Question

tert-Pentylbenzene is the major product of the reaction of benzene in the presence of \(\mathrm{BF}_{3}\) with each of the following alcohols; (a) 2 -methyl-1-butanol, (b) 3 -methyl-2-butanol, (c) 3 -methyl-1-butanol, and (d) neopentyl alcohol. Account for its formation in each case.

Short answer

The Friedel-Crafts alkylation reaction of benzene with each of the given alcohols (2-methyl-1-butanol, 3-methyl-2-butanol, 3-methyl-1-butanol, and neopentyl alcohol) in the presence of \(BF_{3}\) leads to the formation of the same major product, tert-Pentylbenzene, due to carbocation rearrangements. The rearrangements shift the positive charge to the most stable tertiary carbon in each case. These carbocations then undergo electrophilic substitution with benzene, resulting in the formation of tert-Pentylbenzene regardless of the different starting alcohols.

Step-by-step solution

Understand the Reaction Involved

Friedel-Crafts alkylation is the reaction of benzene with an alcohol in the presence of \(BF_{3}\) which acts as a Lewis acid. The first step is the formation of carbocation which is promoted by \(BF_{3}\).

Reaction of 2-methyl-1-butanol with \(BF_{3}\)

When 2-methyl-1-butanol reacts with \(BF_{3}\), the alcohol group is protonated and leaves as a water molecule, forming a primary carbocation at the carbon atom from which OH has left. However, this carbocation can rearrange as a hydride shift from the neighbouring carbon causes migration of the positive charge to form a more stable tertiary carbocation, which forms the tert-Pentylbenzene after bonding with benzene.

Reaction of 3-methyl-2-butanol and 3-methyl-1-butanol with \(BF_{3}\)

The reactions of 3-methyl-2-butanol and 3-methyl-1-butanol parallel that of 2-methyl-1-butanol. Protonation of the alcohol group leads to the formation of a secondary and primary carbocation respectively, which undergo rearrangement to form the same tertiary carbocation and thus the same product, tert-Pentylbenzene.

Reaction of Neopentyl Alcohol with \(BF_{3}\)

Neopentyl alcohol has a branched chain and the carbocation formed after protonation of the alcohol group and leaving of water molecule, also undergoes rearrangement to form the more stable tertiary carbocation, which then combines with benzene leading to the formation of tert-Pentylbenzene.

Generalization

Despite the differing structures of the alcohols used, their reaction with \(BF_{3}\) and benzene lead to the formation of the same product due to carbocation rearrangements. The rearrangements occur to move the carbocation to the most stable carbon, which is usually the tertiary carbon in these alcohols. The carbocations then undergo electrophilic substitution with benzene to form the final product, tert-Pentylbenzene.

Key concepts

Carbocation Rearrangement

During complex organic reactions like the Friedel-Crafts alkylation, an often-encountered phenomenon is the carbocation rearrangement. This process entails the migration of alkyl groups or hydrides within a molecule, with the aim of forming a more stable carbocation. For instance, when we consider alcohols that can generate a primary carbocation in the presence of a strong Lewis acid such as \(BF_{3}\), the positively charged carbon atom often finds itself in an energetically unfavorable position.

However, nature prefers stability, so the molecule undergoes a rearrangement. This could involve the shift of a hydride (a hydrogen atom along with its bonding electrons) or an alkyl group from a neighboring carbon to the carbocation, transforming it into a secondary or tertiary carbocation, which are more stable due to additional alkyl substituents that provide electron-donating effects through hyperconjugation and inductive effects.

Importance of Stability

The concept of stability is crucial because reactions that lead to more stable intermediates are more favorable, which is why a hydrocarbon chain will rearrange to form a tertiary carbocation whenever possible. These rearrangements can drastically change the course of a reaction and dictate the identity of the final product in synthetic chemistry.

tert-Pentylbenzene Formation

tert-Pentylbenzene, a specific type of alkylbenzene, is the end goal of the reactions discussed in our problem. Formation of this compound is illustrative of how the right conditions can steer a reaction towards a particular product. Initially, alcohols such as 2-methyl-1-butanol, 3-methyl-2-butanol, 3-methyl-1-butanol, or neopentyl alcohol, when introduced to \(BF_{3}\), generate their corresponding carbocations. Regardless of the primary structure of these alcohols, their carbocations undergo rearrangements turning primary or secondary carbocations into a more stable tertiary form.

After stabilization, each forms the same tertiary carbocation which reacts with benzene to yield tert-Pentylbenzene. This underscores the predictability of chemical processes; despite varying starting materials, through controlled steps, we can guide the reaction to produce a specific and desired alkylated benzene derivative.

Role of Lewis Acids

It’s the presence of a strong Lewis acid like \(BF_{3}\) that sets the stage for tert-Pentylbenzene formation, acting as a catalyst that both propels the generation of the initial carbocation and enables its ultimate attachment to the benzene ring.

Electrophilic Substitution

The term 'electrophilic substitution' describes a reaction where an electrophile, a species seeking electron density, replaces a substituent on an aromatic compound. In the context of tert-Pentylbenzene formation, electrophilic substitution plays a pivotal role. The tertiary carbocation, a potent electrophile due to its positive charge, interacts with benzene, a molecule with a rich pi electron cloud that acts as a nucleophile.

Benzene donates electron density to the carbocation, forming a bond, and in turn, one of its hydrogen atoms is abstracted to restore its stable aromatic nature. This interplay is fundamental in the synthesis of various aromatic compounds within organic chemistry and showcases the versatility of benzene as a substrate for myriad chemical transformations.

Relevance to Synthesis

Understanding electrophilic substitution is vital not just for laboratories but also for industrial chemistry, as it is used in synthesizing dyes, pharmaceuticals, and other aromatic compounds. The concept is simple but profound — an aromatic ring like benzene serves as a host for new functional groups, dramatically altering the chemical properties and potential applications of the molecule.