Sciencemadness Discussion Board

Sulfanilic Acid

benzylchloride1 - 10-2-2009 at 22:19

I am currently working on the synthesis of sulfanilic acid, which I plan to use for the synthesis of o-nitroaniline. I started with aniline sulfate and dissolved it in concentrated sulfuric acid. I heated it with a heating mantle at around 190 celsius for 8 hours. The temperature exceeded 208 degrees for about 20 minutes. The mixture turned to a purple black colored and gave off fumes of sulfur dioxide near the end of the heating period. The mixture was poured into water and a solid precipitated and the solution was a distanctive purple in color. The procedure I used mentioned some discoloring, but the color seems excessive. I think that the temperature caused the sulfuric acid to oxidize the aniline to aniline black, producing sulfur dioxide in small quantity. I doubled the scale of the procedure in Laboratory Experiments in Organic Chemistry by Adams, Johnson and Wilcox. This procedure mentions a yield of around 50% after heating for five hours. I am afraid that my attempt at increasing the yield by heating for a longer peroid may have decreased my yield substantially. I will post the yield as soon as the product is isolated and dried.

[Edited on 10-2-2009 by benzylchloride1]

benzylchloride1 - 12-2-2009 at 19:02

I then poured the reaction mixture into water and then filtered the crude sulfanilic acid. The crude acid was then washed with water and was a greyish purple color. The sulfanilic acid was then suspended in water and sodium hydroxide solution added until the solution was neutral. Powdered decolorizing charcoal was added and the mixture was heated to boiling. The mixture was than filtered and the slightly discolored solution acidified with hydrochloric acid. Crystals of sulfanilic acid formed and the solution was cooled in an ice bath. The crystals were than filtered off and dried in a 80 celsius oven. The mother liquer from the crystallization was then concentrated to half of its volume and crystals formed after seeding and cooling.

Axt - 13-2-2009 at 04:24

Here a section of a writeup that I'm yet to, if ever complete. I dont recommend using your mums kitchen microwave like I did (it stinks) but it sure beats heating it conventionaly for 8 hours. I dont seem to have recorded the yield though it was quite good. I added concentrated H2SO4 to the water-aniline here but it would be better to add aniline to the diluted acid instead.

<b>Sulphanilic Acid</b>

Sulphanilic acid a clear, crystalline compound that is typically prepared by heating aniline with concentrated sulphuric acid. It decomposes below its melting point at 280-300°C and is slowly soluble in about 90 parts water, 15 parts boiling water; though almost insoluble in alcohol, benzene and ether. The typical preparation by heating aniline in concentrated sulphuric acid is undesirable, due to the high temperatures and long reaction time that is necessary to complete the sulphonation[1].
The sulphonation is reportedly 75 times faster under microwave irradiation, reducing the reaction time from 5 hours to just 4 minutes[3]. This rapid reaction rate is attributed to the selective absorption of microwave energy by polar molecules. Being strongly polar the sulphate salts rapidly reach a high temperature and undergo dehydration and rearrangement forming the sulphonated product.

<i>Preparation</i>: 20g aniline (215mmol) is added to 38ml water in a 400ml beaker. 25g (255mmol) sulphuric acid is added slowly; forming a thick slurry of aniline sulphate (figure?). The slurry was then irradiated in a kitchen microwave oven for 4 minutes.
After the irradiation the product remained as a hard, purple, pumice like solid in the bottom of the beaker (figure ?) which was broken up with a knife.

30g sodium carbonate in 200ml water was added, with heating, to dissolve the product as its sodium salt. The dark purple solution was filtered removing some of the purple by-product, and then 35ml of 32% hydrochloric acid was added to precipitate the sulphanilic acid as glistening light purple crystals with a pearly lustre (figure ?). The precipitate was filtered and washed on the filter paper with 100ml ethanol, which removed much of the purple by-product but the crystals retained a faint lilac colour.

[Edited on 13-2-2009 by Axt]

sulphanilicacid.jpg - 27kB

benzylchloride1 - 13-2-2009 at 19:14

That is impressive! Thank you for sharing this on Sciencemadness. Im planning on getting a cheap used microwave for my home lab for my experiments. I purchased a kilogram of aniline sulfate off of Ebay for around $25.00. Did you prepare your aniline from benzene or purchase it? My sulfanilic acid is currently drying in a drying oven to remove the water of crystallization. It is currently a gray color. Your reaction mixture looks similar to what I obtained. The crystals look just like the crystals that I obtained. Axt do you have a reference for your procedure, I would like to read more about this method as it seems excellent. The yield from my experiment seems to be way lower than 50%.

Axt - 13-2-2009 at 19:55

In this case the aniline was purchased, old and near black by now. This was investigated on route to OTC benzotrifuroxan, which is a 10 step procedure, thus the reason my interest in completing it fizzled :P

Sorry I cant find the bloody article though I know I had it at some stage, the citation is “A Study on the sulfonation of Aromatic Amines with Sulfuric acid Under Microwave Irradiation”, Journal of Chemical Research, 493-494 (2003).

