I wanted to make hydroquinone from p-DCB. At first glance, it seems simple; add two moles of NaOH and one mole of p-DCB and heat to get products of
2NaCl and hydroquinone. After doing a bit of research, I discovered that this was not the case. The chlorine atoms on the p-DCB are not reactive
enough to undergo this substitution. Something is needed to get the reaction going. I continued reading and found that DMSO (dimethylsulfoxide)
increases the reactivity of bases and could allow the substitution to take place.
Objective: Synthesize Hydroquinone from p-DCB
Materials:
147g p-DCB
80g NaOH
DMSO
Distilled Water
Procedure:
Make a solution of 75mL of DMSO in 350mL of water
Add the solution, p-DCB, and NaOH to the flask
Begin heating and mixing
Continue to heat and stir for 3 hours
Let the solution cool and filter/decant off the crystals
Wash the crystals with distilled water and add the liquid to the filtered liquid
Evaporate off the water until crystals start to appear, then let chill
Filter/Decant off the crystals
Recrystalize then dry the product
Theoretical Yield: 110g
Notes:
The solution was attempted to be kept at 100C for the duration of the reaction. If the temperature increased beyond 168C, the DMSO may start to
decompose and lead to an explosion.
The solution was originally in a 1L RBF, but had to be transferred to a tall-form 1L beaker because of inadequate mixing.
Before the solution was transferred, 3 distinct layers had formed. The top layer was assumed to be the aqueous NaOH layer, and the bottom layer
melted p-DCB. The middle layer is unknown. It could not have been the DMSO because there was not enough DMSO added to create that big of a layer.
After the duration of time had passed in the beaker, two layers formed. Upon cooling, the bottom layer froze into off-white crystals. The top
layer remained aqueous.
To speed up the evaporation, the liquid was transferred to a 1L Erlenmeyer flask and set up for vacuum distillation. (This has not been
completed)
Results:
It may be premature, but I do not think that much hydroquinone formed. This was indicated to me by the abundance of p-DCB crystals that formed when
the mixture cooled. I think the pressence of so much water hindered the reaction and a greater concentration of DMSO was needed to get a good yield. I
had planned to use pure DMSO not as a solution, but I read that, in the presence of basic or acidic conditions, DMSO may decompose at low temperatures
and could lead to an explosion. I didn't want to risk that.
--
What concentration do you think would be enough to warrant a good yield but keep the risk of an explosion at a minimum? halogen - 3-1-2017 at 16:02
Quote:
The chlorine atoms on the p-DCB are not reactive enough to undergo this substitution. Something is needed to get the reaction going.
That's simply not true, it's a matter of preference. The hydrogen ion will come off before the HO- sticks to the ring, whch would be necessary for
ipso (same place) substitution. The ring isn't sticky enough. It might be if you nitrate it, sulfonate, or further halogenate; hexachlorobenzene
probably can be sustituted in this way to hexachlorophenol although both are well known environmental pollutants. In these cases, the sulfonate and
nitro groups can themselves be targets of ipso substitution.
And of course the phenol that forms if one chlorine has been replaced would itself deprotonate and deactivate the ring towards a second reaction. So
of course there was no hope of your aim being reached. You must be mad to have thought there was. Read a book!
There are other simple routes to hydroquinone, in particular by oxidation of phenol and simple reduction by any one of a variety of reducing agents.
Electrical potential is, though I am not aware of these conditions, probably more suitable and convenient for both transformations than chemical
reagents.
[Edited on 4-1-2017 by halogen]AvBaeyer - 3-1-2017 at 16:23
The DMSO effect you are musing about only occurs in anhydrous DMSO. The aqueous solution you used completely negates the effect of DMSO on hydroxide
basicity/nucleophilicity. But no matter, halogen is correct - there is no way what you want to do will work in any practical manner.
AvBMeltonium - 3-1-2017 at 16:41
Thanks for this information! I am an amateur chemist with no formal background in organic chemistry. I thought I might give this experiment a try and
share it. At least I got rid of most of my stinky p-DCB and learned some new things.DDTea - 3-1-2017 at 17:06
Admittedly, most posts in these treads are pretty much demented, but even non-sense has some use in the learning process.
Besides, the product of the reaction between NaOH and p-dichlorobenzene, if performed at the required conditions (>300 °C) and if it works at all,
would not be hydroquinone, but a mixture of para- and meta-chlorophenol (and at least some chlorinated diphenylethers as impurities along with who
knows what else - it would likely not be a clean reaction). Maroboduus - 18-1-2017 at 19:09
Just a note on these proposals:
I think they tend to come up again and again because of people drawing an analogy with similar reactions on aliphatic molecules.
The problem is that these reactions on aliphatic molecules are bimolecular substitution reactions involving a backside attack on the carbon atom. This
requires a sort of 'flip' of the other groups attached to that carbon like an umbrella being turned inside out.
This just can't happen with an aromatic for structural reasons having to do with the rigidity of the benzene ring.
