Things in preparation.
First of all measure out 35 ml of benzene.
Then place this into a 250 ml flask equipped with magnetic stirring.
Use ventilation and caution when handling benzene because it seems to act as a trigger
for some very unpleasant types of cancers.
Best to be on the safe side.
Now measure out 29 grams of solid iodine.
It doesn't matter if the iodine is very slightly damp with water but try to get it as dry as you can.
Add the solid iodine to the flask containing the benzene.
This will partially dissolve in the benzene forming a red color.
Now set up the flask on iodine.
Add the iodine to the hot plate and with a reflux condenser attached.
Get the mixture stirring vigorously and bring up the temperature until the mixture is just starting to reflux.
Now weigh out 46 grams of 68% zeotropic concentrated nitric acid.
The original preparation uses 100% nitric acid but we've found that their reaction works well with regular 68% nitric acid.
acid but just takes a little longer to react. Add a small portion of a few
mils of the acid through the top of the condenser into the flask. The reaction
will start slowly. You'll first see a slight brown color in the flask due to
the production of nitrogen dioxide gas. This will become more intense and the
liquid will start to reflux a little more. Soon you'll see quite a lot of
brown gas being produced. At this point be sure to have very good ventilation
going because these vapors are highly toxic.
At this point add some more acid to the flask through the condenser.
The reaction produces a small amount of heat so keep the acid addition going now
so as to keep
a nice steady reflux going on in the flask like this. Total addition takes
about 30 to 40 minutes. After this keep the flask refluxing on the hot plate for
another hour and a half. At the end of this time you should see the reaction
mixture change color like this indicating that all the iodine present
has been consumed. You can see two layers present in the mixture. The bottom is our
organic layer.
The mixture of benzene are product and some nitro compound impurities. Allow to
cool and then transfer into a separating funnel. Wash out the flask with a little
water and then allow the layers to separate completely. Remove the bottom
organic layer. In theory we could distill this mixture but there is a small amount
of nitro compound impurities.
Allow it to be on the safe side. We'll first get rid of these. Transfer into a 250 ml flask
equipped with a stir bar. Now measure out a couple of grams of tin metal granules.
Add these to the flask. The idea here is that we will reduce the nitro compounds
present into the corresponding aromatic amines. Now add 10 ml of water to the
flask and then 10 ml of concentrated hydrochloric acid. Get the stirring turned up and allow
this to react. There is only a small amount of impurity present so you shouldn't get a
big temperature increase. Allow this to mix thoroughly and react for one and a half hours.
At the end you might notice a subtle difference in the color but not much. A little bit of
the tin has reacted and looks tarnished.
Decant the liquid and pour into a separating funnel and wash the flask out with a little
water. In theory our aromatic amines should form salts with the hydrochloric acid and most
should remain in the aqueous layer, leaving us with mostly benzene and iodobenzene in the
bottom organic layer. Separate off the bottom layer into a small flask and add a small amount
of magnesium sulfate to dry it. Transfer into a round bottomed flask decanting off the magnesium sulfate and set up for simple distillation.
On heating you'll see the liquid in the flask start to boil and benzene starts to come off
around 80 degrees C.
Soon no more benzene is distilled and the temperature of the vapor rapidly rises up.
At this point we insulated the still head using some scrunched up aluminum foil.
And we started to collect the distillate coming off from 180 degrees C.
Distillation was pretty rapid and over in about 30 minutes.
The distillation was pretty rapid and over in about 30 minutes.
The distillation was pretty rapid and over in about 30 minutes.
There was a little black tar left in the boiling flask but thats all.
Here's our product.
21 ml. of a pale yellow distillate were collected.
This weighed 37.1 grams giving us a density which is
This weighed 37.1 grams giving us a density which is
within 3% of the literature value for iodo benzene.
This corresponds to an 81% yield from starting iodine.
This corresponds to an 81% yield from starting iodine.
This corresponds to an 81% yield from starting iodine.
The product is slightly yellow and this is probably due to a very slight iodine
impurity and shouldn't affect any further reactions. Iodobenzene is a
useful precursor to the phenylgrignard reagent, an important synthetic building
block.
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