Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

Higher homologues of ethers can be prepared by (a) diazomethane (b) Grignard reagent (c) alkyl halides (d) none of these

Step-by-Step Solution

Answer

(c) alkyl halides

Step 1: Understand Higher Homologues

Higher homologues refer to ether compounds that have been lengthened by adding more carbon atoms to their alkyl groups. This means we are trying to prepare an ether that has longer carbon chains than the starting material.

Step 2: Analyze Option (a) Diazomethane

Diazomethane is used primarily for methylation processes, converting carboxylic acids to methyl esters, or for converting alcohols to methyl ethers. However, it is not used for lengthening the carbon chain in homologation processes. Therefore, diazomethane is not suitable for preparing higher homologues of ethers.

Step 3: Analyze Option (b) Grignard Reagent

Grignard reagents (RMgX) are organomagnesium compounds used in forming carbon-carbon bonds. Although they are effective in synthesizing alcohols from carbonyl compounds, they are not typically used for the direct synthesis of ethers or their higher homologues. Therefore, Grignard reagents are not suitable for this purpose.

Step 4: Analyze Option (c) Alkyl Halides

Alkyl halides can react with metal alkoxides via the Williamson ether synthesis to form ethers. By choosing longer chain alkyl halides, higher homologues of ethers can be prepared. Therefore, alkyl halides are suitable for preparing higher homologues of ethers.

Step 5: Confirm the Best Choice

Based on the analysis, option (c) Alkyl halides is the most appropriate for preparing higher homologues of ethers because they allow for the extension of carbon chains in ethers through reactions like the Williamson synthesis.

Key Concepts

Homologation

Homologation refers to the process of increasing the length of a carbon chain in a molecule by adding one or more methylene groups (-CH₂-). This process creates higher homologues, which are molecules with longer carbon chains. The concept of homologation is crucial in organic chemistry, especially when designing molecules with specific properties or functionalities.
To achieve homologation, different chemical reactions and methods can be employed. One common approach is through reactions that extend the carbon backbone, often using specific reagents or catalysts. In ether synthesis, homologation is significant because it allows chemists to tailor the length of ether compounds for various applications, improving their properties like solubility and boiling points.

Alkyl Halides

Alkyl halides are a group of organic compounds containing a halogen atom (such as chlorine, bromine, or iodine) attached to an alkyl group. They are highly versatile reactants in organic synthesis due to their ability to participate in nucleophilic substitution reactions.
In ether synthesis, particularly in the preparation of higher homologues of ethers, alkyl halides play a pivotal role. The importance lies in their ability to react with metal alkoxides in the well-known Williamson ether synthesis to extend carbon chains. By selecting alkyl halides with longer carbon chains, chemists can successfully create ethers with extended alkyl groups, thereby forming higher homologues.

• Versatility: Widely used in various types of organic reactions.
• Reactivity: Highly reactive due to the polar C-X bond.

This makes alkyl halides a cornerstone in ether synthesis strategies involving longer carbon chains.

Williamson Ether Synthesis

Williamson ether synthesis is a fundamental reaction in organic chemistry used to produce ethers. This method involves the reaction between an alkoxide ion and an alkyl halide. An alkoxide ion is typically formed by deprotonating an alcohol with a strong base.
The strength of the Williamson ether synthesis lies in its ability to construct ethers with desired alkyl chain lengths. By using different alkyl halides, you can determine the length of the carbon chain on the ether's other side.

• Synthesis Mechanism: Nucleophilic attack by the alkoxide on the alkyl halide.
• Applications: Enables synthesis of symmetric and unsymmetric ethers.

The Williamson synthesis is not only straightforward but also highly adaptable, making it a go-to method for creating diverse ether compounds.

Grignard Reagent

The Grignard reagent is an organomagnesium compound typically represented as RMgX, where R is an alkyl or aryl group and X is a halogen. These reagents are crucial in the formation of carbon-carbon bonds in organic synthesis. Created from the reaction of magnesium metal with an alkyl or aryl halide, Grignard reagents are highly reactive and useful in a variety of synthetic applications.
While they are not directly used for the synthesis of ethers, Grignard reagents are fundamental in organic chemistry for their role in creating alcohols from carbonyl compounds after nucleophilic addition. Their utility in homologation lies more with building longer carbon chains in alcohols, rather than ethers. Hence, while Grignard reagents are not suitable for direct ether synthesis, they contribute significantly to homologation reactions and the overall toolkit of organic chemists.