Q.Name the electrophile produced in the reaction of benzene with benzoyl chloride in the presence of anhydrous $AlCl_3$. Name the reaction also.

The key idea here is Friedel-Crafts acylation of benzene. Benzoyl chloride ($C_6H_5COCl$) reacts with benzene in the presence of anhydrous $AlCl_3$ (a Lewis acid catalyst) to form benzophenone.

Reasoning steps:

1. Anhydrous $AlCl_3$ coordinates to the chlorine atom of benzoyl chloride, polarising the $C-Cl$ bond and generating a highly reactive acylium ion.
2. The acylium ion formed is $C_6H_5-\overset{+}{C}=O$ (the benzoyl cation), which acts as the electrophile. …

The question asks for two things: the name of the reaction and the identity of the electrophile. Let’s build the understanding from the ground up.

1. What kind of reaction is this?

Benzene is an aromatic ring — it’s electron-rich but not reactive enough to attack most electrophiles on its own. To substitute a hydrogen on benzene with an acyl group ($\ce{-COR}$), we need a strong electrophile. The classic method is Friedel–Crafts acylation, where an acyl chloride (here benzoyl chloride, $\ce{C6H5COCl}$) reacts with benzene in the presence of a Lewis acid catalyst like anhydrous $\ce{AlCl3}$.

2. How is the electrophile generated?

The catalyst $\ce{AlCl3}$ is a Lewis acid — it accepts a lone pair from the chlorine atom of benzoyl chloride. This weakens the $\ce{C-Cl}$ bond, and the chloride ion leaves as $\ce{AlCl4-}$. What remains is a positively charged species:

The cation $\ce{C6H5CO+}$ is called the acylium ion (specifically, the benzoylium ion here). It is resonance-stabilised:

This delocalisation makes it a stable, effective electrophile.

3. What happens next? …

Concept: Friedel–Crafts Acylation of Benzene

This is an electrophilic aromatic substitution reaction where an acyl group is introduced onto the benzene ring.

Method: Friedel–Crafts Acylation

Step 1 — Formation of the electrophile

Anhydrous $AlCl_3$ (a Lewis acid) reacts with benzoyl chloride ($C_6H_5COCl$). The $AlCl_3$ abstracts a chloride ion, generating the acylium ion (also called benzoylium ion).

Step 2 — Electrophilic attack

The acylium ion acts as the electrophile and attacks the benzene ring, forming a resonance-stabilized carbocation intermediate (sigma complex).

Step 3 — Restoration of aromaticity …

Here are the common mistakes students make on this question, along with how to avoid each one.

1. Mistake: Naming the wrong electrophile (e.g., $Cl^+$ or $AlCl_3$)

• Why it happens: Students often confuse the Friedel-Crafts acylation mechanism with alkylation. In alkylation, the electrophile is often a carbocation (or a polarized complex). Here, they might think $AlCl_3$ just "makes" $Cl^+$ or that $AlCl_3$ itself attacks the ring.
• How to avoid: Remember the role of $AlCl_3$ — it is a Lewis acid catalyst. It does not become the electrophile. Its job is to help generate the acylium ion from the acyl chloride.
  • The actual electrophile is the benzoylium ion (or benzoyl cation): $\ce{C6H5CO+}$.
  • Key check: The electrophile must have a positive charge on the carbon attached to the benzene ring (the carbonyl carbon). Draw the mechanism: $AlCl_3$ pulls off the $Cl$ from $\ce{C6H5COCl}$, leaving $\ce{C6H5CO+}$.

2. Mistake: Naming the reaction incorrectly (e.g., "Friedel-Crafts alkylation")

• Why it happens: Both reactions use $AlCl_3$ and a benzene ring, so students mix them up. The presence of a carbonyl group ($\ce{C=O}$) in the reagent is the giveaway.
• How to avoid: Look at the reagent name: benzoyl chloride contains a carbonyl group. If the reagent is an acyl chloride ($\ce{RCOCl}$), the reaction is Friedel-Crafts acylation.
  • Mnemonic: "Acyl" = carbonyl = acylation. "Alkyl" = no carbonyl = alkylation.
  • Correct answer: Friedel-Crafts acylation.

3. Mistake: Writing the electrophile as $\ce{C6H5COCl}$ (unchanged) or $\ce{C6H5CO+AlCl3-}$

• Why it happens: Students think the complex formed between $AlCl_3$ and benzoyl chloride is the attacking species. While the complex exists, the actual attacking species is the free acylium ion.
• How to avoid: Understand that the complex $\ce{C6H5COCl-AlCl3}$ is highly polarized and dissociates to give the acylium ion ($\ce{C6H5CO+}$). This ion is the true electrophile because it has a complete positive charge and is highly reactive.
  • Correct answer: Benzoylium ion ($\ce{C6H5CO+}$) or benzoyl cation.

4. Mistake: Forgetting to specify the "benzoyl" part (e.g., just writing "acylium ion")

• Why it happens: Students learn the general mechanism but fail to apply it to the specific reagent. …