Friedel-Crafts alkylation, rearrangement during

All these kinetic results can be accommodated by a general mechanism that incorporates the following fundamental components (1) complexation of the alkylating agent and the Lewis acid (2) electrophilic attack on the aromatic substrate to form the a-complex and (3) deprotonation. In many systems, there m be an ionization of the complex to yield a discrete carbocation. This step accounts for the fact that rearrangement of the alkyl group is frequently observed during Friedel-Crafts alkylation. 

Transalkylation and Dealkylation. In addition to isomerizations (side-chain rearrangement and positional isomerization), transalkylation (disproportionation) [Eq. (5.56)] and dealkylation [Eq. (5.57)] are side reactions during Friedel-Crafts alkylation however, they can be brought about as significant selective hydrocarbon transformations under appropriate conditions. Transalkylation (disproportionation) is of great practical importance in the manufacture of benzene and xylenes (see Section 5.5.4) ... 

Carbocations can rearrange during the Friedel-Crafts alkylation reaction, leading to the formation of unpredicted products. One example is the formation of isopropyl benzene by the reaction of propyl chloride with benzene. 

Rearrangement often occurs during Friedel-Crafts alkylations using primary halides. 

Carbocation rearrangement during Friedel-Crafts alkylation (Section 16.3)... 

The present experiment demonstrates Friedel-Crafts alkylation and allows you to examine the following hypothesis The amount of alkyl rearrangement that occurs during a Friedel-Crafts alkylation of a series of arenes depends on the nucle-ophilicity of the arene, Ar-R. This hypothesis is derived as follows. The rate constant, ky is for a unimolecular process of 3 that does not involve the arene (Scheme 15.1), so its value is independent of the nucleophilicity of the arene. In contrast, 2 is the rate constant for a bimolecular reaction between 3 and the arene, so its value is dependent on the nucleophilicity of the arene. Substituents, R, that make the arene more nucleophilic yield higher values of the bimolecular rate constant k2, thereby increasing the overall rate of the electrophilic substitution. Consequently, formation of the product derived from rearrangement becomes a relatively less important reaction pathway. 

Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. 

Positional isomerization occurs similarly as during alkylation with alkyl halides. HF and H2S04, which are weaker catalysts than the conjugate Friedel-Crafts acids, however, do not bring about ready positional isomerization of the alkylated products. Rearrangement in the side chain always takes place before the attachment of the substituent to the aromatic ring when these catalysts are used. 

Tne rearrangement of a primary to a tertiary urb a )on during Friedel-Crafts reaction of benzene with 1-chloro-2.2-dlmethylpropane occurs by shift of an alkyl nup with its electron... 

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