Question

There are fundamentally different mechanism possible for following reaction, If \(\mathrm{O}^{18}\) is present once in phenolic \(\mathrm{OH}\) and once in carboxylate group of product on using \(\mathrm{NaOH} / \mathrm{H}_{2} \mathrm{O}^{18}\) then mechanism used by following reaction in both cases are respectively : (A) Ester hydrolysis, Nucleophilic aromatic substitution (B) Nucleophilic aromatic substitution, Base catalysed acyl oxygen cleavage mechanism (C) Electrophilic aromatic substitution, Base catalysed acyl oxygen cleavage mechanism (D) Nucleophilic aromatic substitution, Nucleophilic aromatic substitution

Short answer

The correct mechanisms for the given reaction conditions are (B) Nucleophilic aromatic substitution, Base catalysed acyl oxygen cleavage mechanism.

Step-by-step solution

Analyze Option A

Option A suggests that the mechanisms used are Ester Hydrolysis and Nucleophilic Aromatic Substitution. Ester hydrolysis involves breaking the ester bond with the addition of water to form a carboxylic acid and an alcohol. Nucleophilic aromatic substitution involves replacing a group on an aromatic ring with a nucleophile, with temporary loss of the aromaticity of the ring.

Analyze Option B

Option B proposes that the mechanisms are Nucleophilic Aromatic Substitution and Base Catalyzed Acyl Oxygen Cleavage Mechanism. We have already analyzed the first mechanism. The Base catalyzed acyl oxygen cleavage mechanism involves the cleavage of an acyl-oxygen bond, often forming a carboxylic acid and a ketone or aldehyde.

Analyze Option C

Option C suggests that the mechanisms are Electrophilic Aromatic Substitution and Base Catalyzed Acyl Oxygen Cleavage Mechanism. Electrophilic Aromatic Substitution involves replacing a group on an aromatic ring with an electrophile, like a halogen. We have already analyzed the second mechanism.

Analyze Option D

Option D proposes that the mechanisms are Nucleophilic Aromatic Substitution twice. This means that the reaction goes through two different nucleophilic aromatic substitution steps, with each step resulting in a substitution of a group on the aromatic ring.

Find the suitable mechanisms in given reaction conditions

Given that \(\mathrm{O}^{18}\) is present once in phenolic \(\mathrm{OH}\) and once in the carboxylate group of the product, we can eliminate option C, as electrophilic aromatic substitution does not involve taking the oxygen from the phenolic \(\mathrm{OH}\). Option D is also unsuitable as the second step involves substitution of carboxylate group which does not support Nucleophilic Aromatic Substitution mechanism. Now, let's look at options A and B. Comparing both options, option B seems more viable as in the base catalyzed acyl oxygen cleavage mechanism, the O18 can be transferred from phenolic OH to the carboxylate group. Ester hydrolysis in option A does not involve transfer of oxygen from phenolic \(\mathrm{OH}\) to the carboxylate group. Therefore, the correct answer is: (B) Nucleophilic aromatic substitution, Base catalysed acyl oxygen cleavage mechanism

Key concepts

Nucleophilic Aromatic Substitution

Nucleophilic aromatic substitution is a key reaction in organic chemistry, allowing for the modification of aromatic compounds. A staple in the synthesis of pharmaceuticals and dyes, it involves the displacement of a leaving group by a nucleophile on an aromatic ring. The fascinating part of this reaction is its temporary disruption of the ring's aromaticity for the nucleophile to add and the subsequent restoration of aromaticity once the leaving group is defected.

Typically, good leaving groups in this mechanism are those like a halide or a nitro group that can stabilize the negative charge in the transition state. The nucleophile, such as a hydroxide ion, often attacks the carbon atom that's been activated by electron-withdrawing substituents. This process is usually facilitated by the use of a strong base, which can deprotonate the nucleophile, increasing its reactivity.

Base-catalyzed Acyl Oxygen Cleavage

Base-catalyzed acyl oxygen cleavage is particularly important in the context of breaking down esters and amides. For students dealing with this concept, imagine it as a process where the acyl-oxygen bond is cleaved, and the electron-rich oxygen is allowed to adopt a more stable state.

This mechanism starts with a base, like hydroxide, attacking the carbonyl carbon, subsequently pushing electrons up onto the oxygen, and then back down to expel the leaving group, often resulting in the formation of a carboxylate anion and an alcohol, or sometimes a ketone or aldehyde. These reactions are not only important in synthetic laboratories but also in biological systems where esters are frequently cleaved during metabolism.

Ester Hydrolysis

Ester hydrolysis is a classic example of how nature and chemists break down complex molecules. It involves slicing an ester into a carboxylic acid and an alcohol, a reaction reminiscent of common biological processes such as digestion.

In aqueous solutions with a hint of acid or base, esters can undergo hydrolysis. Under acidic conditions, the acid acts as a catalyst, protonating the carbonyl oxygen which makes the carbonyl carbon more electrophilic and susceptible to attack by water. The reaction's base-catalyzed counterpart entails a hydroxide ion attacking the carbonyl carbon directly. This diversity in mechanism allows for hydrolysis under a range of conditions, making it a versatile tool for chemists.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution, beloved for constructing carbon rings with a variety of functional groups, is where an electrophile replaces a hydrogen atom on an aromatic ring. This is a cornerstone of organic synthesis, crafting molecules for materials, drugs, and more.

The process involves the generation of an electrophile, which could be a positively charged ion or an atom with a partial positive charge. Curiously, the normally unreactive aromatic ring is activated by this electrophile, leading to the formation of a carbocation intermediate. Stability returns with the loss of a proton, reinstating aromaticity. This mechanism is crucial for students who aspire to design complex aromatic compounds in their future scientific endeavors.