Grignard reagent should be prepared under anhydrous condition because:

It reacts with \(H_2O\) to form alkane

The correct answer is option 4. It reacts with \(H_2O\) to form alkane.

Grignard reagents are highly reactive organometallic compounds composed of a carbon atom bonded to a magnesium atom and typically an alkyl or aryl group. These compounds are prepared by the reaction of an organic halide \((RX)\) with magnesium metal \((Mg)\) in anhydrous (water-free) conditions.

However, in the presence of water, Grignard reagents react with the proton \((H^+)\) from water (\(H_2O\)). This reaction, known as hydrolysis or quenching, proceeds as follows:

In this reaction, the carbon-magnesium \((C-Mg)\) bond is broken, and the organic group \((R)\) is protonated, leading to the formation of an alkane \((RH)\) and magnesium hydroxide (\(Mg(OH)X\)).

For example, if the Grignard reagent is ethylmagnesium bromide (\(C_2H_5MgBr\)), and it reacts with water, it forms ethane (\(C_2H_6\)) and magnesium hydroxide (\(Mg(OH)Br\)):

This unintended reaction with water not only reduces the yield of the desired Grignard reagent but also consumes the reagent, making it unavailable for the desired organic synthesis. Therefore, it's essential to prepare Grignard reagents under anhydrous conditions to avoid this hydrolysis reaction and ensure the success of subsequent organic transformations.