Alcohols
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Can you tell me about the reaction between ethanol, red phosphorus and iodine? Why is iodine added at intervals?
Ulex replies
This reaction is the iodine equivalent of treating an alcohol with phosphorus trichloride to produce a chloroalkane. Phosphorus tri-iodide is not readily available, so it is made in situ by reacting iodine with red phosphorus. The red form is used because its reactions are more controllable and slower than the very reactive white form.
I have to tell you that I have never done this reaction with ethanol - my preparation of choice has been 1-iodobutane from butan-1-ol. I should imagine that ethanol reacts more quickly so the point of adding the iodine a little at a time is to allow the reaction to subside between additions. It also enables you to tell when sufficient iodine has been added so that subsequent purification becomes less messy.
In the preparation of 1-iodobutane it is necessary, after the initial reaction has subsided, to reflux the mixture to ensure complete reaction. I would suppose this to be unnecessary with iodoethane.
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updated: 17 June 2007
