Problem 33 - A Complete Resource Book in Chemistry for JEE Main

Question

The ether that undergoes electrophilic substitution reaction is (a) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{OC}_{2} \mathrm{H}_{5}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\)

Step-by-Step Solution

Answer

The ether that undergoes electrophilic substitution is (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\), anisole.

Step 1: Identify the Structure of Each Ether

The given options consist of different ethers. Analyze the structural components of each: (a) is dimethyl ether, (b) is ethyl methyl ether, (c) is diethyl ether, and (d) is anisole.

Step 2: Understand Electrophilic Substitution

Electrophilic substitution typically occurs in aromatic compounds due to the high electron density in aromatic rings that attract electrophiles. It is important to determine which ether contains an aromatic ring.

Step 3: Identify Aromatic Ether

Among the options, only option (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\), known as anisole, contains a benzene ring. Anisole can undergo electrophilic aromatic substitution.

Step 4: Examine Non-Aromatic Ethers

Options (a), (b), and (c) are aliphatic ethers, which lack the aromatic ring needed for electrophilic substitution. They do not readily undergo this type of reaction due to the absence of a benzene ring.

Key Concepts

Aromatic Compounds

Aromatic compounds are a special class of hydrocarbons. They feature a unique structure known as an aromatic ring. This is most commonly a benzene ring, a six-member ring where atoms share electrons in a specific way that creates a system of conjugated double bonds. This structure not only provides stability but also increases the electron density of the molecule.
Because of this high electron density, aromatic compounds are particularly reactive to electrophiles. Electrophiles are species that seek out electrons, making aromatic rings an ideal reaction site. The stability provided by the delocalized electrons in these rings is why electrophilic substitution reactions predominantly occur here. Aromaticity imparts distinct chemical properties which facilitate many important reaction types in organic chemistry. When it comes to electrophilic substitution in ethers, the presence of an aromatic ring is crucial.

• Electron-rich benzene rings attract electrophiles.
• Aromatics exhibit stability through resonance.
• This resonance makes the substitution reactions more feasible.

Anisole

Anisole, known chemically as methoxybenzene, is a valuable aromatic ether. It has a molecular formula of \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{OCH}_{3}\). In its structure, it contains a methoxy group \(\mathrm{OCH}_{3}\) attached to an aromatic benzene ring. Anisole is a good example of how the presence of an aromatic ring in ethers influences chemical reactivity.
The benzene ring in anisole makes it susceptible to electrophilic aromatic substitution reactions. This is because the methoxy group is electron-donating. It further increases the electron density on the benzene ring through resonance. As a result, the ring is activated toward electrophiles, making reactions like nitration, halogenation, and sulfonation possible.

• The methoxy group enhances ring activation.
• Anisole participates in typical substitution reactions.
• It is a model compound in studying ether-reactivity.

Aliphatic Ethers

Aliphatic ethers, unlike anisole, do not have an aromatic ring. Instead, they consist of alkyl groups linked by an oxygen atom. Examples of aliphatic ethers include dimethyl ether \(\mathrm{CH}_{3}\mathrm{OCH}_{3}\), ethyl methyl ether \(\mathrm{CH}_{3}\mathrm{OC}_{2}\mathrm{H}_{5}\), and diethyl ether \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OC}_{2}\mathrm{H}_{5}\).
These ethers do not undergo electrophilic substitution reactions as their structure is less reactive without the stability and electron density found in aromatic systems. Aliphatic ethers are more commonly involved in reactions like cleavage rather than substitution, due to their lower electron density.

• No aromatic rings mean no typical electrophilic substitution.
• Reactions focus more on bond cleavage or functional group changes.
• Aliphatic ethers play a key role in many synthetic applications.

This makes aliphatic ethers important but fundamentally different from aromatic ethers in behavior and reactivity.