Phenolphthalein is an extremely useful acid-base indicator and medically it
used to be used as a laxative.
For this experiment I used: 2 grams of phenol, 1.5 grams of phtalic anhydride.
And, I think having about 50 milliliters of DCM would be good
As a side note, in previous videos I've shown how all three of these chemicals
can be obtained from household products.
Two grams phenol was added to a 25 millimeter round bottom flask.
This was followed by the addition of 1.5 grams of phthalic anhydride.
Then, using a beaker, a few drops of concentrated sulfuric acid was added.
The mixture is then heated to 150 °C for 2 hours.
As the heat increases, the phenol should melt and the mixture should liquify.
At this point, you can already see the red color of phenolphthalein forming.
The reaction occurring is shown above.
Two molecules of phenol react with one molecule of
phthalic anhydride to form the phenolphthalein molecule and water.
Because this takes place at above 100°C the reaction is
pushed forward by boiling the water away.
As the reaction proceeds, it goes from a cloudy red to a clearer red,
and then finally it turns nearly black.
After two hours, the flask was removed from the oil bath and allowed to cool to room temperature
Once it had cooled, 10 milliliters of distilled water was added.
This was followed by 10 milliliters of dichloromethane.
After the addition of both water and the dichloromethane we have an interesting
three-layered mixture.
The mixture is then stirred rapidly.
However, before stirring, it was necessary to break up the solid at the bottom
using a glass stir rod because it was trapping the stir bar.
When the stirring stoped, it quickly reverted to a two phase mixture.
This mixture is then transferred to a small separatory funnel.
The round bottom flask was washed with a couple milliliters of DCM.
The contents are mixed thoroughly and then the layers were allowed to separate.
The lower layer containing our desired product phenolphthalein is drained into a beaker.
Then, 10 more milliliters of DCM is added to the aqueous layer in the separatory funnel.
Again the separatory funnel was capped, shaken and vented and then the
layers were allowed to separate.
This time, the lower DCM layer is yellow and
not a dark red, indicating that there is much less phenolphthalein present.
The lower DCM layer is combined with the previously drained DCM.
The aqueous layer-- which contains mostly unreacted phenol and sulfuric acid-- is disposed of
The dichloromethane extract is poured back into the separatory
funnel and the beakers washed twice with a small amount of DCM.
To this was added 5 milliliters of 2 molar sodium hydroxide.
In this step we're converting phenolphthalein into a water soluble
salt which has its characteristic purple color.
The separatory funnel is capped and shaken vigorously to make sure that we
react as much of the phenolphthalein as possible.
Afterwards 10 milliliters of water is added to make sure we dissolve as much
of the salt as possible in the water layer.
And for one final time, the separatory funnel is capped, shaken
and vented, and then the layers are allowed to separate.
Even after leaving it for a while, the layers will be very hard to see but they are there.
The lower layer-- which contains some phenolphthalein but mostly unreacted
phthalic anhydride-- is drained.
I did try washing this DCM layer once with water but the yield of phenolphthalein
from that is very low and it's not really worth the time.
The upper layer-- which contains a high concentration of phenolphthalein-- is
drained into a beaker.
The separatory funnel is washed with a little bit of water and also drained into the beaker.
This solution is very concentrated in phenolphthalein and its almost black.
The phenolphthalein solution is then poured into a hundred milliliters of two
molar hydrochloric acid.
This converts phenolphthalein back to its non-salt form which is insoluble in
water and causes it to crash out.
This was then vacuum filtered and then dried under vacuum.
After letting it dry overnight I'm not sure why but the phenolphthalein turned brown.
Shown in this video is only about 0.1 grams but the final yield was
actually around 1 gram.
So, in theory, you could expect about ten times the amount that you see here.
Now to test it, a couple small grains of the phenolphthalein was added to a basic solution.
And immediately upon the addition the solution quickly turns purple.