Benzoic acid help.

[YT2095]

I`m trying to synth Benzoic Acid from Methyl Benzene (Toluene) as related to this: http://www.scienceforums.net/forums/showthread.php?t=19018 thread.

 

needless to say I`m having a few problems doing it.

 

my findings Thus far....

 

1 methyl benzene (MB from now on), was added to KMnO4, there was slight disolution to the purple that then became brown.

 

2 the addition of 46% H2SO4 to this created 2 layers, the contaminated MB at the top and the muddy brown mix od H2SO4 and KMnO4 at the bottom.

 

3 upon reflux heating, the MB became clear again as did the muddy mix at the bottom.

 

4 upon Cooling, the AQ based mix (muddy that became clear) became cloudy white.

 

 

and so, where is the Benzoic acid? did I even do it correctly?

is there an Easier method than this with a higher yield using less costly chems?

[jdurg]

If you did indeed make benzoic acid, I would venture that it is in the white mixture in the aqueous layer. The fact that your KMnO4 went colorless and then formed a muddy brown ppt makes me believe that the MnO4- was reduced to MnO2 which is a muddy brown color. MnO2 and H2SO4 will readily form MnSO4 which is apparently quite soluble, so that would explain the dissolution of the brown "mud". The excess of H+ ions would make me believe that the benzoic acid is formed in the fully protonated state and dissolved into the aqueous layer. Upon cooling, it would crystalize out of solution giving you the white cloud that you now see.

[YT2095]

well I find that pretty disgusting personaly.

I use 10mg of KMnO4 (as the most expensive chem) and get about 2mg Benzoic in return

 

there simply HAS TO BE a beter synth than This!

 

anyone know a Good one?

[raivo]

I made benzoic acid this way:

 

4.1ml toluene

10.2g potassium permanganate

225ml water

 

this mix was boiled with reflux condenser without stirring during 4 hours. Liquid part was separated by filtration and evaporated to 30ml volume.

 

Aproximately 5ml of 22% HCl was added dropwise to this almost clear liquid. White precipitate immediately formed. Precipitate was collected by filtration and was left to dry. I got 2.1g benzoic acid although i was expecting aproximately 3g.

 

These data are from my lab notebook. Amounts are those what i used and need not be exactly those what i found from org chem books when planning this synth.

[YT2095]

so in effect, 10gr KMnO4 gave you 2 gr Benzoic acid

 

there has to be a Better synth for MB conversion to BA.

 

 

anyone!?????

[woelen]

Theoretically from 100 grams of KMnO4, 48.59 grams of toluene and 93.09 grams of H2SO4 you would obtain almost 64.40 grams of benzoic acid. That would be 100% theoretical yield. You need quite some excess H2SO4 and excess toluene.

So, the yield is not as pathetic as you describe. With 10 grams of KMnO4, 5 grams of toluene and 15 grams of H2SO4, mixed with 25 grams of water you should obtain at least 6 grams of benzoic acid.

 

MnO2 and H2SO4 will readily form MnSO4 which is apparently quite soluble

I cannot agree with that. MnO2 and H2SO4 only readily forms MnSO4 in the presence of a reductor. That reductor in this case is the toluene. The Mn goes from oxidation state +4 to +2.

[raivo]

Do you (Woelen and YT) are absolutely sure that sulfuric acid is added to potassium permenganate when oxidising toluene? I did more research in my papers now and i found two books where this synth is described. Both are without sulfuric acid and theoretical yield is said to be aproximately 3g benzoic acid to 10g permanganate. According to equations one molecule of toluene needs 2 molecules of KMnO4 to get oxidised.

 

I would be very interested in cheaper ways for organic oxidations as permanganate indeed is expensive. Maybe KNO3 could be used someway? Anyone knows?

[encipher]

My guess would be that KNO3 is too weak of an oxidizer. KMnO4 is very strong compared to KNO3

[woelen]

Do you (Woelen and YT) are absolutely sure that sulfuric acid is added to potassium permenganate when oxidising toluene? I did more research in my papers now and i found two books where this synth is described. Both are without sulfuric acid and theoretical yield is said to be aproximately 3g benzoic acid to 10g permanganate. According to equations one molecule of toluene needs 2 molecules of KMnO4 to get oxidised.

 

I would be very interested in cheaper ways for organic oxidations as permanganate indeed is expensive. Maybe KNO3 could be used someway? Anyone knows?

Without the sulphuric acid, the oxidation state goes from +7 to +4 in the manganese, so the compound only gives 3 electrons for oxidation for each permanganate ion.

