Question

How do you account for the fact that benzene in the presence of \(\mathrm{ALCl}_{3}\) reacts: (a) with n-propyl bromide to give isopropylbenzene; (b) with isobutyl bromide to yield tert-butylbenzene; (c) with neopentyl bromide to yield tert-pentylbenzene? (d) By which of the alternative mechanisms for the Friedel-Crafts reaction are these products probably formed?

Short answer

In the Friedel-Crafts alkylation of benzene, the Lewis acid catalyst AlCl3 polarizes the carbon-halogen bond in alkyl halides, making the carbon more electrophilic. Benzene reacts with n-propyl bromide, isobutyl bromide, and neopentyl bromide to form isopropylbenzene, tert-butylbenzene, and tert-pentylbenzene, respectively. These products are formed through a carbocation rearrangement mechanism, in which the primary carbocation rearranges to form a more stable secondary or tertiary carbocation, which then attacks the aromatic ring.

Step-by-step solution

General Friedel-Crafts Alkylation Reaction

In the Friedel-Crafts alkylation reaction, benzene (or other aromatic compounds) reacts with an alkyl halide in the presence of a Lewis acid catalyst such as AlCl3. The catalyst polarizes the carbon-halogen bond, making the carbon more electrophilic. The electrophilic carbon then reacts with the electron-rich aromatic ring, adding the alkyl group to the ring. Now, let's proceed to discuss each case.

Reaction with n-propyl bromide

(a) When benzene reacts with n-propyl bromide (C3H7Br) in the presence of AlCl3, the major product is isopropylbenzene. This happens because during the reaction, the primary carbocation (propyl carbocation) rearranges to a more stable secondary carbocation (isopropyl carbocation) through a hydride shift. The isopropyl carbocation then attacks the benzene ring, forming isopropylbenzene as the major product.

Reaction with isobutyl bromide

(b) When benzene reacts with isobutyl bromide (C4H9Br) in the presence of AlCl3, the major product is tert-butylbenzene. A similar carbocation rearrangement occurs in this case as well, with the secondary isobutyl carbocation rearranging to a more stable tertiary tert-butyl carbocation. The tert-butyl carbocation then attacks the benzene ring, forming tert-butylbenzene as the major product.

Reaction with neopentyl bromide

(c) When benzene reacts with neopentyl bromide (C5H11Br) in the presence of AlCl3, the major product is tert-pentylbenzene. Once again, a carbocation rearrangement occurs, with the primary neopentyl carbocation rearranging to the more stable tertiary tert-pentyl carbocation. The tert-pentyl carbocation then attacks the benzene ring, forming tert-pentylbenzene as the major product.

Alternative Mechanisms for the Friedel-Crafts Reaction

(d) The alternative mechanisms for the Friedel-Crafts reaction are carbocation rearrangement and attack on the aromatic ring. In all three cases we discussed, the products are likely formed through a carbocation rearrangement mechanism. The alkyl halide reacts with the catalyst to form a carbocation which then undergoes rearrangement to form a more stable carbocation. The stable carbocation then attacks the electron-rich benzene ring, resulting in the Friedel-Crafts alkylation product.

Key concepts

Benzene Reaction Mechanisms

Understanding how benzene reacts is crucial for predicting the products of its chemical reactions. One of the most common types of reactions involving benzene is the Friedel-Crafts alkylation. In this process, benzene undergoes a reaction with an alkyl halide, facilitated by a Lewis acid catalyst like AlCl3.

During the reaction, the electron-rich benzene acts as a nucleophile, attacking an electrophilic carbon atom of the alkyl halide that has been activated by the catalyst. The outcome of this reaction is highly influenced by the stability of the intermediary carbocation, which can rearrange to form a more stable species before solidifying its bond with the benzene ring. Students often struggle with understanding the principles behind these mechanisms, and it's essential to grasp the importance of the stability of intermediates and the capability of the aromatic system to preserve its resonance stability during the reaction.

This reaction mechanism falls under a broader class known as electrophilic aromatic substitution, where an electrophile selectively reacts with an aromatic compound, resulting in the substitution of an aromatic hydrogen atom. Benzene's reaction mechanisms allow chemists to modify benzene rings into more complex and functional molecules.

Lewis Acid Catalysts

Lewis acid catalysts, such as aluminum chloride (AlCl3), play a pivotal role in enabling reactions, like the Friedel-Crafts alkylation of benzene. These catalysts work by accepting a pair of electrons and, in this context, polarize the carbon-halogen bond of the alkyl halide.

The role of the Lewis acid is twofold: first, it increases the susceptibility of the alkyl halide to donate the alkyl group, and second, it helps stabilize the intermediate carbocation formed during the reaction. The stronger the Lewis acid, the more effective it is at catalyzing the reaction. However, students should note that the presence of a strong Lewis acid might also prompt further unwanted side reactions, such as carbocation rearrangement, which could lead to different products.

An intuitive approach for educators when explaining Lewis acids is to illustrate how they act as electron-pair acceptors and how this influences the reactivity of other species in the reaction mixture, particularly in electrophilic aromatic substitution processes.

Carbocation Rearrangement

Carbocation rearrangement is a common phenomenon in many chemical reactions, including the Friedel-Crafts alkylation. It occurs when an initially formed carbocation (often unstable) reorganizes its structure to form a more stable carbocation.

Stability is governed by the number of alkyl groups attached to the carbocation (tertiary being more stable than secondary, which is more stable than primary). Understanding rearrangements can be tricky for students, but it’s a critical concept for predicting the correct product of a reaction involving carbocations. In the context of benzene reactions, a primary alkyl group attached to a halide can form a primary carbocation that, due to its instability, quickly rearranges to a more stable secondary or tertiary carbocation before attacking the benzene ring.

This rearrangement is an entropy-driven process where the system moves towards a lower energy and more stable state. It underscores the principle that molecules tend to take the path of least resistance (or energy) in achieving their final form.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a reaction where an electrophile replaces a hydrogen atom on an aromatic ring. It is a cornerstone of aromatic chemistry and involves two main steps: the generation of an electrophile, which is facilitated by a Lewis acid catalyst, and the attack of the electrophile on the aromatic ring followed by restoration of the aromatic system.

In the context of the Friedel-Crafts alkylation, the electrophile is a carbocation generated from an alkyl halide. EAS reactions are characterized by the aromatic ring's high electron density, which attracts electrophilic species. The reaction proceeds through a cyclic intermediate known as a 'sigma complex' or 'arenium ion,' wherein the original resonance-stabilized structure of the aromatic ring is temporarily disrupted. After the substitution, the ring regains its aromaticity, releasing a proton in the process.

For educational purposes, it’s beneficial to focus on the regioselectivity and the directing effects of existing substituents on the aromatic ring, as they influence the position at which the incoming electrophile will attach to the ring. The EAS mechanism forms the basis for many synthetic transformations applied in the production of various compounds, from medicines to polymers.