Iodoform reaction

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The iodoform test or iodoform reaction is a qualitative chemical test for the detection of ketones and aldehydes carrying an alpha methyl group. The reagents are iodine and sodium hydroxide.

Only methyl ketones, or alcohols with the feature:

CH₃CH(OH)-R

may undergo this reaction.

There are three steps to the iodoform reaction when starting from an alcohol. If the reaction starts from a methyl ketone, the first step is not needed.

• Addition of I₂(aq) (usually I₂ dissolved in KI, shown here reacting with 2-propanol)

2 NaOH + I₂ + CH₃CH(OH)CH₃ → 2 NaI + CH₃COCH₃ + 2 H₂O

oxidizes the hydroxyl group to a ketone,

• The ketone formed reacts with the I_2(aq) in a substitution reaction to produce a triiodoketone, for example triiodoacetone.

CH₃COCH₃ + 3I₂ → CH₃COCI₃ + 3HI

(Thus, ketone and aldehyde with structure -COCH₃ also show positive results). Ethanal is the only aldehyde to give the iodoform reaction.

• Finally, OH^- (aqueous) is added, which reacts with, for example, the triiodoacetone.

CH₃COCI₃ + OH^- → CHI₃ + CH₃COO^-

Iodoform, or triiodomethane (I₃CH) is a pale yellow substance with a relatively high molar mass due to the iodine atoms. It is therefore a solid at room temperature. It is insoluble in water and has an antiseptic smell. A visible precipitate of this compound will form from a sample only when a methyl ketone is present.

This type of reactivity conforms to the more general Haloform reaction.

[edit] External links

• Experimental procedure I
• Experimental procedure II
• Experimental procedure III