Chemical reactions involve interactions between the molecules of the reactants to form products. This interaction is possible only if there is air in the medium because reactions have to take place at a constant air pressure, which is standard 1 atm.

In vacuum, the molecules of the reactants, instead of reacting amongst themselves, will come out into the surrounding as there is no air perssure to keep them limited to the reaction vessel. So it would be difficult to achieve a chemical reaction in vacuum.