Preparation of benzoic acid

[vulture]

I want to prepare benzoic acid to make benzoates for use in pyrotechnics. Benzoic acid is also an interesting precursor for other aromatic compounds like phenol, benzaldehyde or even benzene.

I tried the reaction of toluene with potassiumpermanganate, but this didn't work, at least under normal conditions.
Does anybody have any experiences with this compound?

Reply's are greatly appreciated.

[mr.evil]

i've tried to make it last evening(as you know) but it didn't work either! :( The reaction didn't show up so i putted some HCl into the liquid, maybe a little to much(1ml on 40ml).. And as (many) of you know: KMnO4 + HCl ==> H2 + Cl (and some other stuff) So i breathed alot of Cl, my heartbeat was after 5minutes around 195/m! (i seriously thought i was gonna die!!)

Anyway, i like to hear to if anyone has succes with preparing Benzoic acid.

(edit; some spelling mistakes)

[Alchemist]

Benzoic acid

Here is some detailed info on making benzoic acid .

Mix the following chemicals in a flask with vertical water condensor attached (reflux
setup).

Toluene - 3 cubic centimeters
Potassium Permanganate - 10 grams
Dilute Sodium Hydroxide Solution - 20 cubic centimeters

Reflux for 3 to 4 hours until oily toluene disapears.Then cool and filter the manganese
(IV) oxide from the solution. Now by addition of concentrated HCl, benzoic acid will
precipitate. Either filter and re-crystallize the benzoic acid from hot H2O, or extract
the benzoic acid with ether and evapourate.

[vulture]

Neat wouldn't it be that I do not happen to own reflux equipment. There must be some other way, like catalized air oxidation.

[VX]

Benzoic acid can also be made by oxidising phenol, with acidified potassium dichromate (or other similar oxidiser). Unfortunately refluxing conditions are needed again, just like whenever you want to oxidise a primary alcohol to a carboxylic acid. This is because the reaction proceeds via the formation of the aldehyde which has a lower boiling point than both phenol and benzoic acid, due to it's inability to form H bonds. (Oh yeah, I’ve been revising: Chemistry A level exams on Wed :( )

I only say this because some people (like me) have better access to phenol than they do to toluene.

[Mr Cool]

Benzoic acid can not be made by oxidising phenol. For one thing, benzoic acid has 7 carbon atoms compared to phenol's 6.
Phenol is not a primary alcohol. I suppose it is closest to a 3* alcohol, although I'm not even sure that that is a correct term for it due to the resonance in the benzene ring... but we will assume that it is 3* (the OH is on a C that has three bonds to other C's). It therefore can't be oxidised easily, and certainly not to a carboxylic arene.

[VX]

Ah yeah, thats a bloody good point! :rolleyes: Why did I say that, It's obviously wrong! :confused:

I was tired when posting that, but it's a good job it didn't come up in the exam. <img border="0" title="" alt="[Wink]" src="wink.gif" />

[rikkitikkitavi]

sodium benzoate is frequently used as a food preservation agent, can probably be found in a well stocked super market. (atleas in Sweden)
It is readily soluble in water (1 part per 2 parts of water w/w)

Benzoic acid has little solubility in cold water(higher in warm water) so by adding a mineral acid to a sodium bensoate solution benzoic acid precitpates. Unfortunately I dont have the exact solubility data...

/rickard

[megalomania]

Solubility data of benzoic acid in water (grams per liter):
0 C = 1.7
10 C = 2.1
20 C = 2.9
30 C = 4.2
40 C = 6.0
50 C = 9.5
60 C = 12.0
70 C = 17.7
80 C = 27.5
90 C = 45.5
95 C = 68.0

I would like to hear from vulture and mr. evil as to how you have tried to make benzoic acid. Refluxing ic certainly important. I have a procedure nearly the same as the one Alchemist posted. Try using 1 mole of toluene per 2 mol of potassium permanganate (plus a 20% excess) and dilute with 10 times this volume of water. You will need to reflux for 3-4 hours. A good rule of thumb to judge when the reaction is done is to wait until the purple color of the potassium permanganate is gone.
You will then have to distill the reaction mixture to remove most of the water, along with a little unreacted toluene. Only a simple distillation is necessary, you can stop when the bulk of the liquid volume has been removed, or little water remains. While still hot, filter the remaining liquid to remove the manganese dioxide. I suggest keeping it to remake potassium permanganate again. Wash the manganese dioxide to remove any product that may have come along for the ride.
Add the wash to the liquid and acidify with about 125 mL of 35% hydrochloric acid per mole ratio as above. Make sure you are stirring this during the addition. A precipitate of benzoic acid will form as soon as you add the acid. Cool the liquid to push as much benzoic acid as possible out of solution, and then filter to collect it. Wash the benzoic acid with cold water to remove some impurities.
The addition of the acid forms potassium chloride. This can be kept and used with the manganese dioxide to reform potassium permanganate. Boiling down the liquid may yield an additional crop of crystals. Zyou may try to purify the benzoic by recrystallizing from alcohol or acetone. Starting with freshly distilled toluene is also recommended (if you use the store bought stuff).

