Triiodomethane Test

A2 Triiodomethane Test: 

A2 Triiodomethane Test Hydroxy and Carbonyl compounds only

Objectives:: 

Objectives: OH I deduce the presence of a CH3CH- group in an alcohol from its reaction with alkaline aqueous iodine to form tri-iodomethane O II describe the reaction of CH3C- compounds with alkaline aqueous iodine to give tri-iodomethane

Triiodomethane rxn: 

Triiodomethane rxn is a test for the existence of the CH3-CO- group in a molecule. E.g. part of an alcohol (C-O single bond) and part of a carbonyl compound (C=O double bond).

Hydroxy compounds: 

Hydroxy compounds Reagent : iodine and aqueous NaOH Condition : warm Product : iodoform, CHI3 Observation : yellow crystals of CHI3

The reaction: 

The reaction + 4I2 → + NaOH CHI3 + + 5HI Yellow crystals 2o alcohol

Steps in iodomethane for 2o alcohol: 

Steps in iodomethane for 2o alcohol 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 I2(aq) in a substitution reaction to produce a triiodoketone, for example triiodoacetone. CH3COCH3 + 3I2 → CH3COCI3 + 3HI

Slide7: 

(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-

Triiodomethane test is only for:: 

Triiodomethane test is only for: 1o alcohol – yellow crystals 2o alcohols - having a methyl group attached to the carbon with the -OH group. Not 3o alcohols – because 3o alcohols do not have a hydrogen atom attached to the carbon with the -OH group.

Carbonyl compounds: 

Carbonyl compounds Reagent : iodine and aqueous NaOH Condition : warm Product : iodoform, CHI3 Observation : yellow crystals of CHI3 insoluble in water

The reaction taken from www.chemguide.org.uk:: 

The reaction taken from www.chemguide.org.uk: Yellow crystals Methyl ketone

Triiodomethane test is for :: 

Triiodomethane test is for : Ethanal CH3CHO All methyl ketones.

Summary : 

Summary Hydroxy compounds: 1o alcohol – yellow crystals 2o alcohols with a methyl group attached to the carbon with the -OH group. 3o alcohols – negative result because 3o alcohols do not have a hydrogen atom attached to the carbon with the -OH group. Carbonyl compounds: 1. Ethanal CH3CHO 2. All methyl ketones.

Slide13: 

Go to Phenol