Friedel–Crafts Acylation

• Aromatic Stability: Benzene is exceptionally stable due to its delocalized $\pi$ electron system. This stability means it does not undergo addition reactions (which would break the delocalization) but instead undergoes substitution to preserve the aromatic ring.

• Lewis Acid Catalysis: The $AlCl_3$ catalyst acts as a Lewis acid by accepting a lone pair of electrons from the chlorine atom of the acyl chloride. This polarization weakens the $C-Cl$ bond, eventually leading to the formation of a highly reactive acylium ion ($R-C^+=O$).

• Electrophilicity: The acylium ion is a potent electrophile because the carbon atom carries a full positive charge. This charge makes it sufficiently attractive to the high electron density of the benzene ring to initiate an attack.

Step 1: Generation of the Electrophile

• The acyl chloride reacts with the $AlCl_3$ catalyst to form the reactive electrophile and a complex ion.
• Equation: $RCOCl + AlCl_3 \rightarrow RCO^+ + AlCl_4^-$

Step 2: Electrophilic Attack

• Two electrons from the delocalized $\pi$ system of the benzene ring move to form a covalent bond with the $RCO^+$ electrophile.
• This creates a carbocation intermediate (often called a Wheland intermediate or sigma complex) where the delocalization is temporarily broken, represented by a 'horseshoe' shape inside the ring with a positive charge.

Step 3: Regeneration of Aromaticity

• The $AlCl_4^-$ ion removes a proton ($H^+$) from the carbon atom that was attacked. The electrons from the $C-H$ bond move back into the ring to restore the stable delocalized $\pi$ system.
• Final Products: The aromatic ketone, $HCl$ gas, and the regenerated $AlCl_3$ catalyst.

• It is vital to distinguish between Acylation and Alkylation to choose the correct synthetic route.

• Acylation is often preferred because the resulting carbonyl group is electron-withdrawing, which 'deactivates' the ring and prevents further substitutions, ensuring a cleaner product yield.

• The Horseshoe Rule: When drawing the mechanism, ensure the open end of the 'horseshoe' in the intermediate faces the carbon atom where the substitution is occurring. The horseshoe must span at least four carbon atoms to represent the remaining delocalization.

• Arrow Precision: Curly arrows must start exactly from the delocalized ring (the circle or a double bond) and point directly to the positive carbon of the electrophile. In the final step, the arrow must start from the $C-H$ bond and point back into the ring.

• Catalyst Regeneration: Always include the final step where $AlCl_4^-$ reacts with $H^+$ to show that $AlCl_3$ is a catalyst and not a reagent. Forgetting to show the regeneration of $AlCl_3$ and the formation of $HCl$ is a common way to lose marks.

• Moisture Sensitivity: A common error is failing to specify anhydrous conditions. $AlCl_3$ reacts violently with water to form $Al(OH)_3$ and $HCl$, which destroys its catalytic ability.

• Reaction Type Confusion: Students often mistake this for an addition reaction. Remember that addition would result in the loss of the circle/delocalized system in the final product, which is energetically unfavorable for benzene.

• Electrophile Identity: Ensure the positive charge in the electrophile is placed on the carbon atom, not the oxygen. The carbon is the center that bonds to the benzene ring.