Is this some form of cabbage?
The Brassicaceae family (of which cabbage and mustard are members) has a characteristic pattern of flowers - each flower has four equal sized petals, arranged in a cross-shape hence their other name - crucifers.

[h/t wikimedia commons]

It's not easy to see from your photos whether this is true, but I'm going to say, yes, it does look like a member of the Brassicaceae family. The cross-shape of the petals would confirm this.
I don't think these plants are native to Illinois, and it also does look somewhat like a crop plant, rather than a weed, so maybe it is an escape from a farm/garden. It could be canola, possibly.
Wild cabbages are native to the Mediterranean and Atlantic coasts of Europe , so it is probably not a wild cabbage, unless someone planted it in Illinois.