Problem 113 - A Complete Resource Book in Chemistry for JEE Main

Question

The reagent used for the preparation of higher ethers from halogenated ethers is (a) sodium alkoxide (b) Grignard reagent (c) concutrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (d) dry silver oxide

Step-by-Step Solution

Answer

The reagent is (a) sodium alkoxide.

Step 1: Identify the Reaction Type

To solve this problem, we need to identify the reaction type for the preparation of higher ethers from halogenated ethers. This is typically a nucleophilic substitution reaction where an alkoxide ion attacks a halogenated ether to form a higher ether.

Step 2: Evaluate the Reagents

Let's evaluate each reagent option provided: 1. Sodium alkoxide 2. Grignard reagent 3. Concentrated \(\mathrm{H_{2}SO_{4}}\) 4. Dry silver oxide. We need to determine which reagent can effectively act as a nucleophile to facilitate the reaction.

Step 3: Select the Correct Reagent

Alkoxide ions, such as those provided by sodium alkoxide, are strong nucleophiles. They readily participate in nucleophilic substitution reactions to form ethers. This aligns with the requirement for the preparation of higher ethers from halogenated ethers.

Step 4: Confirm the Mechanism Suitability

The reaction mechanism involves an alkoxide (RO-) attacking the halogenated ether (R'-X) to form the higher ether (R-O-R'). Sodium alkoxide fits this mechanism, enhancing the likelihood of ether formation without undesirable side reactions.

Key Concepts

Nucleophilic Substitution

In the realm of organic chemistry, nucleophilic substitution is a key reaction type that involves the replacement of an atom or group by a nucleophile. A nucleophile is a molecule or ion with a pair of electrons ready to make a new bond. This process typically happens in molecules where a leaving group, often a halogen, is present. The reaction is pivotal because it allows the conversion of one functional group into another, providing diverse pathways in synthetic chemistry.

• Two common types of nucleophilic substitution mechanisms are the SN1 and SN2 reactions.
• In SN1 reactions, the leaving group departs before the nucleophile attacks. This usually occurs in structurally bulky molecules.
• In SN2 reactions, the nucleophile attacks as the leaving group departs in a single simultaneous step.

Understanding whether a reaction is SN1 or SN2 helps in predicting the product and reaction conditions required.

Halogenated Ethers

Halogenated ethers are a specific class of compounds where an ether functionality is bonded to at least one halogen atom directly or indirectly. These compounds play a central role in ether synthesis due to their ability to act as electrophiles in reactions. Electrophiles are atoms or molecules that accept an electron pair during a chemical reaction.

• The halogen atom makes the carbon to which it is attached more reactive by pulling away electrons, making the carbon susceptible to nucleophilic attack.
• This property is particularly advantageous in nucleophilic substitution reactions, where the halogen serves as a good leaving group.

By using halogenated ethers, chemists can efficiently introduce ether linkages into molecules, which is a crucial step in the synthesis of many organic compounds.

Sodium Alkoxide

Sodium alkoxide is a valuable compound in organic synthesis, particularly known for being a strong nucleophile. It is formed by the reaction of sodium with an alcohol, where the hydrogen in the hydroxyl group is replaced by a sodium ion, resulting in the formation of an alkoxide ion (RO⁻).

• The alkoxide ion is highly reactive and seeks out electrophilic centers, such as those present in halogenated ethers.
• Upon encountering a halogenated ether, the alkoxide ion can efficiently attack the carbon atom bonded to the halogen.
• This results in the expulsion of the halogen and formation of a new ether bond.

Because of its reactivity and effectiveness as a nucleophile, sodium alkoxide is commonly used in ether synthesis, particularly for forming larger or "higher" ethers from smaller molecules.

Reaction Mechanism

Understanding the reaction mechanism is essential for predicting the products and conditions of a chemical reaction. In the preparation of higher ethers from halogenated ethers using sodium alkoxide, a nucleophilic substitution mechanism is at play. Here's how it goes:

• The sodium alkoxide (RO⁻) acts as a potent nucleophile and targets the carbon-halogen bond in the halogenated ether (R'-X).
• The alkoxide ion attacks the electrophilic carbon, facilitating the release of the halogen as a leaving group.
• This attack results in the formation of a new bond between the oxygen of the alkoxide and the carbon, generating the higher ether (R-O-R').

This mechanistic pathway minimizes unwanted side reactions, ensuring the targeted ether synthesis proceeds efficiently. Understanding these steps provides clarity on how molecular transformations occur and why particular reagents, like sodium alkoxide, are so effective in these processes.