Magpie - 19-10-2010 at 14:44

Quote: Originally posted by entropy51  
Quote: Originally posted by Magpie  

BTW entropy, you will be glad to know that I'm now tending a little batch of sulfanilic acid per Brewster.
Yes! I finally ran a Brewster prep before Magpie.:D

I'm sure you will get a higher yield, however.:(

Did you distill your aniline first? If not, your product may be a tad off-color.

I'm not so sure about that yield. I got lazy and am just using antifreeze straight out of the jug as heat transfer fluid. The stuff bumps! Next time I will use purified ethylene glycol.

Lazy on the second count also - did not distill my amber aniline. I just want enough good product to make some methyl orange. I'll post a picture on edit.

Edit: The thermometer is reading 160-162C.

[Edited on 19-10-2010 by Magpie]

sulfanilic acid.JPG - 40kB

entropy51 - 19-10-2010 at 16:11

Quote: Originally posted by Magpie  
I'm not so sure about that yield. I got lazy and am just using antifreeze straight out of the jug as heat transfer fluid. The stuff bumps! Next time I will use purified ethylene glycol.
It will bump less if you put a little stir bar it, or at least that's true when distilling antifreeze.

MagicJigPipe - 19-10-2010 at 16:49

Wouldn't there be some ortho-sulfanilic acid created as well? I suppose it would be much less than 50% due to the bulky sulfonate group, but surely it's not all the para, right?

I'm referring to Axt's picture.

I've been trying to apply what I've been learning in organic to interpreting things on the forum so some input would be really helpful and appreciated.

entropy51 - 19-10-2010 at 17:20

Heating the aniline sulfate to about 180 C yields only the para isomer. Milder conditions are said to yield some ortho as well.

I'm sure Nicodem knows why this is so. I do not.

Sandmeyer - 20-10-2010 at 08:55

Great work Axt, though your mum would probably disagree! :D

Quote: Originally posted by entropy51  
Heating the aniline sulfate to about 180 C yields only the para isomer. Milder conditions are said to yield some ortho as well.

I'm sure Nicodem knows why this is so. I do not.

So, at lower temp the reaction gives some ortho prod while at higher temp it gives exclusively para prod with no ortho - what's the reference? It can be argued that the ortho-product should be less stable due to the unfavourable steric interactions, the para product is free from this steric effect hence the para product should be a more stable system. Sulfonation is a reversible process and at 180*C the major product should be that which is the most stable one (i.e para), or if you like fancy words - the reaction will be under thermodynamic control.

[Edited on 20-10-2010 by Sandmeyer]

Magpie - 21-10-2010 at 18:53

Here's some preliminary results on the sulfanilic acid synthesis I began earlier this week. As I indicated I used antifreeze straight out of the jug as heat transfer fluid for the heating period. This boils at 166C so I did not get the high soak temperature of ~195C that I would have if I had used pure ethylene glycol. I tried to make up for this by using a long (8 hour) soak time.

At the end of the soak the product was poured into a beaker of crushed ice as shown below. This resulted in the precipitation of a somewhat gummy suspension. This was then placed on a 7cm Buchner and pumped dry, saving the filtrate. The solids were then redissolved in hot water and some activated charcoal added to remove some slight discoloration. This could be described as a very light red or purple. Suspecting that a lot of product was in the filtrate from before I added this to the hot water solution followed by a little more charcoal. This was then heated to boiling and filtered to remove the charcoal.

The filtrate flask was then cooled in an ice bath and the gummy product recovered on the Buchner funnel. It was washed with 5mL denatured alcohol. Then it was placed on a dinner plate to dry at 55C. Then I turned up the oven to 80C. After several hours it was still gummy. It was then placed in a desiccator over CaCl2 (per Vogel). After several hours I weighed it at 15.4g (see 2nd picture).

Tentatively assuming this is sulfanilic acid dihydrate, the yield would be 68%. I'm suspecting that I didn't get it washed well, especially in regard to residual sulfuric acid. Monday I will recrystallize it and try to isolate a more pure sulfanilic acid product.

sulfanilic acid on ice.JPG - 40kBsulfanilic acid dihydrate.JPG - 41kB

Magpie - 25-10-2010 at 18:00

The 15.4g of preliminary product sulfanilic acid was washed on a 7cm Buchner funnel as follows:

1. mix with 10mL ice cold water, stir, pump dry
2. mix with 5mL of 95% ethanol, stir, pump dry
3. wash with 10mL of ether and pump dry.

The washed product looked a lot better, ie, whiter and dryer, not gummy. It was placed on a piece of paper and dried at room temperature for 6 hours. The chunks were chopped up using a razor blade. It looked quite dry. See picture below.

A lot of weight was lost during this washing, especially during the water wash. The final product weighed 5.6g. Assuming it is anhydrous sulfanilic acid, the yield is 29.8%.

As a check on purity, 0.173g of the product was titrated to a phenolphthalein endpoint using 0.11N NaOH. This indicated a MW of 134 vs a book value of 173. Although not as pure as hoped for it is a good improvement over the MW of 90 calculated for the unwashed preliminary product.

washed sulfanilic acid.JPG - 35kB