What is Friedel Crafts Alkylation and Acylation Mechanism Examples and Key Differences
Friedel-Crafts Alkylation and Acylation is essential in chemistry and helps students understand various practical and theoretical applications related to this topic.
These reactions are key types of electrophilic aromatic substitution and play a crucial role in naming, making, and modifying aromatic compounds that appear in daily life, and, industrial syntheses.
What is Friedel-Crafts Alkylation and Acylation in Chemistry?
A Friedel-Crafts Alkylation or Acylation refers to reactions where an aromatic compound (like benzene) reacts with either an alkyl halide or acyl halide in the presence of a Lewis acid catalyst, most commonly aluminium chloride (AlCl3).
This concept appears in chapters related to electrophilic aromatic substitution, nomenclature of organic compounds, and reaction mechanisms, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
The molecular formula involved in Friedel-Crafts reactions depends on the specific reactants used. Typical formulas include C6H6 (benzene), RX (alkyl halide), or RCOCl (acyl halide), with the catalyst being AlCl3.
The products are either alkylbenzenes (in alkylation) or acylbenzenes (in acylation), classified as aromatic hydrocarbons and ketones, respectively.
Preparation and Synthesis Methods
In the laboratory and industries, Friedel-Crafts alkylation is carried out by reacting benzene (or another aromatic hydrocarbon) with an alkyl halide in presence of a dry Lewis acid catalyst (like AlCl3).
For Friedel-Crafts acylation, the process uses an acyl chloride instead of an alkyl halide, again with a Lewis acid. Large-scale preparations are essential for making detergents, pharmaceuticals, dyes, and plastics.
Physical Properties of Friedel-Crafts Products
The main products are alkylated and acylated aromatic hydrocarbons. Alkylbenzenes are colorless liquids or solids, non-polar, less dense than water, and have characteristic aromatic odors.
Acylbenzenes (like acetophenone) are usually ketones, have higher boiling points than hydrocarbons, and are also aromatic in nature.
Chemical Properties and Reactions
Both Friedel-Crafts alkylation and acylation are classic examples of electrophilic aromatic substitution. In alkylation, an alkyl group replaces a hydrogen atom on the aromatic ring, producing alkylbenzenes; in acylation, an acyl group substitutes a hydrogen, giving aryl ketones.
Acylation is not prone to further substitution while alkylation products can undergo polyalkylation due to activation of the ring.
Frequent Related Errors
- Thinking vinyl or aryl halides can be used (they cannot; their carbocations are not stable).
- Forgetting that formyl chloride (HCOCl) decomposes and cannot give aldehyde by Friedel-Crafts.
- Mistaking acylation for reduction—reduction is needed to convert acylated products to alkylbenzenes.
- Ignoring the effect of deactivating groups (like NO2, SO3H, COOH) which prevent Friedel-Crafts reactions.
- Assuming every Lewis acid has the same reactivity or selectivity as AlCl3.
Uses of Friedel-Crafts Alkylation and Acylation in Real Life
Friedel-Crafts reactions are widely used in the manufacture of perfumes, detergents, and pharmaceuticals (hydrocarbons). Acetophenone and cumene are key ingredients produced this way.
These reactions are also used to create high-octane fuels, flavors, and industrial feedstock chemicals, making them vital to the chemical and petrochemical industries.
Relation with Other Chemistry Concepts
Friedel-Crafts alkylation and acylation is closely related to topics such as nitration of benzene, electrophilic aromatic substitution, and Gattermann-Koch reaction. Understanding these reactions helps to visualize the reactivity of aromatic compounds, build on the idea of electron-donating and withdrawing groups, and develop overall skills in organic reaction mechanisms.
Step-by-Step Reaction Example
- Start with the reaction setup.
For alkylation: C6H6 (benzene) + CH3Cl (methyl chloride) in presence of AlCl3. - Write the balanced equation.
C6H6 + CH3Cl →[AlCl3]→ C6H5CH3 + HCl - Explain each intermediate or by-product.
AlCl3 generates a methyl cation (CH3+) which attacks the benzene ring. Aluminium chloride is regenerated at the end of the reaction. - For acylation: C6H6 + CH3COCl (acetyl chloride) in presence of AlCl3 gives C6H5COCH3 (acetophenone) and HCl.
Lab or Experimental Tips
Remember Friedel-Crafts reactions by the rule: "Always use anhydrous AlCl3 and keep the reaction vessel dry." Vedantu educators often use the tip "Acylation prevents further substitution, but alkylation does not" to simplify this concept for students. Benzene must be used in excess for alkylation to avoid polyalkylation.
Try This Yourself
- Write the IUPAC name of the product from benzene + propanoyl chloride in Friedel-Crafts acylation.
- Predict if nitrobenzene will react under Friedel-Crafts alkylation.
