Question

An aromatic ether which is not cleaved by HI even at \(525 \mathrm{~K}\) is (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OC}_{6} \mathrm{H}_{5}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OC}_{3} \mathrm{H}_{7}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OCH}_{3}\) (d) tetrahydrofuran

Short answer

Option (a) diphenyl ether is not cleaved by HI at 525 K.

Step-by-step solution

Understand the reaction conditions

The problem involves identifying an ether that is not cleaved by hydroiodic acid (HI) even at high temperatures (525 K). Typically, ethers can be cleaved by HI, but some specific ether structures offer resistance to this reaction.

Evaluate given options

Consider each option: - (a) \( ext{Diphenyl ether (} ext{C}_{6} ext{H}_{5} ext{OC}_{6} ext{H}_{5} \)- (b) \( ext{Phenyl propyl ether (} ext{C}_{6} ext{H}_{5} ext{OC}_{3} ext{H}_{7} \)- (c) \( ext{Anisole (} ext{C}_{6} ext{H}_{5} ext{OCH}_{3} \)- (d) \(\ ext{Tetrahydrofuran}\).Among these, diphenyl ether is known for its high resistance to cleavage by HI due to resonance stabilization, while the others can be cleaved more readily.

Analyze the structural stability

In diphenyl ether, the presence of two aromatic rings linking the oxygen atom provides stability through resonance. This makes the C-O bond much stronger and less susceptible to the cleavage by HI at high temperatures. On the other hand, ethers like anisole and phenyl propyl ether have one aromatic ring, and their aliphatic part can be cleaved more easily.

Determine the correct option

Based on the structural analysis, diphenyl ether (option a) is resistant to cleavage by HI even at elevated temperatures. The stability and resistance come from the resonance provided by the two phenyl groups bonded to the oxygen atom.

Key concepts

Chemical Stability

Chemical stability refers to the ability of a substance to maintain its chemical structure under a variety of conditions. Aromatic ethers, like diphenyl ether, are particularly stable. This enhanced stability arises due to their specific structural configuration. Bonds in aromatic compounds are known for their strength, which contributes significantly to the ether's resistance to reactions involving cleavage, such as those with hydroiodic acid (HI). Ethers, in general, resist breaking apart because they lack the reactivity seen in other chemical groups. The chemical stability in some ethers, like diphenyl ether, stems from strong C-O bonds that remain unaffected under conditions that would easily break other ethers.

Resonance Stabilization

Resonance stabilization is a phenomenon whereby electron delocalization over a molecule's structure reduces its energy, contributing to its stability. In aromatic ethers like diphenyl ether, the oxygen atom is bonded to two phenyl groups. These phenyl groups participate in resonance, where electrons are shared across the structure, enhancing its stability. When electrons are delocalized, it strengthens the C-O bonds, making them less prone to breaking under chemical reactions such as cleavage by HI. The aromatic nature of the rings allows movement of electrons in such a way that the entire molecule gains energy stability, protecting it from unwanted reactions.

Ethers Cleavage

Ethers cleavage is a chemical reaction process where the C-O bond in the ether is broken, leading to two separate components. For many simpler ethers, this can readily happen under the conditions involving hydroiodic acid (HI), especially at elevated temperatures. However, in the presence of stabilizing structures, like what we see in aromatic ethers, this cleavage reaction is inhibited. In ethers like anisole or phenyl propyl ether, the absence of double contributing aromatic rings makes them more vulnerable to cleavage by HI. The difference in cleavage susceptibility is mainly due to the strength of the C-O bond, which is impeded by resonance effects in more stable aromatic ethers.

Hydroiodic Acid Reaction

The reaction of hydroiodic acid (HI) with ethers primarily involves cleaving the C-O bond. This reaction can proceed easily in many aliphatic ethers due to the lower bond strength of the C-O linkage. However, in aromatic ethers, the reactivity with HI changes significantly. At high temperatures, typically around 525 K, HI can generate a powerful enough environment to potentially break down weaker ether links. Yet, in cases like diphenyl ether, the presence of aromatic rings results in a formidable resistance against HI. The resonance stabilization in these molecules acts as a protective barrier, greatly reducing the accessibility of HI to the ether linkage, preventing cleavage even under these otherwise formidable conditions.