Iodoform Reaction - Test for the alpha-Methyl Carbonyl Group

Demonstration Experiment on Video

Iodoform Reaction

Objectives: Nucleophilic Carbonyl alpha-Substitution, Test for the alpha-Methyl Carbonyl Group

Peter Keusch

German version

Chemicals:

potassium iodide

iodine

ethanol 95%

2-propanol

acetone

2 N NaOH

Preparation of a 0.2 M iodine solution:
Using an Erlenmeyer flask, 37.35 g of potassium iodide are dissolved in 375 mL of dist. water. After the addition of 19.05 g of iodine the mixture must be stirred until it is homogeneous.

Apparatus and glass wares:
	hot plate
	2 thermometers
	3 conical measures, graduated, 500 mL
	3 glass stirring rods
	Erlenmeyer flask 500 mL
	3 beakers 200 mL
	1 beaker 100 mL
	2 pipettes 5 mL, graduated in 0.1 mL
	1 pipette bulb
	2 snap-cap vials 20 mL
	2 measuring cylinders 200 mL

Hazards and safety precautions:

Iodine is toxic - may be fatal if swallowed or inhaled. Harmful by inhalation and through skin absorption. Readily absorbed through skin. Very destructive of mucous membranes and upper respiratory tract, eyes and skin. Severe irritant.

Acetone, ethanol and 2-propanol are highly flammable.

Iodoform is harmful by ingestion, inhalation and through skin contact. Eye, skin and respiratory irritant.

Safety glasses, nitrile gloves and effective ventilation required. The 0.2 M iodine solution should be prepared inside a fumehood!

Experimental procedure:

conical measure 1: 200 ml of 0.2 M iodine solution, warmed up to 40°C

conical measure 2: 200 ml of 0.1 M iodine solution

conical measure 3: 200 ml of 0.1 M iodine solution

Iodoform test on ethanol:
60 mL of ethanol are added to the iodine solution in conical measure 1. Afterwards 150 mL of 2 N NaOH (warmed up to 40°C) are added while stirring.

Iodoform test on 2-propanol:
The iodine solution in conical measure 2 is mixed with 3.8 mL of 2-propanol and 150 mL of 2 N NaOH (room temperature).

Iodoform probe on acetone:
3.7 mL of acetone and 150 mL of 2 N NaOH (room temperature) are added to the iodine solution in conical measure 3.

Results:

A yellow, crystalline precipitate is formed in each of the three conical measures.

Video clip (Download RealPlayer .rm file)

Discussion:

The iodoform reaction is characteristic for methylketones as well as for alcohols (e.g. ethanol, 2-propanol), that can be oxidized to a methyl carbonyl compounds. The iodoform test is a test for the existence of the CH₃-CO group in a molecule. The group to which the CH₃-CO group is attached can be aryl, alkyl and hydrogen.

Both ethanol and 2-propanol are oxidized by iodine to give ethanale or acetone. (1).

equation 1

When a-methyl carbonyl compounds react with iodine in the presence of a base, the hydrogen atoms on the carbon adjacent to the carbonyl group (a hydrogens) are subsituted by iodine to form tri iodo methyl carbonyl compounds which react with OH ^- to produce iodoform and carboxylic acid (2):

Reaction mechanism:

The hydrogen atoms on the methyl group are slightly acidic and can be removed with hydroxide. The carbanion formed then react with iodine molecules to give a iodide ion and a monoiodonated methyl carbonyl derivate. Introduction of the first iodine atom (owing to its electronegativity) makes the remaining hydrogens of the methyl group more acidic. Hence a base-catalized iodination of a monohalogenated methyl carbonyl derivate occurs at the carbon that is already substituted. Finally a tri iodo methyl carbonyl derivate is formed.

equation 3

The next step is a nucleophilic attack by hydroxide on the carbonyl carbon atom. A carbon-carbon bond cleavage occurs and a triiodomethanide ion departs. The triiodomethanide ion is unusually stable. Its negative charge is dispersed by the three negative iodine atoms. In the last step a proton transfer takes place between carboxylic acid and triiodomethanide ion to form ultimately carboxylate ion and iodoform.

equation 4

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