Will a crystal of iodine lower the yield of an Grignard Reaction
No. Iodine is used to help magnesium react with the bromoderivate. If pure magnesium is added to a solution containing a bromoderivate, it should react quickly. But sometimes it does not react at all, because magnesium is covered by a thin and protective layer of magnesium oxide. Added iodine will etch this layer and the underlying metal. This reaction breaks the oxide layer, so that the metal can get in contact with the bromoderivate, and quickly react.