Benzocaine Synthesis

Benzocaine (total) synthesis Aim This brief write up attempts to illustrate the facile and relatively quick synthesis of a known local anesthetic, Benzocaine. The target compound will be synthesis using commonly available materials or which can be acquired with ease. General info PABA or para-aminobenzoic acid is a very common compound which is used in health and fitness food sort of things. It can be easily found online or bought from shops, it is not watched and is not suspicious. As we can see it is basically a benzene ring with an amino group and a carboxylic acid. Instead Benzocaine, ethyl 4aminobenzoate is the ester version we could say:

As you can see to get from PABA to Benzocaine is not hard, we are going to use what is industrially used, the Fischer esterification. What it is is forming an ester by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst.

The reaction What your going to need: -Sulphuric acid 90% or more (concentration wise) -PABA (pure or from gel capsules which can be purified) -Ethanol, 90% or more (concentration wise) -DCM (optional) -Sodium bicarbonate/ Sodium carbonate (Sodium carbonate is preferred)

From left to right: Sodium bicarbonate, Sulphuric acid, Ethanol and PABA (DCM is missing)

Cleaning The pills I bought where 500 mg gel caps one, but contained some crap :( The ingredients where: PABA, Magnesium stearate and rice flour. After a little reading and thinking I attempted different things in order to get the job done. I dissolve approx 2g of PABA (and crap) in 90% IPA, shook around, filtered any solids and left to crystallize. Needless to say I was not impressed, I got some bright yellow crystals which smelled like perfume, my IPA was full of shit. As I couldn’t get my hands on some clean one at the moment I read some more and did the following, which in my opinion is the best I have got so far: 1) Get 2 g of PABA(with the crap) and place it in a beaker with 250 ml of water 2) Shake the shit out of it and make sure the you dissolve all you can 3) Filter of all the white paste (rice flour and magnesium stearate 4) Take your filtered water and boil it down to around 50 ml 5) Pour half of the boiled down extract out and place it on the side 6) Boil down 25 ml of the 50 ml, crystals will start to appear as white/slight yellow 7) Dissolve the obtained crystals in the previously set aside extract

8) Boil all down, crystals will start to precipitate out of the solution 9) Stop heating, place on the side to cool down 10)Recover your crystals 11)85% yields if done correctly

IPA+PABA, noticed the yellow color :(

Scraped up and put aside, you never know!

Boiling down the extract,

All boiled down placed aside to evaporate

All dried out and crunchy. They are not that yellow, just my camera sucks.

Placed into a glass vial for further use

Now all the above can be omitted and I omitted it the first time I did this and I still manage to get pure Benzocaine, but doing the cleaning now will cut down other cleaning you have to do latter!

PABA---> Benzocaine Note: This was my first run and I did not clean the Benzocaine as I described above, I will be posting the follow up when I have time to see how the reaction differs with “clean” PABA. Most of the stuff I have read use a single neck flask, but I like to have control over what is going on inside so I decided to use a Three neck one. A Liebig column was use to reflux. Here we go: In a 250 ml Three neck flask where placed 1 g of PABA and 7.6 ml of EtOH. It was lightly heated and swirled so all the PABA dissolves.

When all was dissolved 0.76 ml of 95% H2S04 where added inside and a white precipitate formed, stirring was continued.

The reaction was run for 1 hour and 30 minutes at around 85-95°C. Stirring was done every 15 min. The picture below show the change in color, It turned such a dark read because of the additional crap in the PABA, this is why I plan on using the “cleaned” one next time.

At 1 hour and 20 minutes the heating was stopped and the flask was left to cool, meanwhile a 20% solution of sodium bicarbonate and water was prepared.

The flask cooling down

The solution made

Once it was all cooled down enough the reaction fluid was placed in a beaker and the Sodium bicarbonate solution was added until a ph of 8/9 was reached. Now from here you have two choices: basify, crash out the crystals, filter them out and clean them up or basify crystals crash out, shake up, add DCM, evaporate/rotovap DCM, clean crystals. I chose the latter one but other people have successfully done the first one.

Basified and DCM added

Removed top layer

Tadaaa! 80 mg of un-pure Benzocaine Yes, the scale was tared before placing the plate and the coin!

As we can see it is red and it is suppose to be white. I tested the red one on my tongue and it went numb, so yes works but not pure so the following was done: 1) Washed with 100 ml of cold water 2) Washed with 100 ml of warm water * a single cold water wash is fine too 3) Dried 4) Dissolved in EtOH+ 2 ml of water

5) Evaporate and….

There you go around 70 mg of pretty pure Benzocaine!