for everyone's viewing pleasure i'll just share my personal notes about this reaction. this is a full write-up i made and tried to break down as much as i could to clear up any and all confusion from this reaction. as i said i'm no expert, so take it for what it's worth. hopefully it will help some people. i attached a picture that is a snapshot of a table including information about all the chemicals involved in this reaction.
Synthesis of Benzoquinone via Hydroquinone/H2O2/I2 Chemicals:
• Precursor:• 55g Hydroquinone (0.5088 moles)
• Catalyst:• 30ml 2% Iodine Tincture (to compensate for 1g Iodine which is 0.0039 moles of I2)
• Oxidizer:• 58g/51ml 35% H2O2 (1.7053 moles)
• Reaction Solvent• 103.5g/131ml 99% Isopropyl Alcohol (1.7224 moles)
• Recrystallization Solvent• 64.56g/51ml 91% Isopropyl Alcohol (same amount of ml as h2o2 used above)
• Washing Solvent• 91% Isopropyl Alcohol (eyeball it)
Introduction:
C6H4(OH)2 + H2O2 i2> C6H4O2 + 2H2OAll credit here goes out to Bond_DoubleBond and who modified the drip-rate of the h2o2 to purposely elevate the exothermic reaction which is what the patent synthesis (found on Rhodium) advises against doing. Here’s what makes Bond_DoubleBond’s procedure so great:
After a lot of searching/reading old hive archives, it seems a lot of bees had a problem with the original Rhodium write-up of benzoquinone via the route of hydroquinone/h2o2/i2 due to them yielding the intermediate quinhydrone as the final product. The color of quinhydrone can from olive green, a dark green that’s so dark it looks almost black, and it can also be gold; however all of these colors have a metallic look to it. Benzoquinone is bright canary yellow, not metallic looking at all. Quinhydrone is a complex between hydroquinone and benzoquinone (ref:
http://www.jtbaker.com/msds/englishhtml/q3250.htm) that forms when hydroquinone comes into contact with benzoquinone (ref: hive\hiveboard\chemistrydiscourse\000313044.html). So quinhydrone is an intermediate product between hydroquinone and benzoquinone, it’s the signal that you’re halfway through the reaction. Mountaingirl (via the hive) says that quinhydrone works in the benzo wacker as well but would have to scale up 2x due to quinhydrone only having half the amount of benzoquinone, but why have to do guesswork and why deal with 2x the amount of quinone tar? I’d also have to argue that the solution in the p-benzoquinone wacker gets very thick as it is without doubling the amount of quinone. Just shoot for nice pure yellow benzoquinone and guarantee yourself a successful benzoquinone wacker! Bond_DoubleBond uses an aggressive drip-rate paired with external heating after addition of the h2o2 to keep the temp high enough so that when the quinhydrone crashes out upon formation it will re-dissolve back into the solution due to high heat of the water bath, allowing the rxn to progress further and continue oxidation, while solution boils for an additional 5 or so minutes after all the quinhydrone is formed. Note: Benzoquinone is highly irritating to the eyes and to inhale, goggles & respirator at all times, long-sleeve shirt is a good idea. It must be handled with care also, if it touches your skin then it will turn it black/brown and stain it for several weeks until the skins wears off so use typical lab safety procedures except you may want to also double-glove as benzo can go through a nitrile or latex glove.
Another change that has been made to this procedure is the amount of oxidizing solvent. This is because the original method used by the patent and perfected by Bond_DoubleBond were used when pure Iodine could be obtained. The original ratios were 55g Hydroquinone (0.5088 moles), 1g Iodine (0.0039 moles), 58g/51ml 35% H2O2 (1.7053 moles), and 115g/145ml 91% Isopropyl Alcohol (1.9137 moles). Now that pure Iodine is no longer accessible it can only be sourced in tinctures. Most iodine tinctures are 30ml and consist of a volume of 2% iodine solvated in 51% water and 49% alcohol, therefore 2 changes must be made. First is that the use of 91% IPA must be replaced with 99% IPA (iso-heet) to compensate for the water present in the iodine tincture. The second change is the amount of alcohol (IPA) must be reduced from 115g/145ml 91% IPA to 103.5g/131ml 99% IPA due to some alcohol already being present in the tincture.
