Presentation Transcript
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.
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