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: What the question asks:
We need the reagent that Friedel-Crafts acylation uses to put an acyl group on benzene. Option (A) is an acid chloride, (B) an ester, (C) a carboxylic acid and (D) a ketone.

Step 2: Think about the leaving group:
The Lewis acid $AlCl_3$ needs a halogen to grab. Only $R-CO-Cl$ has a halogen on the carbonyl carbon, so it alone loses $Cl^-$ (as $AlCl_4^-$) and leaves the electrophile $R-C^+=O$.

Step 3: Check the rest:
The ester, acid and ketone carry $-OCH_3$, $-OH$ and $-CH_3$ on the carbonyl carbon. None of these leaves easily with $AlCl_3$, and $-OH$ or $-OCH_3$ would just coordinate with the catalyst and weaken it. So they are not used.

Step 4: Overall reaction:
Benzene plus the acid chloride gives an aryl ketone: \[ C_6H_6 + RCOCl \xrightarrow{AlCl_3} C_6H_5COR + HCl \]

Final Answer:
Option (A), the acid chloride $R-CO-Cl$, is the acylating agent. \[ \boxed{R-CO-Cl\ \text{(A)}} \]