Problem 45 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In the sulphonation, acetylation and formylation of benzene the group of effective electrophiles would be (a) \(\mathrm{SO}_{3}^{\oplus}, \mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{O}^{\oplus}, \mathrm{HCO}\) (b) \(\mathrm{SO}_{3}, \mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{O}, \mathrm{HCO}\) (c) \(\mathrm{SO}_{3}, \mathrm{CH}_{3} \mathrm{CHO}, \mathrm{CO}+\mathrm{HCl}\) (d) \(\mathrm{HSO}_{3}, \mathrm{CH}_{3} \mathrm{CO}, \mathrm{HCO}\)

Step-by-Step Solution

Answer

The effective electrophiles are in option (b): \(\mathrm{SO}_3\), \(\mathrm{CH}_3\mathrm{C}\equiv\mathrm{O}\), \(\mathrm{HCO}\).

Step 1: Identify the Electrophiles

In sulphonation, acetylation, and formylation reactions, the effective electrophiles are responsible for the attack on the benzene ring. These reactions involve specific groups that can accept an electron pair from benzene.

Step 2: Sulphonation Electrophile

For sulphonation, the effective electrophile is sulphur trioxide (\(\mathrm{SO}_3\)). It acts as a Lewis acid, accepting electrons to form sulphonated benzene.

Step 3: Acetylation Electrophile

During acetylation, the effective electrophile is the acylium ion, typically represented as \(\mathrm{CH}_3\mathrm{C}\equiv\mathrm{O}^+\) or in other cases as \(\mathrm{CH}_3\mathrm{C}\equiv\mathrm{O}\). This ion forms from an acid anhydride in the presence of a catalyst.

Step 4: Formylation Electrophile

In formylation reactions, the electrophile is the formyl cation \(\mathrm{HCO}^+\), often generated from reagents like carbon monoxide in a strong acid environment.

Step 5: Compare Options

Let's compare each option provided: - \(\mathrm{SO}_3\), \(\mathrm{CH}_3\mathrm{C}\equiv\mathrm{O}\), \(\mathrm{HCO}\) in option (b) is correct, as these correspond to the electrophiles for sulphonation, acetylation, and formylation respectively.- Other options either contain incorrect or incomplete forms of these electrophiles for the stated reactions.

Key Concepts

Sulphonation

In sulphonation, the electrophilic aromatic substitution reaction involves the introduction of a sulfonyl group into an aromatic ring such as benzene. The key electrophile responsible for this process is sulfur trioxide \((\text{SO}_3)\). This molecule acts as a Lewis acid, meaning it can accept a pair of electrons from the benzene ring. Typically, sulphonation is carried out using concentrated sulfuric acid, which is often composed of dissolved sulfur trioxide in sulfuric acid, known as `oleum`. This reaction is significant because it allows the introduction of a sulfonic acid group, which is an essential functional group in various industrial applications like detergents and dyes.
Sulphonation proceeds through the formation of an intermediate sigma complex, where the sulfur atom forms a bond with a carbon atom on the benzene ring. This is followed by a deprotonation step, resulting in a stable sulfonated aromatic compound. These reactions are reversible, and in some cases, the reverse reaction can be useful when removing the sulfonyl group is necessary.

Acetylation

Acetylation is another important electrophilic aromatic substitution reaction, where an acyl group is introduced into the aromatic ring. The effective electrophile in this process is the acylium ion, which can be represented as \(\text{CH}_3\text{C} \equiv \text{O}^+\) or \(\text{CH}_3\text{C} \equiv \text{O}\). This cation forms when acid anhydrides or acyl chlorides react with a Lewis acid catalyst, often aluminum chloride (\(\text{AlCl}_3\)).
The acylium ion is particularly reactive and readily attacks the electron-rich benzene ring to form a new carbon-carbon bond. This reaction is commonly known as the Friedel-Crafts acylation and is a key method for installing acyl groups into aromatic compounds. Products of this reaction are ketones, making it an important step in the synthesis of many pharmaceuticals and fragrances.

• Friedel-Crafts acylation provides control over multiple substitutions by avoiding unwanted polyacylations, which are common in the Friedel-Crafts alkylation.
• The reaction doesn't typically suffer from carbocation rearrangements, allowing for more predictable outcomes.

Formylation

Formylation involves the introduction of a formyl group \((\text{HCO}^+)\) into an aromatic compound, forming an aldehyde. The formyl cation serves as the effective electrophile in this reaction. However, due to its high reactivity and instability, it is typically generated in situ from other reagents like carbon monoxide and hydrogen chloride in the presence of a catalyst, such as aluminum chloride. This specific type of reaction is known as the Gattermann-Koch reaction.
Unlike other electrophilic substitution reactions, formylation is used less frequently due to its demanding conditions and potential side reactions. Yet, it remains an essential method for directly introducing aldehyde functionalities into aromatic rings, which is highly valuable in organic synthesis. The introduction of a formyl group adds versatility to a molecule because it can be further transformed into a variety of functional groups, such as alcohols or carboxylic acids. By understanding these key aspects of formylation, students can appreciate the complexities and strategic uses of this electrophilic substitution reaction in chemical synthesis.