A player in my campaign is interested in figuring out how to manufacture chemicals. How do I implement this and where could I find these rules?

Arsenal. Be careful: Arsenal specifically warns that the rules can be abused. For one thing, you should probably just completely ban resale. Another suggestion is to have overuse come to the attention of police and crime organizations.

Ah, chemistry. I have loved chemistry since I was eight and my Dad gave me a highschool textbook from 1950 called "All about the wonders of Chemistry." In fact my current character uses his chem shop to great effect. In our last session he made a batch of Fentenol(sp?) to plant in order to manufacture blackmail evidence.

Fentanyl?

I'm not sure that Fentanyl is easy to make. It's not in SR4 anyway, so I guess you just invented a Threshold and ingredient list?

It'd be on par with Methadone, so that's 16+, and you'd need the formula, ingredients, and tools. Legality F, and Availability? Pretty rare. The fluff indicates that a Facility or Microfac is the appropriate tool-size, so you'll be at a penalty without them (if it's even possible).

I believe if you don't have a facility, you can't make it.

Yes, the book says, 'if you don't have the right tools, you get the penalty, or the GM may simply say it's impossible'. It's important to use all appropriate limitations, because it's no good crying that things are overpowered if you forgot all the intended ways to rein them in.

I wouldn't rate a legally prescribed pain medicine as forbidden.... though its precursors are generally illegal.

That's why it's forbidden. It can only be used under prescription, and it's very tightly controlled. You can't get a license for it, as it were. But, I can see your point, compared to some of the other 2070 examples. I guess it's sort of like Slab, although it's closer to heroin's deadlier, less fun cousin.

The GM I am playing with is more into story than number crunching. Once we determined it was possible she just handwaved it. My character had to spend time and money but we mostly just moved on with the game. I am sorry if this is not a good example rules-wise, I was just sharing personal insights on chemistry in gaming.

So banning resale sounds like a good idea, but how do I sell the character on this (his idea was to sell them)?

Tell him he's a shadowrunner, not a day-jobber. Shadowrunners perform exciting missions. If he wants to be a farmer, mechanic, or other wageslave, wrong game.

Depends on what chemicals he wants to sell, really.

Drugs? Explain to him that the current players in the drug market really, really don't like independent operators.

Chemical weapons? Governments take interest in that in a hurry.

Personally, I allow resale in my games, with the strict understanding that it's more for roleplay than actual money-making. Start moving any significant quantity, and the hammers are ready to come down. So, people in the know come this character for custom-made, specialty stock; in return the character gets a little money, and some sweet contacts, and I get plot hook galore^^

There are some many cool things you can do with this stuff, and not just combat related. If he wants to make money off it. Give him the flaw day Job. He has to spend X amount of time in the lab or on the corner selling it. Then he can use the spare space to cook up the cool toy you use for running.
BugKiller Rounds, DMSO/Narkojet, Acid/Contact poison, the infamous K-10/Dopadrine Panic Pen, fake Skin, the many various solvents and dozens of other cool compounds you can use

Hmm not sure how that happened again. :\

That's fine: limit him to the Day Job income, and the rest goes to his syndicate masters.

If you want the player to have buy-in with you on the restricted use of chemistry, have a discussion with him on your concerns that its use may unbalance the game for the other players. Ask him what he thinks is a reasonable amount of "extra" income earned between runs and come to a set Nuyen amount that you both can handle (and yes, ask him first if you really want his buy-in).

Once the reasonable amount is set, suggest that he use the normal rules for buying/brewing/selling his goods and map out the time spent for your benefit (have him do the heavy lifting). If he generates more income than the amount you set, he can expect to attract the attention of rivals (both street & corporate) that may burn down his shop, steal his stuff, sell him out to the cops, etc.

Lastly, let him know this is new ground so it may need some modification going forward. But try not to over-react if the first Nuyen amount you both came to was off. If the character is making too much NuYen for a couple games in a row, it won't kill your game right away. After a bit, come back and ask him if he feels it is still balanced. After thinking through his response, you can suggest an alternate amount (or a completely new tactic) that will bring your game to "balance".

... at least that's what I would try if I was in a good mood.

P.S. consider modifying the base 30% selling price down to 24% using the "Item Counterfeit" modifier since he is making these on his own. After a while you can reduce it to 21% from "market flooded". And lastly, make sure you are taking some off the top both when buying raw materials & selling the goods if he is going through a contact (I believe the standard is 5% of the base cost per point of connection and even a bottom level street fixer has to have a connection of 3+).