Question

Alkyl halides can be converted into Grignard reagents by:
A. Boiling them with \[{\rm{Mg}}\]ribbon in alcoholic solution
B. Warming them with magnesium powder in dry ether
C. Refluxing them with \[{\rm{MgC}}{{\rm{l}}_2}\]solution
D. Warming them with \[{\rm{MgC}}{{\rm{l}}_2}\]

Answer 1

Hint: A Grignard reagent may be defined as an organomagnesium compound having the general formula \[{\rm{RMgX}}\], where \[{\rm{R}}\]is an alkyl group or an aryl group and \[{\rm{X}}\]is a halogen atom.

Complete Step by Step Solution:
Grignard reagents are generally prepared by reacting an alkyl halide or an aryl halide with magnesium in dry ether.
In general, the reaction may be represented as:

where, \[{\rm{RX}}\,\]= alkyl halide or aryl halide
\[{\rm{Mg}}\]= magnesium
\[{\rm{RMgX}}\]= Grignard reagent
Ether is generally used as a solvent since it has the capacity to solvate the reagent by acting as a base for the acidic magnesium. However, for obtaining Grignard reagents from inactive halogen compounds THF (tetrahydrofuran) is used as the solvent instead of dry ether. The magnesium metal disappears and the Grignard reagents are formed with the evolution of heat. These reagents are seldom isolated in the solid state as they generally ignite spontaneously in the air. Grignard reagents are usually used in the solution.
Hence, it is evident that Grignard reagents are prepared by reacting an alkyl halide or an aryl halide with magnesium in dry ether.
Therefore, option B is correct.

Note: The mechanism for the formation of Grignard reagents is still not fully understood and it is believed to proceed via a free radical mechanism. The reagents used must be completely pure and dried since moisture tends to stop the reactions. Ether must be washed with water to free it from alcohol and then it is dried for a few days to remove moisture and alcohol. Also, magnesium turning is treated with ether to remove the grease and also with dilute hydrochloric acid to remove the oxide film after which it is dried.