Which of the following Grignard reagent can be prepared ?

Options

A. `Br-Mg-CH_(2)-CH_(2)-CH_(2)-O-H`
B. `Br-Mg-CH_(2)-CH_(2)-SH`
C. `BrMg-CH_(2)-CH_(2)-NH_(2)`
D. `BrMg-CH_(2)-CH_(2)-underset(CH_(3))underset(|)(N)-CH_(3)`

Answer

D

Explanation

The Grignard reagent is a very powerful base, in effect it contains a carbanion. Thus, it is not possible to prepare a Grignard reagent from an organic group that contains an acidic hydrogen, by an acidic hydrogen, we mean any hydrogen more acidic than the hydrogen atoms of an alkane or alkene. We cannot, for example, prepare a Grignard reagent from a compound containing an – OH group, an-NH- group, an - SH group, a `-CO_(2)H` H group, or an `-SO_(3)H` group. If we were to attempt to prepare a Grignard reagent from an organic halide containing any of these groups, the formation of the Grignard reagent would simply fail to take place. (Even if a Grignard reagent were to form, it would immediately react with the acidic group.)
Since Grignard reagents are powerful nucleophiles, we cannot prepare a Grignard reagent from any organic halide that contains a carbonyl, epoxy, nitro, or cyano (CN) group. If we were to attempt to carry out this kind of reaction, any Grignard reagent that formed would only react with the unreacted starting material :

  This means that when we prepare Grignard reagents, we are effectively limited to alkyl halides or to analogous organic halides containing carbon - carbon double bonds, internal triple bonds, ether linkages, and `-NR_(2)`  groups.