Pretty sure the answer is mercury fulminate. It's a ridiculously sensitive explosive that detonates on impact so is a perfect candidate for cartridges.

In general, chemical complexity has very little to do with a function like "detonate on impact" and there's actually a broad range of chemicals that form touch sensitive explosives (which is why chemists tend to be mortally afraid of old bottles of peroxide formers or picric acid). Many of them are surprisingly easy to make and significantly more research has gone into making safer explosives rather than the other way around. A classic example being nitroglycerin (also a shock sensitive explosive, which is a large part of why dynamite was such a success compared to pure nitroglycerin).

Also, just a side note, but chemistry was surprisingly advanced at the time period your asking about, considering the basic structure of atoms wasn't even (accurately) known yet. Still, there was plenty of trial and error experience with great effect. For example, there is a reaction used even today that is enormously useful known as a Friedel Crafts alkylation/acylation that was discovered around 1870. Many basic gases and elements were being isolated and characterized around that time (which isn't easy work). Kekule was even able to more or less accurately describe the structure of benzene which is pretty amazing given the methods available to him.

edit: spelling

The earliest potential primer was gold fulminate, discovered by at least 1603. But it was both expensive and too unstable and there were still attempts being made to use it 60 years alter, Pepys mentions it in his diary. Silver fulminate was found in 1788, mercury fulminate in 1800 - but they were still too unstable for practical uses.

The first practical example seem to have been made by one Alexander Forsyth in late 1805. Although he found mercury fulminate unsuitable, a mixture of potassium chlorate, charcoal and sulphur would work well, and (military research being rejected after a change in government) he went on to design gun locks for a private manufacturer. Shortly after, a Swiss gunmaker in Paris called Pauly added some gum to the mixture and put a pellet between disks of paper (like the old cap gun ammo) which made it easier to handle than the previous powder.

By then there is a lot of interest and research into primers and the timeline gets a bit vague and who actually invented the copper cap is debatable, my source considers the inventor to be Joshua Shaw, a landscape painter who emigrated to the US to sell the idea as Forsyth still owned the patent in Britain. Mercury fulminate was being used now, but only in small amounts as part of a mixture, having been moderated with other ingredients like gunpowder, potassium chlorate, antimony sulphide or powdered glass. All of this is happening before 1866, by the way.

Mixtures with mercury fulminate starts to be replaced by ones with lead azide around the turn of the century, and things are about where we're still at today.

Source, The Big Bang, a History of Explosives, GI Brown, 1998.