What is the purpose of the chemical in store-brand rooting hormone? I’d rather not use anything with chemicals when I’m propagating plants. Are there alternatives?
The chemical in rooting hormone (usually Indole-3-Butyric acid) acts as a growth stimulator. In commercial rooting hormone formulations, it may be combined with fungicide to prevent the development of fungus/fungal diseases during the rooting process, as is the case with a common brand, Rootone, which contains Thiram (a fungicide). The Environmental Protection Agency has more information about Indole-3-Butyric acid.
If you would rather not use synthetic rooting hormone, you can skip this stage altogether, or you can try making willow water to encourage rooting instead. Oregon State University (now archived) explains how to make a rooting tonic using willow.