Brown's GCE Chemistry
Advanced Level Organic Chemistry
GCE A Level Revision Notes PART 10
Summary of organic reaction mechanisms
A mechanistic introduction to organic chemistry and
explanations of different types of organic reactions
iodination of ketones
(substitution reaction, not a nucleophilic addition)
The organic synthesis of iodoketones from the reaction of ketones with
What is the mechanism for the
iodine substitution reaction of ketones?
Propanone readily forms
1–iodopropanone on reaction with acidified iodine solution, as do all 2–ones
('methyl ketones') I assume?
However, from experimental
observations, the rate
rate = k [ketone/propane] [acid
rate = k2[CH3COCH3(aq)][H+(aq)]
and iodine is not in
the rate expression but one of the products is!
reaction is zero order for iodine and it also zero order for bromine in
the similar bromination reaction.
This suggests there
is a slow rate determining step involving the ketone and the hydrogen ion in
the mechanism and what ever happens next e.g. involving the iodine, is
much faster. A proposed mechanism is shown below, where R = CH3
if it was propanone. Note that very little can be absolutely proved in
mechanistic detail and you will find variations of this diagram on the web.
Other examples of
iodine reacting with ketones (specifically '2-ones')
So, what is the
reaction mechanism for propanone reacting with iodine under acidic
keywords phrases: reaction conditions formula
intermediates organic chemistry reaction mechanisms
reaction of propanone and iodineCH3COCH3(aq) + I2(aq) ==> CH3COCH2I(aq) + H+(aq) + I–(aq) (CH3)2C=O + H3O+ <=>
(CH3)2C=O+H + H2O (CH3)2C=O+H <=> (CH3)2C+–OH (CH3)2C+–OH + H2O <=> CH3C(OH)=CH2
+ H3O+ CH3C(OH)=CH2 + I2 CH3C(=O+H)–CH2I + I– CH3C(=O+H)–CH2I + H2O <=>
CH3COCH2I + H3O+ (CH3)2C=O + H+ <=> (CH3)2C=O+H (CH3)2C=O+H <=> CH3C(OH)=CH2 +
I2 CH3COCH2ICH3C <=> (OH)=CH2 + H+
and Organic Synthesis INDEX