With the sulphuric acid the oxidation state can go from +7 to +2, so this adds another two electrons and this allows 5/3 times as much to be oxidized.

 

KNO3 is not really suitable. It either does nothing, or it starts to nitrate the material, but that occurs at very high concentrations in a situation where hardly any water is present.

Another oxidizer could be 30% H2O2 + 30% H2SO4, but I'm not sure about that. One could try that. Also, NaClO3 could do the trick, but I'm afraid that give too much side reactions. With this, Cl2 and ClO2 can be formed as well and especially the first may lead to chloro-substituted products. Chlorate is not a very clean oxidizer.

[YT2095]

I tried the NaNO3 reaction anyway, and all you get is a runaway nitration that leaves you with alot of NO2 and Dinitromethylbenzene.

worth a try for experiments sake, but somewhat predictable and not at all usefull for making benzoic acid, Might make an interesting Fuel addative though

 

unfortunately I don`t have any 30% H2O2, I only have 9% (30 vols) available, so the other synth`s probably out the question too.

[raivo]

Thanks woelen. What if to use H2O2 without sulfuric acid? Do you think this will work? Or what if we use some other acid instead of sulfuric? I still suppose that sulfuric acid may be unsuitable for this kind of oxidation. Toluene gets easily sulfonated, this may affect end products.

 

85% sulfuric acid at room temperature sulfonates it in great extent when left to stand for some weeks. First crystalls of sulfonic acid can sometimes be seen few hours after liquids are put together. At higher temperatures sulfonation goes much faster.

[woelen]

Raivo, with such high concentrations of acid I indeed think sulfonation is a serious issue, but when the concentration is just 15 ... 20% (which is the case when 30% H2O2 and 30% H2SO4 are mixed 1 : 1) then I see no problem with that. With the H2O2 mix, I'm quite sure that H2O2 alone will not work easily and that a lot of the H2O2 simply will decompose. Acid stabilises H2O2 and it helps making it a stronger oxidizer. The same is true for permanganate. In acidic conditions this is a much stronger oxidizer.

 

With the H2O2 I would first try it in small amounts, before spending a lot of chemicals. I just gave it as a suggestion and I expect it to work, but I've notr tried that personally. That is why I wrote I'm not sure about that.

 

@YT: You could mix in some conc. H2SO4 in some of your 9% H2O2. That does not dilute the material too much and it may be succesful as well.

[YT2095]

Hmm... an Interesting note, when I left today (taking my cat to the vets for surgery) the DNT I`de made with the runaway nitration, left a clear liquid at the bottom and the yellowish DNT floating at the top.

when I came back, the clear liquid at the bottom is now a deep but clear brown and the DNT has stayed the same color!?

 

the smell is rather pleasant also, although it would be almost impossible to describe, other than quite sweet and little bit like roast Lamb?

 

what a bizare little experiment!

[woelen]

The clear layer of liquid, was this concentrated H2SO4 with nitrate in it, or did it also contain large quantities of water?

If it is concentrated H2SO4, then the brown color can be explained by assuming impurities in the toluene. Chemicals like acetone, MEK, certain aromatics, tend to be dehydrated and then form polymeric species, which are dark brown (they can even become black) and quite viscous. It might be that this also happened with your liquid.

 

Once, I did an experiment by adding concentrated sulphuric acid to acetone. I mixed the liquids in a 1 : 1 ratio by volume. Initially, the liquid remains clear and on mixing quite some heat is produced (although not as much as with water). I dripped the acid into the acetone and swirled occasionally, letting it cool down in between. When the acid was added, the bottle was tightly capped.

One day later, the liquid was orange. Another day later, it was dark red. A week later, it was very dark brown and also very viscous. It had changed into a syrup. What happens is that the acetone molecules are partly hydrated and form longer chains with all kinds of side branches. This ill-defined polymeric mix is viscous and these large molecules look brown. It was a nice experiment though.

[YT2095]

Hmmm... be Very Carefull with that acetome sulphuric mix, the gasses that can give of even when mixed with water are are actualy a precursor (and still quite effective on its own) for a reasonably well-known Chemical weapon!

 

I`ll not go into Any further detail for obvious reasons.

 

anyway, other than the sweet roast lamb smell, the other synth with the KMnO4 and H2SO4 with MB smells like something vaguely familiar also, I can`t place the smell but I`m sure it`s some sort of stuff you put on shoes???

it`s not a sweet smell (nor unpleasant) and doesn`t smell like Marker Pens anymore either.

 

I`de still like to know how to get any (if any) Benzoic acid Out of this mix though, I still have 2 distinct layers, both are clear now.