- Give two real-life aromatic compounds made by Friedel-Crafts reactions.
Final Wrap-Up
We explored Friedel-Crafts alkylation and acylation—their definitions, differences, chemical equations, common mistakes, and real-world applications.
For more in-depth explanations and exam-prep tips, explore live classes and notes on Vedantu, where these reactions are often explained with interactive mechanisms, concept maps, and practice questions.
For deeper learning, check out: Electrophilic Aromatic Substitution, Gattermann-Koch Reaction, and Hydrocarbons.
FAQs on Friedel Crafts Alkylation and Acylation in Aromatic Chemistry
1. What is Friedel–Crafts alkylation?
Friedel–Crafts alkylation is an electrophilic aromatic substitution reaction in which an alkyl group is introduced into an aromatic ring using an alkyl halide and a Lewis acid catalyst such as AlCl3. It forms a new carbon–carbon bond between the aromatic ring and the alkyl group.
- General reaction: C6H6 + CH3Cl → C6H5CH3 + HCl (in presence of AlCl3)
- Electrophile: carbocation (R+) generated from alkyl halide
- Type of reaction: electrophilic aromatic substitution (EAS)
2. What is Friedel–Crafts acylation?
Friedel–Crafts acylation is an electrophilic aromatic substitution reaction that introduces an acyl group (–COR) into an aromatic ring using an acyl chloride and a Lewis acid such as AlCl3. It forms an aromatic ketone.
- General reaction: C6H6 + CH3COCl → C6H5COCH3 + HCl (in presence of AlCl3)
- Electrophile: acylium ion (RCO+)
- Product: aromatic ketone (e.g., acetophenone)
3. What is the difference between Friedel–Crafts alkylation and acylation?
Friedel–Crafts alkylation introduces an alkyl group (–R), whereas Friedel–Crafts acylation introduces an acyl group (–COR) into an aromatic ring.
- Reagent: Alkyl halide (R–Cl) vs acyl chloride (RCOCl)
- Electrophile: Carbocation (R+) vs acylium ion (RCO+)
- Rearrangement: Possible in alkylation; not observed in acylation
- Poly-substitution: Common in alkylation; rare in acylation due to deactivating acyl group
4. What catalyst is used in Friedel–Crafts reactions?
Aluminum chloride (AlCl3) is the most commonly used Lewis acid catalyst in Friedel–Crafts alkylation and acylation reactions.
- Acts as a Lewis acid by accepting an electron pair
- Generates the electrophile (R+ or RCO+)
- Other catalysts: FeCl3, BF3
5. What is the mechanism of Friedel–Crafts alkylation?
The mechanism of Friedel–Crafts alkylation involves electrophile formation, attack on the aromatic ring, and restoration of aromaticity.
- Step 1: Formation of carbocation: R–Cl + AlCl3 → R+ + AlCl4-
- Step 2: Electrophilic attack on benzene forming a sigma complex
- Step 3: Deprotonation restores aromaticity and regenerates catalyst
6. Why does rearrangement occur in Friedel–Crafts alkylation?
Rearrangement occurs in Friedel–Crafts alkylation because unstable carbocations rearrange to form more stable carbocations before attacking the aromatic ring.
- Primary carbocations may rearrange to secondary or tertiary
- Hydride or alkyl shifts can occur
- Example: A primary carbocation may shift to form a more stable tertiary carbocation before substitution
7. Why does Friedel–Crafts acylation not show rearrangement?
Friedel–Crafts acylation does not show rearrangement because the acylium ion (RCO+) is resonance-stabilized and does not rearrange.
- Acylium ion structure: R–C≡O+ ↔ R–C+=O
- Resonance stabilization prevents carbocation shifts
- Gives predictable ketone products
8. What are the limitations of Friedel–Crafts reactions?
Friedel–Crafts reactions have limitations such as polyalkylation, carbocation rearrangement, and failure with strongly deactivated aromatic rings.
- Do not occur with strongly electron-withdrawing groups (e.g., –NO2)
- Amines (–NH2) form complexes with AlCl3
- Alkylation can lead to multiple substitutions
9. Can you give an example of Friedel–Crafts acylation reaction?
A common example of Friedel–Crafts acylation is the reaction of benzene with acetyl chloride to form acetophenone.
- Balanced reaction: C6H6 + CH3COCl → C6H5COCH3 + HCl (in presence of AlCl3)
- Product: acetophenone (an aromatic ketone)
- Type: electrophilic aromatic substitution
10. Why is Friedel–Crafts reaction important in organic chemistry?
Friedel–Crafts reactions are important because they form new carbon–carbon bonds and are widely used to synthesize substituted aromatic compounds.
- Used in preparation of alkylbenzenes and aromatic ketones
- Important in pharmaceutical, dye, and polymer industries
- Key method in electrophilic aromatic substitution chemistry
