Question

Grignard reagent is prepared by the reaction between:
A) Zinc and alkynes
B) Zinc and alkyl halide
C) Magnesium and alkane
D) Magnesium and alkyl halide

Answer 1

Grignard reagent is an organometallic reagent which means it contains both organic part (alkyl or aryl group) and metal part in it. Grignard reagents consist of Magnesium as the metallic part. These reagents are very well known for carbon-carbon bond formation. Victor Grignard won the Nobel Prize for the discovery of the Grignard reaction.

Complete step by step answer:

1. Since Grignard reagent contains magnesium metal option A and B can be eliminated.
2. But let’s see each option in detail:
   In option A, when Zn and alkynes are put together along with Cu and $C{H_2}{I_2}$, cyclopropanation takes place with carbene as intermediate. This reaction is called the ‘Simon Smith reaction’.
3. In option B, if Zn metal is treated with alkyl halide we get an organometallic compound called dialkylzinc$\left( {{R_2}Zn} \right)$, it the compound similar to Grignard reagent but less powerful when compared to Grignard reagent.
   4)In option C, Magnesium is treated with alkane, nothing much is going to happen as alkanes are without functional groups and hence are less reactive.
   5)In option D, When Magnesium is treated with alkyl halide$\left( {RX} \right)$ in dry ether or Tetrahydrofuran (THF) solvent, we get $RMgX$ (Grignard reagent).The reaction can be written as
   $RX + Mg\xrightarrow{{Dryether}}RMgX$
   Where $R$ can be any alkyl or aryl group and $X$ can be $Cl,Br,I$.
   Therefore option D is correct.

Note: Grignard reagents are very important industrially as they provide negatively charged alkyl or aryl group which are very useful in nucleophilic addition reactions of formaldehyde, acetaldehyde, and ketone to form primary, secondary and tertiary alcohol respectively. They also take part in nucleophilic substitution reactions. They sometimes even act as base if the substrate is acidic.