Ch. 18 - Reactions of Aromatics: EAS and BeyondSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Advantages of Friedel-Crafts Acylation

See all sections
Sections
Electrophilic Aromatic Substitution
Benzene Reactions
EAS: Halogenation Mechanism
EAS: Nitration Mechanism
EAS: Friedel-Crafts Alkylation Mechanism
EAS: Friedel-Crafts Acylation Mechanism
EAS: Any Carbocation Mechanism
Electron Withdrawing Groups
EAS: Ortho vs. Para Positions
Acylation of Aniline
Limitations of Friedel-Crafts Alkyation
Advantages of Friedel-Crafts Acylation
Blocking Groups - Sulfonic Acid
EAS: Synergistic and Competitive Groups
Side-Chain Halogenation
Side-Chain Oxidation
Birch Reduction
EAS: Sequence Groups
EAS: Retrosynthesis
Diazo Replacement Reactions
Diazo Sequence Groups
Diazo Retrosynthesis
Nucleophilic Aromatic Substitution
Benzyne
Additional Practice
EAS: Sulfonation Mechanism
EAS: Gatterman–Koch Reaction
EAS: Total Benzene Isomers
EAS: Polycyclic Aromatic Hydrocarbons
EAS: Directing Effects
Resonance Theory of EAS Directing Effects
Activated Benzene and Polysubstitutions
Clemmensen Reduction
EAS: Dueling Benzenes
Hydrogenation of Benzene
EAS: Missing Reagent
EAS: Synthesis
Diazonization of Aniline
Diazo Coupling Reactions
SNAr vs. Benzyne
Aromatic Missing Reagent
Aromatic Synthesis
Aromatic Retrosynthesis
EAS on 5-membered Heterocycles
Johnny Betancourt

Friedel-Crafts acylation has a few advantages over Friedel-Crafts alkylation and uses a Lewis acid catalyst and an acyl chloride to add an acyl group to benzene. The ketones produced can be reduced to alkyl groups using Clemmensen reduction. 

Acylation vs alkylation

Say you want to get to n-propylbenzene. How would you do it? Could you use Friedel-Crafts alkylation? Nope! The carbocation that would result would rearrange from a primary carbocation to a secondary like in the mechanism below: 

Friedel-Crafts alkylation mechanismFriedel-Crafts alkylation mechanism

Notice that the substituent added is actually an isopropyl instead of a straight-chain propyl group because of the hydride shift. Additionally, the added alkyl group would actually activate the benzene to react further and add more isopropyl groups. Let’s actually look at this chart to see the relative disadvantages of Friedel-Crafts alkylation:

Limitation

Alkylation

                Acylation                

No reaction with deactivated benzene

Aniline complexes               

Polysubstitution


Carbocation rearrangements  


*Another limitation is that acylation doesn't work with vinyl or aryl halides.

Okay, so we know that our direct and obvious limitation in this case is the carbocation rearrangement. Friedel-Crafts acylation is not subject to carbocation rearrangement, so let’s try that instead: 

Friedel-Crafts acylation mechanismFriedel-Crafts acylation mechanism

Let’s break this down step by step. The chloride first dissociates to form a bond with the AlCl3 to form the acylium ion (the carbocation on the carbonyl carbon) which does not rearrange. The benzene then attacks the cation, and the chlorine removes a hydrogen to restore aromaticity. 

That ketone makes that substituent an electron-withdrawing group, making the benzene deactivated, so the benzene will only be monoacylated. From there, all that’s left is to remove that carbonyl by using Clemmensen reduction (zinc amalgam in HCl) or Wolff-Kishner reduction



Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.