Some of the most useful compounds for organic synthesis are Grignard reagents (general formula R±MgX, where X is a halogen), which are made by combining an alkyl halide, R±X, with Mg. They are used to change the carbon skeleton of a starting carbonyl compound in a reaction similar to that with R - Li:(c) There are often two (or more) combinations of Grignard reagent and carbonyl compound that will give the same product. Choose another pair of reactants to give the product in (a).