Grignard reagent is usually prepared by
`R-X+Mg overset(Et_(2)O)rarr RMgX " Grignard reagent"`
`Ar-X+Mg overset(Et_(2)O)rarr ArMgX " Grignard reagent"`
Grignard reagent acts as a strong base. Grignard reagent carry out nucleophilic attack in absence of acidic hydrogen. Grignard reagent form complex with its ether solvent. Complex formation with molecular of ether is an important factor in the formation and stability of Grignard reagent.
What is the correct order of reactivity of halides with magnesium ?

Options

A. `R - Cl gt R - Br gt R-I`
B. `R - Br gt R - Cl gt R-I`
C. `R-I gt R-Br gt R-Cl`
D. `R-I=R-Br=R-Cl`

Answer

C

Explanation

Grignard reagents are usually prepared by the reacton of an organic halide and magnesium metal (turnings) in an ether solvent.
`{:(RX + Mg overset(Et_(2)O)rarr RMgX),(ArX + Mg overset(Et_(2)O)rarr ArMgX):}}`Grignard reagents
The order of reactivity of halides with magnesium is also `RI gt RBr gt RCl`. Very few organomagnesium fluorides have been prepared. Aryl Grignard reagents are more easily prepared from aryl bromide and aryl iodides than from aryl chlorides, which react very sluggishly.
Grignard reagents are seldom isolated but are used for further reactions in ether solution. The ether solutions can be analyzed for the content of the Grignard reagent, however, and the yields of Grignard reagents are almost always very high (85-95%). Two examples are shown are :
`CH_(3)I + Mg underset(35^(@)C)overset(Et_(2)O)rarr underset((95%))underset("Methylmagnesium iodide")(CH_(3)MgI)`
`C_(6)H_(5)Br + Mg underset(36^(@)C)overset(Et_(2)O)rarr underset((95%))underset("Phenylmagnesium bromide")(C_(6)H_(5)MgBr)`