Explain reason:

(a) Grignard reagent should be prepared under anhydrous conditions.

(b) C6H5CHClCH3 is hydrolysed more easily with KOH than C6H5CH2Cl.

When discussing Grignard reagents and their reactivity, it's essential to understand the underlying chemistry that dictates their behavior. Let's break down your questions regarding the preparation of Grignard reagents and the hydrolysis of specific alkyl halides.

Why Grignard Reagents Require Anhydrous Conditions

Grignard reagents, which are organomagnesium compounds, are highly reactive and sensitive to moisture. The reason they must be prepared under anhydrous (dry) conditions can be explained through the following points:

• Reactivity with Water: Grignard reagents are formed by the reaction of magnesium with an organic halide. If water is present, the Grignard reagent will react with it to produce hydrocarbons and magnesium hydroxide. This reaction not only consumes the Grignard reagent but also leads to the formation of unwanted byproducts.
• Formation of Hydrocarbons: For example, if phenylmagnesium bromide (C6H5MgBr) encounters water, it will yield benzene (C6H6) and magnesium hydroxide (Mg(OH)Br). This reaction effectively destroys the Grignard reagent, preventing it from participating in further reactions.
• Controlled Environment: Preparing Grignard reagents in an anhydrous environment ensures that the desired reagent is obtained in a pure form, allowing for more predictable and efficient chemical reactions in subsequent steps.

Comparative Hydrolysis of C6H5CHClCH3 and C6H5CH2Cl

Now, let’s delve into the hydrolysis of the two alkyl halides you mentioned: C6H5CHClCH3 (1-phenyl-2-chloroethane) and C6H5CH2Cl (benzyl chloride). The difference in their reactivity towards hydrolysis with KOH can be attributed to the structure and stability of the intermediates formed during the reaction.

• Stability of Carbocations: When C6H5CHClCH3 undergoes hydrolysis, it can form a secondary carbocation (C6H5C^+(Cl)CH3) upon the departure of the chloride ion. Secondary carbocations are more stable than primary carbocations due to hyperconjugation and inductive effects from adjacent carbon atoms.
• Mechanism of Reaction: The hydrolysis of C6H5CH2Cl, on the other hand, leads to the formation of a primary carbocation (C6H5C^+(Cl)H2). Primary carbocations are less stable and more prone to rearrangement or elimination reactions, making the hydrolysis slower and less favorable.
• Reaction Conditions: KOH acts as a strong base and nucleophile in these reactions. The presence of a more stable carbocation intermediate in the case of C6H5CHClCH3 allows for a quicker and more efficient reaction with hydroxide ions, leading to faster hydrolysis compared to benzyl chloride.

In summary, the requirement for anhydrous conditions in the preparation of Grignard reagents is crucial to prevent their destruction by water, while the difference in hydrolysis rates between the two alkyl halides can be attributed to the stability of the carbocation intermediates formed during the reaction. Understanding these concepts helps in predicting the behavior of various organic compounds in chemical reactions.