It depends on how you change history
In short the answer is: since it did not happen, they could not do it. Back then people were a lot less squeamish about what kind of weapons they used. So if Napoleon or anyone else had had access to "real" chemical weapons, we would have known about it. We do not, so it could not happen.
Why is that?
Napoleon I reigned from 1804 to 1814. At that point, chemistry was blooming and lots of discoveries were made. Chlorine for instance — the first "effective" chemical weapon — was not identified as a pure element until 6 years after he placed the crown on his head.
This means that there were no chemical industries that could produce chemical warfare agents on a large scale, which is why poison gas was not employed with any considerable effect until World War I. That is not to say that no attempts were made before; there were. But in general, in the Napoleonic era they lacked the manufacturing capabilities to get the stuff made.
As for delivery... no. This is what the weapons of the Napoleonic era were like. What we today consider to be "artillery" — that is to say long range delivery of shells that you can actually fill with stuff like poison gas — were not yet in use. The cannons were for line of sight use, and were much too close to the own troops to be used for chemical warfare.
So if you want chemical warfare in the Napoleonic era, you need to change history. You need to have the belligerents stumble upon some discovery that allows them to mass produce poison gas, and preferably in such a way that it can be catapulted towards the enemy to give the range they need to be out of the danger zone themselves.