Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will not form a Grignard reagent on treatment with \(\mathrm{Mg}\) in dry ether? (a) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (b) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2}^{2} \mathrm{Br}\) (c) \(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br}\) (d) All the three above can form Grignard reagent.

Step-by-Step Solution

Answer

Option (b) will not form a Grignard reagent on treatment with Mg in dry ether.

Step 1: Understand Grignard Reagent Formation

Grignard reagents are formed by the reaction of organic halides, typically bromides or chlorides, with magnesium metal in dry ether. These reactions require the halogen to be primary or secondary, as tertiary halides and some highly substituted vicinal dihalides may not react as expected.

Step 2: Analyze Option (a)

Option (a) is 1,3-dibromopropane: \( \mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} \). This molecule contains two bromine atoms bonded to the primary carbons in the chain, which can each potentially form a Grignard reagent by reacting with magnesium.

Step 3: Analyze Option (b)

Option (b) is \( \mathrm{BrCH}_{2} \mathrm{CH}_{2}^{2} \mathrm{Br} \), which seems to be a typographical error. However, if it represents 1,2-dibromoethane (vicinal dibromide), this type of compound reacts differently. Instead of forming a Grignard reagent, it tends to undergo elimination, leading to the formation of ethylene and magnesium bromide.

Step 4: Analyze Option (c)

Option (c) represents 1,4-dibromobutane: \( \mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} \). Both bromine atoms in this molecule are bonded to primary carbons. Therefore, this compound can form a Grignard reagent effectively.

Step 5: Compare Findings

Both option (a) and option (c) can form Grignard reagents as they have bromines attached to primary carbons. However, option (b) does not form a Grignard reagent due to potential elimination reactions. Therefore, option (d) is incorrect.

Key Concepts

Organic Halides

Organic halides are compounds containing one or more halogen atoms (such as bromine or chlorine) attached to carbon atoms in an organic molecule. These compounds are crucial in many chemical reactions, including the formation of Grignard reagents. The type of halide—whether it's a bromide or chloride—impacts how these compounds behave in chemical reactions. Typically, bromides and chlorides are used to create Grignard reagents because they readily react with magnesium.

The reactivity of organic halides depends on the type of carbon they are connected to:

• Primary halides react well with magnesium to form Grignard reagents.
• Secondary halides can also form Grignard reagents, but the reaction may be less efficient.
• With tertiary halides, forming Grignard reagents becomes challenging due to steric hindrance and the possibility of other side reactions like elimination.

Understanding the nature of the halide is essential when predicting the possible outcomes of reactions involving organic halides.

Magnesium Reaction

The reaction of organic halides with magnesium is fundamental in creating Grignard reagents, which are important tools in organic synthesis. In this process, magnesium metal is introduced to the organic halide in a solution of dry ether. The role of dry ether is crucial as it acts as a solvent and protects the reaction from moisture.

When magnesium comes into contact with the organic halide:

• It inserts itself into the carbon-halogen bond.
• This creates a carbon-magnesium bond, transforming the organic halide into an organomagnesium compound, known as a Grignard reagent.
• Grignard reagents are highly reactive and function as strong nucleophiles, targeting electrophilic centers in subsequent reactions.

During this process, the magnesium effectively converts the halogen-carbon bond into something much more reactive, enabling the formation of complex organic molecules.

Elimination Reactions

Elimination reactions are another pathway that organic halides can undergo, especially when the conditions aren't right for forming Grignard reagents. In an elimination reaction, the halogen, along with a hydrogen atom from an adjacent carbon, is removed, forming a double bond between the carbons.

An example of an elimination reaction is what likely happens with vicinal dihalides, like 1,2-dibromoethane, when exposed to magnesium. Instead of forming a Grignard reagent, these undergo elimination to produce ethylene and release magnesium bromide. Factors influencing elimination include:

• The presence of vicinal dihalides, which are particularly prone to this process.
• The type of solvent and conditions under which the reaction occurs.

Elimination reactions are significant because they provide an alternative route in organic synthesis, especially when reactions with magnesium are not viable. Understanding when and why elimination reactions occur can help predict and control the outcomes of various chemical processes.