 2-naphthol

Literature

Extension of experiment 298, "Eisen(III)-chloridprobe auf aromatische Hydroxylverbindungen", p. 176, [1]