Return to Contents  Chapter 18: Enols and Enolates  Ch 18 contents
The Haloform reaction
haloform reaction of methyl ketones
Reaction type : Nucleophilic substitution

Summary

iodofrom test for methyl ketones
A yellow precipitate indicates a positive result in the iodoform test (centre tube).

 

QUESTION Could a methyl aldehyde undergo this reaction ? ANSWER

Related Reactions

MECHANISM OF THE HALOFORM REACTION OF METHYL KETONES

Step 1:
First, an acid-base reaction. Hydroxide functions as a base and removes the acidic α-hydrogen giving the enolate.
mechanism of the haloform reaction
Step 2:
The nucleophilic enolate reacts with the iodine giving the halogenated ketone and an iodide ion.

Step 3:
Steps 1 and 2 repeat twice more yielding the trihalogenated ketone.
Step 4:
The hydroxide now reacts as a nucleophile at the electrophilic carbonyl carbon, with the C=O becoming a C-O single bond and the oxygen is now anionic.
Step 5:
Reform the favourable C=O and displace a leaving group, the trihalomethyl system which is stabilised by the 3 halogens. This gives the carboxylic acid.
Step 6:
An acid-base reaction. The trihalomethyl anion is protonated by the carboxylic acid, giving the carboxylate and the haloform (trihalomethane).

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© Dr. Ian Hunt, Department of Chemistry, University of Calgary