Grignard reagent should be prepared under anhydrous condition because:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It reacts with H2O to form alkane
In a Grignard reagent the C–Mg bond is covalent but highly polar, with carbon carrying a partial negative charge. The carbon behaves like a carbanion and reacts with any source of proton, including water, to give a hydrocarbon:
\( \displaystyle \ce{RMgX + H2O -> RH + Mg(OH)X} \)
Even traces of moisture would destroy the reagent as it forms, so it is prepared in dry ether under anhydrous conditions.
Why the other options are incorrect
Option (1) carbon - Mg bond is not highly polar — Incorrect. The C–Mg bond is highly polar, which is what makes the reagent so reactive.
Option (2) Mg-X bond is essentially covalent — Incorrect. The Mg–X bond is essentially ionic.
Option (3) It reacts with proton of water to form HX — Incorrect. The proton combines with the alkyl group to give RH; the halogen ends up in Mg(OH)X, not as HX.
NCERT (2026-27 edition):
Reaction with Metals – “… In the Grignard reagent, the carbon-magnesium bond is covalent but highly polar, with carbon pulling electrons from electropositive magnesium; the magnesium halogen bond is essentially ionic. Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.”
\( \displaystyle \ce{RMgX + H2O -> RH + Mg(OH)X} \)