Difference Between Friedel-Crafts Alkylation and Acylation (Table & Examples)
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: Mechanisms, Differences & Examples
1. What is the difference between Friedel-Crafts alkylation and Friedel-Crafts acylation?
Friedel-Crafts alkylation introduces an alkyl group to an aromatic ring using an alkyl halide and a Lewis acid catalyst (like AlCl3), while Friedel-Crafts acylation introduces an acyl group using an acyl chloride and AlCl3.
Key Differences:
- Alkylation may cause polyalkylation and carbocation rearrangement; acylation avoids these issues.
- Acylation products are generally more stable and cannot undergo further substitution easily.
- Reagents: Alkyl halide for alkylation, acyl chloride (or acid anhydride) for acylation.
2. What is the role of AlCl3 in Friedel-Crafts reactions?
AlCl3 acts as a Lewis acid catalyst in Friedel-Crafts reactions.
Main functions:
- It helps generate the electrophile—carbocation (for alkylation) or acylium ion (for acylation)—by reacting with the halide or acyl chloride.
- It increases the reactivity of the aromatic ring toward electrophilic substitution.
3. Why is polyalkylation a problem in Friedel-Crafts alkylation?
Polyalkylation occurs because the product formed after the first alkylation is more reactive than benzene itself.
Key Points:
- The alkyl group increases electron density, activating the ring for further alkylation.
- This can lead to multiple alkyl groups attaching, making it hard to control the reaction for a single product.
4. Can phenol undergo Friedel-Crafts alkylation and acylation?
Phenol generally does not undergo Friedel-Crafts alkylation or acylation under normal conditions.
Reasons:
- The –OH group in phenol forms a strong complex with AlCl3, deactivating the catalyst.
- This prevents the formation of the required electrophile for the reaction to proceed.
5. Give one application of Friedel-Crafts reaction in industry.
Friedel-Crafts reactions are widely used in industrial synthesis.
Example:
- Production of ethylbenzene, which is a key precursor in manufacturing styrene for making polystyrene plastics.
6. Why doesn't nitrobenzene undergo Friedel-Crafts alkylation or acylation?
Nitrobenzene does not undergo Friedel-Crafts alkylation or acylation because the nitro group is a strong electron-withdrawing group.
Main Effects:
- It deactivates the aromatic ring toward electrophilic substitution.
- The reaction is therefore highly unfavorable or does not occur under normal conditions.
7. What are the limitations of Friedel-Crafts alkylation?
Friedel-Crafts alkylation has several limitations:
- Polyalkylation (multiple alkyl groups adding to the ring)
- Carbocation rearrangement, leading to unexpected products
- Does not proceed with deactivated aromatic compounds (like nitrobenzene)
- Poor yields with strongly electron-withdrawing substituents on the ring
8. How do electron-donating and withdrawing groups affect Friedel-Crafts reactions?
Electron-donating groups (like –OH, –OCH3) activate the ring, making it more reactive toward Friedel-Crafts reactions.
Electron-withdrawing groups (like –NO2, –COOH) deactivate the ring, reducing or preventing reaction.
- Activation leads to faster and easier substitution.
- Deactivation may stop the reaction or give low yield.
9. Can other Lewis acids replace AlCl3 in Friedel-Crafts reactions?
Yes, other Lewis acids such as FeCl3, BF3, and AlBr3 can also be used as catalysts in Friedel-Crafts reactions.
AlCl3 is most commonly used due to its high effectiveness and availability, but the choice may vary based on the substrate and reaction conditions.
10. What are the main steps in the mechanism of the Friedel-Crafts acylation reaction?
The Friedel-Crafts acylation mechanism involves several key steps:
- Formation of the acylium ion (RCO+) by reaction of acyl chloride with AlCl3
- Electrophilic attack by the acylium ion on the aromatic ring to form a carbocation intermediate
- Loss of a proton to regenerate aromaticity, producing the acyl-substituted aromatic compound
- Regeneration of the AlCl3 catalyst
11. Are Friedel-Crafts reactions reversible? Under what conditions?
Friedel-Crafts reactions are generally not reversible under standard conditions.
Exceptions:
- Alkylation can be reversible under strong acidic conditions via dealkylation (cleavage of alkyl groups).
- Acylation products are typically stable and resist reversal due to the electron-withdrawing effect of the acyl group.
12. What type of questions are asked on Friedel-Crafts reactions in exams?
Common exam questions on Friedel-Crafts reactions include:
- Drawing and explaining mechanisms for alkylation and acylation
- Discussing limitations and applications
- Comparing alkylation and acylation in a table
- Predicting products with different substrates
- Short MCQs on reagents, catalyst, and mechanism steps
