Question

The treatment of benzene with isobutene in the presence of sulphuric acid gives: (a) Isobutyl benzene (b) n-butyl benzene (c) tert-butyl benzene (d) Cumene

Short answer

tert-butyl benzene

Step-by-step solution

Understanding the Reaction

This reaction involves the treatment of benzene with isobutene in the presence of sulfuric acid. Sulfuric acid acts as a catalyst, facilitating electrophilic aromatic substitution reactions. Isobutene, a type of alkene, will form a carbocation intermediate under these conditions.

Formation of the Carbocation

When isobutene is in the presence of sulfuric acid, a proton (H+) from sulfuric acid will add to the double bond, resulting in the formation of an isobutyl carbocation. This is a key intermediate in the reaction.

Electrophilic Attack on Benzene

The formed isobutyl carbocation acts as an electrophile and attacks the benzen ring. This type of reaction is known as Friedel-Crafts alkylation. However, due to the stability preference of carbocations, the isobutyl cation rearranges to a more stable carbocation before the attack occurs.

Rearrangement to a Tert-butyl Carbocation

The isobutyl carbocation will undergo a rearrangement to form a more stable tert-butyl carbocation. This is because a tertiary carbocation is more stable than a secondary one. This rearranged, more stable carbocation will then facilitate the reaction with benzene.

Formation of tert-Butyl Benzene

The tert-butyl carbocation then attacks the benzene ring at one of the carbon atoms, resulting in the introduction of a tert-butyl group to the benzene. The result is the formation of tert-butyl benzene as the major product.

Key concepts

Friedel-Crafts Alkylation

Friedel-Crafts Alkylation is a common method to introduce alkyl groups to an aromatic ring. In this process, an electrophile is generated, usually from an alkyl halide and a Lewis acid catalyst like AlCl₃, which then attacks the benzene ring.
In our scenario, iso-butene forms a carbocation with the aid of sulfuric acid and acts as the electrophile. This reaction is a key example of electrophilic aromatic substitution where the benzene ring donates electrons to bond with the electrophile.

• The reaction enhances the substitution on the benzene, maintaining the aromaticity in the intermediate states.
• It usually proceeds with significant reactivity and selectivity due to the nature of the carbocation.

The Friedel-Crafts alkylation presents a versatile pathway for modifying benzene, though care must be observed to avoid polyalkylation and ensure the stability of the intermediates.
By targeting the correct conditions, tert-Butyl benzene becomes the major product in the given exercise.

Carbocation Rearrangement

Carbocation rearrangement is a notable phenomenon during electrophilic aromatic substitution reactions. When an alkylation process occurs, sometimes the initial carbocation formed may not be the most stable. In these cases, the carbocation will rearrange itself to reach a more stable configuration.
Such was the case with the isobutyl carbocation in our example; it transformed into a tert-butyl carbocation.

• This rearrangement typically involves a hydride or alkyl shift, moving the positive charge to a more stable carbon.
• The stability of carbocations generally increases from primary to secondary to tertiary structures.
• In our example, isobutyl (secondary) rearranges to tert-butyl (tertiary), providing a better electrophile for the benzene ring.

This process is vital for achieving the desired product given the context of substitutions. It assures that the final compound is electronically and structurally balanced with maximum stability.

Alkene Chemistry

Alkene Chemistry is essential in understanding the behavior of isobutene in reactions like Friedel-Crafts alkylation. Alkenes, containing a C=C bond, are reactive due to their high electron density. In the presence of a strong acid, such as sulfuric acid, alkenes will undergo protonation, forming a carbocation.
This protonation step is crucial since it makes the alkene a participant in further chemical transformations such as substitution reactions.

• The proton adds to one of the carbon atoms in the double bond, converting the alkene into a more reactive carbocation.
• This carbocation can then act as an electrophile, ready to react with electron-rich species like benzene.
• Alkene reactions are driven by the transformation potential from the initial unsaturated complex to various other organic structures.

Understanding these properties of alkenes aids in grasping how isobutene acts as a precursor to different carbocations in the beyond-simplistic electrophilic aromatics reactions.

Benzene Reactions

Benzene Reactions, particularly electrophilic aromatic substitutions, showcase benzene's unique reactivity despite its notorious stability. Benzene's electrons in the aromatic ring provide the capacity for substitution without losing its aromatic nature.
This stability arises from the delocalized π electrons, allowing benzene to act as a nucleophile.

• In electrophilic aromatic substitution, an electrophile like a carbocation will replace a hydrogen on the benzene ring.
• The reaction proceeds through a carbocation intermediate, briefly disrupting benzene's aromaticity before being restored in the final product.
• This process keeps benzene's core intact while transforming it into a more complex molecule.

The versatility and predictability of benzene reactions allow chemists to create diverse and complex structures, optimizing conditions for the formation of the desired product, such as tert-butyl benzene, while preserving benzene's stable, aromatic framework.