Question

Some organomagnesium halides (Grignard reagents) can be challenging to synthesize even when glass- ware is rigorously dried. Several improvements were developed to increase chances of successful gen- eration of organomagnesium halide. One method is to stir magnesium turnings vigorously under nitrogen atmosphere for several hours before adding alkyl or aryl halide. During that time the color of Mg turnings darkens significantly (picture change from Silver to Space Gray in iPhone color pallette).

• Explainthecolorchange.

• Explain why the reaction of organic halide with “Space Gray” magnesium is faster than with regular magnesium turnings.

Answer 1

The elemental magnesium is coated with a layer of magnesium oxide (MgO). Magnesium oxide is formed by areal oxidation of magnesium metal. The layer of magenisum oxide over magnesium metal reduces the reactivity of magnesium.

If you stir the metal in inert atmosphere for severalk hours, the magneisum oxide is separated from the metal and the pure magnesium metal is obtained. The colour of pure metal darkens.

The pure metal (free from any oxide) is more reactive and hence reacts faster with the alkyl halide.