Draw the major organic product of the following Friedel-Crafts alkylation. (An excess of benzene is present.) The drawn product would require the formation of a rather unstable primary carbocation intermediate. The aluminum chloride polarizes the alkyl halide’s chlorine-carbon bond. Consider the possibility of a rearrangement reaction.
The Correct Answer And EXplanation is
To address this Friedel–Crafts alkylation problem, let’s walk through the full process carefully, especially focusing on the carbocation rearrangement, which is a crucial feature in such reactions.
Reaction Setup:
A Friedel–Crafts alkylation typically involves:
- Aromatic compound: Benzene (C₆H₆), in excess here.
- Alkyl halide: Let’s assume the alkyl halide is 1-chlorobutane (CH₃CH₂CH₂CH₂Cl).
- Lewis acid catalyst: AlCl₃, which facilitates the formation of a carbocation from the alkyl halide.
Step-by-Step Reaction Mechanism:
- Activation of Alkyl Halide:
- Aluminum chloride (AlCl₃), a strong Lewis acid, coordinates with the chlorine atom in the alkyl halide.
- This polarization weakens the C–Cl bond, promoting cleavage and carbocation formation.
- Carbocation Formation:
- Initially, a 1° (primary) carbocation would form: CH₃CH₂CH₂CH₂⁺.
- But this 1° carbocation is highly unstable.
- Rearrangement occurs: a hydride shift from the adjacent carbon occurs (specifically, from the β-carbon, forming a more stable carbocation).
- Carbocation Rearrangement:
- A 1,2-hydride shift from carbon 2 to carbon 1 converts the primary carbocation into a more stable 2° (secondary) carbocation: CH₃CH⁺CH₂CH₃ (sec-butyl cation).
- Electrophilic Aromatic Substitution:
- The benzene ring acts as a nucleophile and attacks the more stable secondary carbocation.
- This leads to the formation of sec-butylbenzene as the major product.
Final Answer:
The major product is sec-butylbenzene (C₆H₅CH(CH₃)CH₂CH₃).
Explanation Summary (300+ words):
The Friedel–Crafts alkylation is a classic electrophilic aromatic substitution (EAS) reaction that installs an alkyl group onto an aromatic ring, such as benzene. This reaction requires a Lewis acid catalyst (commonly AlCl₃) and an alkyl halide. In this case, we are working with 1-chlorobutane as the alkyl halide.
When AlCl₃ reacts with the alkyl halide, it polarizes the C–Cl bond, making the carbon more electrophilic. This facilitates the cleavage of the bond, leading to the formation of a carbocation. The initial carbocation formed from 1-chlorobutane would be a primary (1°) carbocation, which is highly unstable and unlikely to exist for long. As a result, a rearrangement occurs almost immediately to form a more stable intermediate.
The rearrangement in this scenario is a 1,2-hydride shift, where a hydrogen atom with its bonding electrons moves from the β-carbon to the carbocation center. This stabilizes the carbocation by moving the positive charge to a secondary (2°) carbon—one that is bonded to two other carbon atoms—which is more stable than a primary (1°) one. This intermediate is known as the sec-butyl cation.
The benzene ring then reacts with this carbocation in a typical electrophilic substitution mechanism. The π electrons of benzene attack the carbocation, forming a new C–C bond. Subsequent loss of a proton restores aromaticity, resulting in the final product: sec-butylbenzene.
Thus, although a primary carbocation would be expected from the alkyl halide, the reaction proceeds through a carbocation rearrangement, leading to a more stable carbocation and a different substitution product. This explains why sec-butylbenzene is the correct major product, not n-butylbenzene.