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Q6.12(iii):
Explain why (iii) Grignard reagents should be prepared under anhydrous conditions?

Solution :

Chemical Foundation of Grignard Reagents

Grignard reagents are organometallic compounds represented by the general formula $R-Mg-X$ (where $R$ is an alkyl or aryl group and $X$ is a halogen). The bond between the Carbon atom and the Magnesium atom ($C-Mg$) is highly polarized, possessing significant ionic character, which renders the carbon atom a strong nucleophile and a potent Brønsted base.

Step 1: Analyzing the Electronic Structure

Due to the electropositivity of Magnesium ($\chi \approx 1.31$) relative to Carbon ($\chi \approx 2.55$), the electron density is localized on the carbon atom [Per Pauling Electronegativity scale]. This results in a partial negative charge on the alkyl group ($\delta^-$) and a partial positive charge on the magnesium atom ($\delta^+$):

$R^{\delta-} - Mg^{\delta+} - X$

Step 2: Interaction with Protic Sources

Water ($H_2O$) is a protic solvent that contains hydrogen atoms bonded to a highly electronegative oxygen atom. These hydrogen atoms possess a partial positive charge ($H^{\delta+}$) and are acidic. When a Grignard reagent encounters even a trace amount of moisture, an acid-base reaction occurs instantaneously [Per Brønsted-Lowry Theory of Acids and Bases]:

$R-Mg-X + H_2O \rightarrow R-H + Mg(OH)X$

Step 3: Analytical Breakdown of the Reaction

  • Formation of Hydrocarbon: The nucleophilic alkyl group ($R^-$) abstracts a proton ($H^+$) from the water molecule, leading to the formation of the corresponding alkane ($R-H$).
  • Decomposition: The original organometallic reagent is effectively destroyed, losing its utility for subsequent nucleophilic addition or substitution reactions.
  • By-product formation: The remaining fragment forms a basic magnesium hydroxy halide ($Mg(OH)X$), which is an inactive precipitate in the context of organic synthesis.

Visual Representation of Nucleophilic Attack

R-Mg-X (Grignard) + H-OH (Water) R-H (Alkane) + Mg(OH)X (Inert Product)

Conclusion on Operational Necessity

Because Grignard reagents are powerful nucleophiles, they react with any substance containing an active hydrogen atom ($O-H$, $N-H$, or $S-H$ bonds). Anhydrous conditions (absence of water) and an inert atmosphere (often diethyl ether or tetrahydrofuran) are strictly required to preserve the structural integrity of the $C-Mg$ bond. Failure to exclude moisture results in the premature protonation of the reagent, leading to yield loss and contamination of the reaction mixture with alkanes.

Final Solution: Grignard reagents are extremely sensitive to protic environments. The highly nucleophilic carbon atom in the $R-Mg-X$ bond acts as a strong base, reacting irreversibly with water to form a stable hydrocarbon ($R-H$) and basic magnesium salts, thereby decomposing the reagent.


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