Question

Which of the following species is used in Friedel - Craft reaction?

• A.

  

• B.

  

• C.

  

• D.

  

Hint:

Friedel-Crafts acylation needs an acyl chloride (or anhydride) with anhydrous AlCl3. Pick the option that has Cl on the carbonyl carbon.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Friedel-Crafts reactions attach a carbon group to a benzene ring using a Lewis acid catalyst such as anhydrous \(AlCl_3\). The four options are carbonyl compounds, each with the group R on the left of the C=O carbon and a different group on the right.

Step 2: Key Formula or Approach:
In Friedel-Crafts acylation, the attacking species is the acylium ion \(R-C^{+}=O\). It forms when \(AlCl_3\) pulls the chlorine off an acid chloride: \[ R-CO-Cl + AlCl_3 \to R-C^{+}=O + AlCl_4^{-} \]

Step 3: Identify the compound in each option:
Option (A) is \(R-CO-Cl\), an acyl chloride (acid chloride). Option (B) is \(R-CO-OCH_3\), an ester. Option (C) is \(R-CO-OH\), a carboxylic acid. Option (D) is \(R-CO-CH_3\), a ketone.

Step 4: Why the other options are wrong:
Chlorine is a good leaving group, so only the acid chloride gives the acylium ion easily. The ester has \(-OCH_3\) and the acid has \(-OH\), which bind to \(AlCl_3\) and do not leave readily. The ketone has a \(-CH_3\) group, which cannot leave at all. So these three are not used as acylating agents.

Step 5: Product:
The acylium ion attacks the benzene ring and gives an aromatic ketone: \[ C_6H_6 + R-CO-Cl \xrightarrow{\text{anhyd. } AlCl_3} C_6H_5-CO-R + HCl \]

Final Answer:
The species used in the Friedel-Crafts reaction is the acid chloride \(R-CO-Cl\), which is option (A). \[ \boxed{R-CO-Cl\ \text{(Option A)}} \]