Waiting
Login-Verarbeitung ...

Trial ends in Request Full Access Tell Your Colleague About Jove

18.15: Limitations of Friedel–Crafts Reactions

TABLE OF
CONTENTS
JoVE Core
Organic Chemistry

Ein Abonnement für JoVE ist erforderlich, um diesen Inhalt ansehen zu können. Melden Sie sich an oder starten Sie Ihre kostenlose Testversion.

Education
Limitations of Friedel–Crafts Reactions
 
TRANSCRIPT

18.15: Limitations of Friedel–Crafts Reactions

Several restrictions limit the use of Friedel–Crafts reactions. First, the halogen in the alkyl halide must be attached to an sp3-hybridized carbon for the Friedel–Crafts reactions to occur. Vinyl or aryl halides do not react since the carbocations formed are unstable under the reaction conditions. Second, Friedel–Crafts alkylation is susceptible to carbocation rearrangement, and the major products obtained have a rearranged carbon skeleton. In contrast, the acylium ion is stabilized by resonance, so no carbocation rearrangement occurs in Friedel–Crafts acylation. Third, polyalkylation frequently occurs with the Friedel–Crafts alkylation reactions. An alkyl group is an activator, and when added to the benzene ring, it activates the ring towards further alkylation. Contrastingly, polyacylation does not occur in Friedel–Crafts acylation. The acyl group is a deactivator toward electrophilic substitution and does not promote further acylation of the benzene ring. Forth, both Friedel–Crafts alkylation and acylation reactions fail on benzene rings bearing either powerful electron-withdrawing groups or a basic amino group that can be protonated. Such substituents make the ring less reactive by making it electron-deficient, which deactivates it towards further electrophilic substitution.

Tags

Limitations Of Friedel-Crafts Reactions: 1. The Halogen In The Alkyl Halide Must Be Attached To An Sp3-hybridized Carbon For The Reaction To Occur. Vinyl Or Aryl Halides Do Not React. 2. Friedel-Crafts Alkylation Is Susceptible To Carbocation Rearrangement Resulting In Products With Rearranged Carbon Skeletons. 3. Polyalkylation Frequently Occurs In Friedel-Crafts Alkylation As The Alkyl Group Activates The Benzene Ring Towards Further Alkylation. 4. Polyacylation Does Not Occur In Friedel-Crafts Acylation As The Acyl Group Is A Deactivator Towards Electrophilic Substitution. 5. Friedel-Crafts Reactions Fail On Benzene Rings Bearing Powerful Electron-withdrawing Groups Or Basic Amino Groups That Can Be Protonated As These Substituents Make The Ring Less Reactive

Get cutting-edge science videos from JoVE sent straight to your inbox every month.

Waiting X
Simple Hit Counter