Preparation:Prepare an ice water bath and keep it in the refrigerator or freezer (if kept in freezer then check on it periodically and break the thin layer of ice that develops on the top to keep it from freezing), you will need this ice bath later. Now, in a 1L 3-neck flask (at the 55g hydroquinone scale only a 1L flask is needed but if you want to you can oversize it if you have a 2L flask handy, just in case of accidental excessive drip-rate and boiling but again a 1L flask will suffice just fine if you keep an eye on the drip-rate): add a large stirbar, 30ml 2% iodine tincture, 55g hydroquinone, and 103.5g 99% IPA to the flask. Then place flask into a water bath of 30-35C (86-95F). Allow it to stir for an hour or so until all the hydroquinone dissolves then turn off the heat, unless room temp is cold like winter time then just leave the water bath on throughout the whole procedure but keep it at ~30C until addition of h2o2 is complete then you will be instructed to turn the heat up higher. Before proceeding to the next step, attach a reflux condenser fed with cold water otherwise the room will smell of benzoquinone and IPA. Now attach a sep funnel to one of the necks and clamp down the flask in multiple places to keep weight on one side of the flask from tipping it over.
Details: Always test (density check) your h2o2 before using it to make sure it has not decomposed, decomposed or lower concentration h2o2 will result in failure or the reaction will stop at the formation of quinhydrone due to only partial oxidation. “The iodine acts as the direct oxidant, oxidizing hydroquinone to benzoquinone and being reduced to I- in the process. The hydrogen peroxide then reoxidizes the I- to I2 and so the cycle continues..." per “hive\hiveboard\newbee\000475341.html”. The initial reason in the patent as to why to keep it under 80C is because part of the peroxide is decomposed before reacting per “hive\hiveboard\chemistrydiscourse\000245368.html”Addition of the H2O2:Now fill the sep funnel with 58g 35% h2o2 and set a drip-rate of 1 drop per second (don’t forget to tighten your stopcock!) until all the h2o2 has been added, some temperature evolution will be noted as the flask gets hot and may begin sweating or a very mild reflux. After all of the h2o2 has been added then you should see gold flakes stirring in the solution (the un-dissolved hydroquinone turns gold) and the solution gets thicker. Stirring may become difficult, if stirring stops then swirl by hand. Now place the flask in a water bath and heat the solution to 88C (192F). Allow the solution to boil for 5-6 minutes. After letting it boil for 5-6 minutes then remove it from the water bath and swirl by hand for 2 minutes.
Details: There will be a strong exothermic reaction, how hot it gets depends entirely on your drip-rate, the faster you drip in the H2O2 the more exothermic the reaction gets. You shouldn’t dump all the H2O2 in at once because H2O2 decomposes at high temperatures. You can use a pretty aggressive drip-rate (more than 1 drop per second) just as long as you don’t over-do it and stream it all in at once then yields will plummet because the H2O2 will decompose faster than it will react even though the reaction’s rate increases at higher heat. As the H2O2 is added the solution will turn to a dark red and eventually quinhydrone will begin to precipitate out of the solution making the solution very thick with gold specks. At this point it is only partially oxidized because quinhydrone is a complex between hydroquinone and p-benzoquinone. As the reaction continues the gold specks will dissolve back into the solution and the solution will begin to thin out again. This is good because the quinhydrone precipitate is dissolving back into the solution and will continue oxidation to p-benzoquinone. You may have to adjust the stirbar’s speed to accommodate the reaction as it gets thicker and thinner.Initial Crystallization:After swirling it by hand for 2 minutes then let it sit and cool off for a few minutes. After it has cooled down somewhat then plunge it into an ice water bath that you prepared earlier (10C or colder) and clamp it down so that it stays in the ice water bath (no stirring). Allow the reflux condenser to continue being fed by ice cold water and leave the flask clamped down in the ice water bath for 30 minutes even though yellow shards will be noticed within about 5 minutes or so. After the 30 minutes have passed then move onto the next step. If you absolutely must stop here then remove the reflux condenser and stopper the neck (or use cyran wrap + rubber band, then aluminum foil over top of that) and put it in the freezer overnight but that is not recommended.
