7.1 Limitations of Friedel - Crafts Reactions

1.1 A Resonance Picture of Benzene
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule

2.1 A General Mechanism for Electrophilic Aromatic Substitution

7.1 Limitations of Friedel - Crafts Reactions

8.1 Donation of electrons into a benzene ring by resonance
8.2 Withdrawal of electrons from a benzene ring by resonance

9.1 Directive influence of the groups during substitutions in benzene ring
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents

10.1 Halogenation of the side chain
10.2 Oxidation of side chain

Several restrictions limit the usefulness of Friedel-Crafts reactions.

1. When the carbocation formed from an alkyl halide, alkene, or alcohol can rearrange to a more stable carbocation, it usually does so, and the major product obtained from the reaction is usually the one from the more stable carbocation.

2. An aromatic ring less reactive than that of halobenzene don’t undergo Friedal Craft’s reaction. Aromatic ring containing - $N{H_2}$ , $-NHR,\; - N{R_2},$ groups does not undergo friedal craft’s alkylation due to formation of anilinum complex which is meta directing and has more electron withdrawing power than halogen in benzene ring.

3. Aryl and Vinylic halides cannot be used as the halide component because they do not form carbocations readily.

4. Polyalkylations often occur - Alkyl groups are electron releasing groups, and once one is introduced into the benzene ring it activates the ring towards further substitution.