[YT2095]

aha, I just had a very interesting Email from a Friend of mine (he`s a Chem Teacher whose garden is next to mine), it reads:

 

"For the oxidation of methylbenzene - I had forgotten that permanganate can be used in alkaline solution (i.e bloody washing soda!) to oxidise any side chain alkane on an aromatic ring to -COOH. I mean even ethylbenzene ends up as benzoic acid! Don't know where the spare bits go. Magic probably involved - 'tis organic chem, after all. Try Chemguide (for UK A level students), perhaps."

 

opinions?

[woelen]

Yes, permanganate in alkaline solution is capable of oxidizing quite some aryl-compounds, but unfortunately, the permanganate itself only is reduced to MnO2 and not further. This means that 40% of the total oxidizing potential of the permanganate is not used and that is a pity. In alkaline media, the oxidation state of manganese goes from +7 to +4 and in acidic media, the oxidation state goes from +7 to +2. There also is a side effect, being that permanganate is not really stable at very high pH. Part of it is converted to manganate, which contains manganese in the +6 oxidation state. That goes to +4 and then only 2 electrons per manganate ion are available for oxidation and a side product is oxygen, which escapes into the air.

 

So, effectively, only appr. 50% of the oxidizing power of permanganate is used in alkaline media.

 

YT, if you want to see that nice manganate effect, then make a concentrated solution of NaOH or KOH (doesn't matter which one), and add a single crystal of permanganate to this. The liquid will turn deep green instead of purple. That green color is the color of manganate ion, MnO4(2-). If you observe very carefully, then you also may see tiny bubbles of oxygen, but the dark solution may make that observation difficult. Here follows a page with properties of manganese. Look at the section on oxidation state +6 for the manganate.

 

http://woelen.scheikunde.net/science/chem/solutions/mn.html

 

This manganate looks neat, but its formation is not good in oxidation experiments, because it causes a loss of 20% of the oxidizing power of the permanganate.

[YT2095]

Bingo!

 

that explains alot, because when I mixed the AQ layer 1`st I saw just the purple (almost black) when I added the MB then I saw the green and got worried! it was so much like manganesse heptoxide!

I`m still here and so No it wasn`t

 

although I didn`t notice any gas liberated?

I used KOH.

 

however you be pleased to note that my 1`st synth (left to stand in the test tube in the rack for about a week) has now produced a beautifully clear crystal about 4mm long and 2mm wide in the AQ layer

[woelen]

Ah, what you observed indeed was the green manganate. It is not dangerous at all. Probably the concentration of KOH was not that high, that the permanganate already decomposed spontaneously. However, with a reductor as MB, even at moderate alkalinity, the permanganate is reduced to manganate (and then no gas can be observed), and finally, the reduction proceeds further to the brown MnO2.

 

What is the color of the crystal? Is it white/transparent, or does it have brown spots in it? That crystal most likely is potassium benzoate in your alkaline liquid. However, it also could be just K2CO3, because of absorbtion of CO2 from the air by the alkaline solution. If you add acid to a small quantity of the aqueous layer, and the solution starts bubbling a lot, then it has absorbed a lot of CO2 from the air. I usually do longer-lasting experiments with alkaline solutions in well-stoppered test tubes or bottles, otherwise within a few days, all hydroxide is converted to carbonate.

[YT2095]

no no no

 

the Crystal I have is NOT from the synth I received in the email (that went down the sink as heating it made it shoot out the test tube into the room and I hade to evacuate until the MB fumes dissipated).

 

the crystal I spoke of is from the KMnO4/H2SO4/MB synth I started a week ago, and yes it`s Clear as glass, but with some manganesse oxide on the one end (probably provided a seed for the growth).

[woelen]

That crystal could indeed be a nice crystal of benzoic acid, but it also could be K2SO4. K2SO4 is only sparingly soluble and easily forms nice glassy crystals. It the aqueous layer of the first synth completely colorless now? If that is the case, then you know that all permanganate is oxidized completely. Benzoic acid is almost insoluble in water, but I do not know its solubility in toluene. You could take out a drop of the toluene layer and let it evaporate. It might be that you get many small crystals. If you don't, then that means that most of the benzoic acid is in the aqueous layer, and then indeed I expect the large crystal to be benzoic acid.

[YT2095]

Hmmm... you present excellent points!

 

I`m not entirely certain what this crystal is either, and look forwards to finding out with tests.

I`m not doing this to make Benzoic acid for a Use, only because I`ve never made it myself before from raw materials, I`ve only ever Extracted it from benzoic salts.

 

it`s more a mission of exploration than anything else