Friedel-Crafts Reactions Flashcards
What are Friedel-Crafts reactions used for in organic chemistry?
They are synthetic processes used to functionalize aromatic compounds, specifically for alkylation and acylation of benzene.
Who developed the Friedel-Crafts reactions and in what year?
Charles Friedel and James Crafts developed them in 1877.
What reagents are required for a Friedel-Crafts alkylation reaction?
An alkyl halide (R-Cl) and a Lewis acid catalyst like AlCl₃.
What is the general equation for Friedel-Crafts alkylation?
Benzene + R-Cl + AlCl₃ → Alkylbenzene + HCl
(requires heat around 30°C)
Describe the mechanism of Friedel-Crafts alkylation.
- R-Cl reacts with AlCl₃ to form R⁺ (carbocation) and AlCl₄⁻
- R⁺ electrophile attacks the benzene ring
- The intermediate loses a proton to regenerate aromaticity
- Final product: alkylbenzene
Why are two products sometimes formed during Friedel-Crafts alkylation?
Due to rearrangement of the R⁺ carbocation before reacting with benzene.
What is a key disadvantage of Friedel-Crafts alkylation?
- The alkyl group activates the benzene ring, causing multiple substitutions.
- Carbocation rearrangement can lead to unexpected products.
How can you reduce disubstitution in Friedel-Crafts alkylation?
Use an excess of benzene during the reaction to minimize multiple substitutions.
What is the stability order of carbocations in Friedel-Crafts alkylation?
Tertiary > Secondary > Primary
Can Friedel-Crafts alkylation occur with alkenes? If yes, how?
Yes, in the presence of HCl and AlCl₃, alkenes can form carbocations that react with benzene.
Name two industrial applications of Friedel-Crafts alkylation.
- Production of cumene (used to make phenol and acetone)
- Production of styrene via dehydrogenation
What are the major and minor products when benzene reacts with 1-chlorobutane using AlCl₃?
Major: sec-butylbenzene (rearranged carbocation)
Minor: n-butylbenzene
What product is formed when benzene reacts with 1-chloro-2,2-dimethylpropane using AlCl₃?
Only one product: tert-butylbenzene, due to stable tertiary carbocation.
What is a Friedel-Crafts acylation?
Substitution of a hydrogen atom on an aromatic ring by an acyl group using an acid chloride and AlCl₃.
What is the preferred method for synthesizing aryl ketones and why?
Friedel-Crafts acylation because it avoids rearrangement and provides clean products like phenyl ethyl ketone.
What are the steps in the Friedel-Crafts acylation mechanism?
- Acyl chloride reacts with AlCl₃ to form the acylium ion (R-C≡O⁺)
- Acylium ion reacts with benzene to form a resonance-stabilized intermediate
- Intermediate loses H⁺ to reform aromaticity
Predict the major products for the reaction:
Benzene + CH₃CH₂CH(CH₃)CH₂CH₂Cl + AlCl₃
Major product: sec-pentylbenzene (due to more stable carbocation)
Minor product: primary pentylbenzene
Predict the major product from the following reaction steps:
Benzene + CH₃CH₂CH(CH₃)COCl + AlCl₃
Zn/Hg, HCl
- Friedel-Crafts acylation forms aryl ketone
- Clemmensen reduction removes carbonyl, forming the alkylbenzene