Details: If you put the flask in the freezer for a few hours or overnight then the solution will get much thicker and harder to deal with when decanting or vacuum filtering, it gets almost syrupy. The benzoquinone crystals seemingly suck up the iodine making the benzoquinone darker and red. This will basically ensure that your benzoquinone isn’t as pure as it would’ve been by not keeping it in the freezer so long but ultimately it will still work fine in the benzoquinone wacker. So only put it in the freezer overnight if you absolutely have to. Otherwise it is best to decant or vacuum filter right after it sitting in the ice bath for 30 minutes.Initial Washes:Take the reaction flask once it has cooled down and the benzoquinone has precipitated out (or remove it from the freezer if you chose to do that) and carefully decant off the post-reaction solution (take your time). Now vacuum filter what is remaining in the reaction flask with a #101 filter until it is dry. Now add some ice cold 91% IPA to the reaction flask and swirl it around to be sure to get all the benzoquinone out of the flask. Use this same ice cold 91% IPA to also wash the filter cake with. Vacuum filter the filter cake dry after each wash then add more ice cold 91% IPA. Repeat this 3-5 times or until the IPA stops picking up color.
Recystallization:Now put the filtered crude benzoquinone back into the 1L RBF, attach a reflux condenser fed with cold water to the reaction flask, add 64.56g/51ml 91% IPA to the flask, and toss in a stirbar. Turn on stirring and heat up the IPA almost to a boil or basically just until all solids in the flask are dissolved, swirl by hand if needed if the stirbar stops. Once everything is dissolved then allow the IPA to continue to boil for a only couple minutes then turn off the heat and remove it from the water bath. Swirl the flask by hand then let it sit and cool off for a few minutes. After it has cooled down somewhat then plunge it into an ice bath that is 10C or colder and clamp it down just like you did after the initial reaction. Be sure to keep the ice bath cold by refilling with extra ice as needed, wait for 30+ minutes before proceeding. Alternatively you can stopper all the necks and put it in the freezer for one hour instead of using an ice bath, if you must stop here then you can leave the flask in the freezer overnight.
Details: The iodine stains the crystals a shade of orange or red. On a side note, again, you can put it in the freezer overnight if necessary but I recommend only putting it in the freezer overnight if absolutely necessary.
Washing & Vacuum Filtering:
After the benzoquinone has precipitated again then take the flask out of the ice bath or freezer and vacuum filter the contents with a #101 filter, you may add some 91% IPA to the reaction flask and swirl it around to be sure to get all the benzo out of the flask. Wash the filter cake with 91% IPA until it stops picking up color. Now dump out the filter cake and filter paper(s) onto a couple layers of paper towels that have aluminum foil underneath it (to keep the iodine/ipa from staining what is underneath it).
Drying the Benzoquinone (Fan or Desiccant Chamber):Fan: Now that you have a pile of benzo and filter papers sitting on the paper towels/aluminum foil, spread out the benzo so that it is thin and not piled up so it can dry out more thoroughly. Now put a fan blowing on it for 1-2 hours to dry them out (Note: if you prefer not to stink up the room and is quite irritating to the eyes/throat to be around then this can be done outside at night as benzo is sensitive to light or you may use a desiccant chamber).
Desiccant Chamber: You may alternatively use a desiccant chamber which will really help contain smells. Put your damp benzoquinone into a large shallow tupperware container sitting on top of newspaper/paper towels/etc within a larger one with CaCl2 (weight your benzo and use 50% of CaCl2, IE: if you have 50g benzo then use 25g CaCl2). In a few hours you will have nice large bright yellow crystals that don’t huff and puff like the powder does.
Details: The directions for the desiccant chamber above are from Chilly_Willy at The Hive.
Storing the Benzoquinone:
Benzoquinone does not store very well or very long, not like hydroquinone which can be stored for decades if done properly. Benzoquinone tends to turn green/brown over time and it will stain whatever it touches brown. However some have claimed to have stored benzoquinone upwards of a year and it is the same color still. Here is the secret to properly storing benzoquinone: airtight container, dry conditions (store with a desiccant), and it is light sensitive so one must block out all light.
Put long bright yellow crystals into baggie with a little bit of CaCl2 (make a little paper towel pouch for the CaCl and tape it shut). Now use put this baggie into another baggie (double-bag it), vacuum sealed is nice. Now put these 2 bags in a light-proof jar or wrap them thoroughly with aluminum foil and put it in the freezer.