First I've heard or thought about stockpiling chemicals for the apocalypse! What an interesting thing to theorycraft about. Here's a few ideas:

Potassium permanganate - A swiss army knife of a chemical. It can be used to clean certain contaminants from water, clean wounds, start fires, and it's cheap as dirt.

Aqua Regia - A mixture of muriatic acid and nitric acid. I can imagine this being useful to smuggle gold through bandit warlord checkpoints. George de Hevesy dissolved a nobel prize medal to hide it from nazis. Acids in general are useful.

Sulfur - Depending on your location, this is the hardest component to obtain for making gunpowder. The other components can be made from pee and cooked wood.

That's all I can think of off the top of my head. Note: I am neither a chemist nor a prepper.