Ethylene is the most produce compound in organic chemistry. Ethylene (also called Ethene; C2H4), the simplest Alkene, is an organic compound containing a C=C double bond. Ethylene is a coplanary unsaturated hydrocarbon (also called olefin) which is the most produced for industrial use.
So put your unripe or unrotten fruit in a brown paper bag and add a store bought banana or apple, tomatoes might get too messy. Leave in a warm place, like on top of the fridge, for a couple days and come back to it, it will be ripe. Leave a few days more and it will rot.