[vulture]

I did the following:
I first prepared a saturated KMnO4 solution and added some 85% H3PO4.
(why I didn't use HCl or H2SO4 should be obvious)
Then I added the toluene and some water. Of course this wasn't miscible with the water, so I added some acetone and isopropyl alcohol. This seemed to increase the miscibility somewhat but not much.

It's been standing there for a week now, haven't looked at it lately.

Could anyone please explain me shortly the principles of refluxing, so maybe I can come up with something improvised.

[Polverone]

I am unsure why you added acid to your permanganate. Benzoic acid is prepared from toluene with alkaline potassium permanganate - at least that's the way I've always seen it. Some weeks ago I tried to prepare it, out of curiosity. I used sodium hydroxide and KMnO4 in hot water and mixed it with a bit of toluene, then shook everything up in a glass flask. I know the reaction was working because the KMnO4 was turning green (being reduced to potassium manganate) but I was unable to complete it since the toluene fumes were making my roommate sick. I really should try this again some day soon.

I have wondered - could you add a bit of detergent so that the toluene and water would emulsify? It seems like this would significantly increase surface area (and reaction rate).

To reflux is to heat/boil something in a closed system so that the vapor is continually recondensed and returned to the source. It allows you to perform reactions at relatively high temperatures without wasting or being menaced by volatile chemicals. You can do a crude sort of reflux by putting your materials in a tall, sealed glass bottle with plenty of room in it. Heat the bottle so that the bottom portion is at or near boiling, and the vapor will continually condense and run down the inside of the bottle back to the source. The condensation can be enhanced by cooling the upper portion of the bottle. I have found that flask-like wine vinegar bottles work quite well for this, and that they also leak around the cap when the pressure rises very much, providing a sort of built-in safety valve.

[vulture]

I added the acid because permanganate is a strong oxidizer in acidic environment, most reactions with permanganate involve an acid (NOT HCL!)

[Polverone]

Alright, I recently made benzoic acid. This writeup is going to be slightly shorter than I'd like because the browser crashed in the middle of my long writeup.

I combined 33.4 g KMnO4, 10 g toluene, 5 g NaOH and 200 ml water in a long-necked wine vinegar bottle. I capped it and wrapped a wet paper towel around the top to complete my improvised reflux apparatus. I initially refluxed for about 3.5 hours, then decided things weren't going fast enough. I let the vessel cool, added an additional 10 g NaOH, and returned it to reflux for an additional 4 hours. The purple liquid had turned green and there was a thick layer of precipitated MnO2 in the bottom of the flask. I turned the heat down and went to bed at about this point.

In the morning I transferred the flask contents to an evaporating dish, accidentally spilling a considerable amount in the process. The dish was left outside in the sun since it's been quite warm here lately. A day later, all the toluene was gone and the liquid was somewhat concentrated, but it was still green. According to my Lange's handbook, potassium manganate (the green stuff) is unstable in water but stable in strongly alkaline solution. I didn't want to add acid at this point, so instead I added methanol. It gets oxidized to formic acid (which presumably formed sodium formate); I reasoned that since formic acid is a liquid, evaporation or rinsing would remove it later when I acidified the solution. I let the dish set in the sun for the day again.

When I returned this afternoon, there was clear liquid sitting atop a thick fudgelike layer of MnO2. I added additional water and stirred it around until the "fudge" had gone away and everything was freely fluid. I filtered the liquid, then added a little more water to the material in the filter to extract remaining goodies.

The clear liquid was returned to the evaporating dish (after the dish was cleaned) and again set on the hot plate. As I heated the liquid, small black flakes gradually appeared in the bottom of the dish. At first I thought I had missed some MnO2 and refiltered the stuff, but they later returned. When the liquid was concentrated enough that materials were starting to crystalize out and float on the surface, I removed it from heat, cooled it with a fan, and then placed it in a basin of ice water. At some point during the cooling, the black flakes disappeared. When it had cooled about 10 minutes I added hydrochloric acid to the liquid.

There was a sort of gentle "phshhh" and white, meringue-like masses of benzoic acid sprang up on the surface. They were light enough to float. I kept adding HCl until no more benzoic acid sprang up and the lower layer of liquid was acidic. The fresh benzoic acid smelled like over the counter acne medication (which contains benzoyl peroxide). I filtered the liquid out and I'm now waiting for the fluffy/creamy mass to dry. I don't know what my yields will be like, but there's certainly enough here to do some small experiments with.

I think that this process could be improved by the addition of a small amount of detergent to the reflux container, to emulsify the mixture and get greater surface area. I have tried this with my other benzoic acid attempt (using dilute nitric acid instead of KMnO4) but I have not yet processed that batch.

In any case it was a fun and satisfying exercise, though probably not economical if you can just buy benzoic acid.

<small>[ June 27, 2002, 03:00 AM: Message edited by: Polverone ]</small>

[stanfield]

Benzoic acid is relatively cheap here, 1kg --&gt; 9 €
Try to buy it first before bugging you... no ?

[mr.evil]

Do you get your Chemicals from Acros.be or from the drug stores in Belgium? (i don't say the names of them...)

[stanfield]

no :) this time this is from an another supplier !
Furthermore, Acros is a lot expensive on benzoic acid :
2kg -&gt; 59.61 €

from my supplier : 2kg for 20 €

see ya !