Problem 80 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

The treatment of benzene with isobatene in the presence of sulphuric acid gives: [AIIMS 2003] (a) isobutyl benzene (b) terf-butyl benzene (c) \(n\)-butyl benzene (d) no reaction

Step-by-Step Solution

Answer

(b) tert-butyl benzene

Step 1: Understanding the Reactants

In this reaction, benzene is treated with isobutene in the presence of sulfuric acid. Sulfuric acid acts as a catalyst in this reaction.

Step 2: Analyzing the Reaction

The reaction involves the formation of a carbocation from isobutene, which is a type of alkene. In the presence of sulfuric acid, isobutene can form a tertiary carbocation by the shift of a hydrogen atom, making it more stable.

Step 3: Determining the Carbocation Intermediate

Isobutene, in the presence of sulfuric acid, forms a tertiary carbocation. This is because the methyl shift leads to a more stable carbocation.

Step 4: Electrophilic Aromatic Substitution

The formed tertiary carbocation acts as an electrophile and attacks the benzene ring, substituting one of its hydrogen atoms. This results in the formation of tert-butyl benzene.

Step 5: Conclusion

The electrophilic substitution takes place primarily at the para position but can also occur at the ortho position. The major product formed is tert-butyl benzene.

Key Concepts

Carbocation Stability

Carbocation stability plays a pivotal role in chemical reactions, especially in those involving alkenes. A carbocation is a positively charged carbon atom with only three bonds instead of four. This positive charge can be unstable, depending on the surrounding atoms or groups. In general, carbocations are more stable when they have more alkyl groups attached. This is due to the electron-donating effects of these groups, which help to spread out the positive charge.

In this particular reaction with isobutene and benzene, a carbocation is formed. Sulfuric acid helps this process by allowing the hydrogen bond shift. The shift in alkenes like isobutene often results in more stable carbocations.

The stability sequence for carbocations is primary (less stable) < secondary < tertiary (most stable). Because tertiary carbocations are surrounded by three alkyl groups, they can best accommodate the positive charge. Therefore, a tertiary carbocation is preferred, making the substitution reaction more likely to proceed.

Tertiary Carbocation

A tertiary carbocation is a type of carbocation where the positively charged carbon is bonded to three other carbon groups. This structure provides optimal stability compared to primary and secondary carbocations. If you imagine the positive charge needing support, the nearest friends, the alkyl groups, provide it.

During the reaction between benzene and isobutene using sulfuric acid, a tertiary carbocation is crucial. Isobutene undergoes a hydrogen shift, resulting in a structure where the positive charge lands on a carbon surrounded by three other carbon atoms. This is now a stable tertiary carbocation and acts as the electrophile.

Electrophiles are electron-loving species, so a stable carbocation will eagerly interact with the electron-rich benzene. Its stability ensures that it won't easily dissipate before it performs its part in the reaction, which sets the stage for the next phase.

Substitution Reaction

A substitution reaction involves replacing an atom or group in a molecule with another atom or group. In electrophilic aromatic substitution, an electrophile replaces a hydrogen atom on the benzene ring. This reaction can lead to various products, depending on the conditions and the electrophile involved.

In the case of benzene and isobutene, the tertiary carbocation formed acts as the electrophile. This positively charged species is crucial as it can attract and form a sigma bond with the electron-rich benzene ring.
This type of substitution occurs because benzene has a high electron density, making it a target for electrophiles. Once the carbocation gets close to benzene, it can kick off a hydrogen atom and take its place.

In this reaction, the major product after substitution is tert-butyl benzene, showing the preferential attack at the para position. These reactions enhance our understanding of organic synthesis and various industrial applications.