Predict the products (if any) of the following reactions.
a. (excess) benzene + isobutyl chloride + AlCl₃

Verified step by step guidance

1. Step 1: Recognize the reaction type. This is a Friedel-Crafts alkylation reaction, where an alkyl halide reacts with an aromatic compound (benzene) in the presence of a Lewis acid catalyst (AlCl3 or FeCl3).
2. Step 2: Understand the role of the catalyst. FeCl3 acts as a Lewis acid, accepting a lone pair from the chlorine atom in isobutyl chloride, forming a carbocation intermediate.
3. Step 3: Analyze the carbocation formation. Isobutyl chloride forms a primary carbocation initially, but due to carbocation rearrangement, it will rearrange to a more stable tertiary carbocation.
4. Step 4: Predict the electrophilic substitution. The tertiary carbocation acts as the electrophile and reacts with benzene, leading to the formation of an alkylbenzene product. The benzene ring donates electrons to the carbocation, forming a sigma complex, which then loses a proton to restore aromaticity.
5. Step 5: Consider the excess benzene. Since benzene is in excess, the reaction will likely produce a mono-substituted product (tert-butylbenzene) rather than polysubstitution, as steric hindrance prevents further substitution.

Key Concepts

Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. In this process, the aromatic system retains its stability while allowing for the introduction of new substituents. The reaction typically involves the formation of a sigma complex, followed by deprotonation to restore aromaticity.

Lewis Acids and Catalysis

Lewis acids, such as AlCl3, are compounds that can accept an electron pair from a Lewis base, facilitating reactions by generating more reactive electrophiles. In the context of EAS, AlCl3 activates alkyl halides like isobutyl chloride, allowing them to form a more potent electrophile that can effectively react with the aromatic ring. This catalytic role is crucial for enhancing the reaction rate and product yield.

Regioselectivity in Substitution Reactions

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In EAS reactions, the nature of the substituents already present on the aromatic ring influences where new substituents will attach. For example, alkyl groups are generally ortho/para-directing, meaning they favor substitution at the ortho or para positions relative to themselves.