Alkylation of Aromatic Rings The Friedel-Crafts Reaction

Alkylation of Aromatic Rings The Friedel-Crafts Reaction 

Charles Friedel (1832-1899) was born in Strasbourg. France, and studied at the Sorbonne 

One of the most useful of all electrophilic aromatic substitution reactions is alkylation, the attachment of an alkyl group to the benzene ring. Charles Friedel and James Crafts reported in 1877 that benzene rings can be alkylated by reaction with an alkyl chloride in the presence of aluminum chloride as catalyst. For example, benzene reacts with 2-chloropropaneand AICI3 to yield isopropylbenzene, also called cumene. 

Though broadly useful for the synthesis of alkylbenzenes, the Friedel-Crafts alkylation nevertheless has strict limitations. One limitation is that only alkyl halides can be used. Alkyl fluorides, chlorides, bromides, and iodides all react well, but aryl halides and vinylic halides do not react, Ary and vinylic carbocations are too high in energy to form under Friedel-Crafts conditions. 

A second limitation is that the Friedel-Crafts reaction doesn t succ 

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Bromination of Aromatic Rings 593 Other Aromatic Substitutions 597 Alkylation of Aromatic Rings The Friedel-Crafts Reaction Acylation of Aromatic Rings 604 Substituent Effects in Substituted Aromatic Rings 605 An Explanation of Substituent Effects 610 Trisubstituted Benzenes Additivity of Effects... 

Alkylation and Acylation of Aromatic Rings The Friedel-Crafts Reaction 555... 

ALKYLATION AND ACYLATION OF AROMATIC RINGS THE FRIEDEL-CRAFTS REACTION JJl... 

The alkylation of aromatic rings, called Friedel-Crafts alkylation, is a reaction of... 

The alkylation of aromatic rings, called Friedel-Crafts alkylation, is a reaction of very broad scope. The most important reagents are alkyl halides, alkenes, and... 

The Friedel-Crafts reaction is a very important method for introducing alkyl substituents on an aromatic ring. It involves generation of a carbocation or related electrophilic species. The most common method of generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The usual Friedel-Crafts catalyst for preparative work is AICI3, but other Lewis acids such as SbFj, TiC, SnCl4, and BF3 can also promote reaction. Alternative routes to alkylating ecies include protonation of alcohols and alkenes. 

Among the most useful electrophilic aromatic substitution reactions In the laboratory is alkylation—the introduction of an alkyl group onto the benzene ring. Called the Friedel-Crafts reaction after its discoverers, the reaction is carried out... 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

The most important means of introducing an alkyl group into an aromatic ring is the Friedel-Crafts reaction. In its simplest form, this is the reaction of an alkyl halide (halogenoalkane) with an aromatic compound, such as benzene, in the presence of a Lewis acid, commonly aluminium chloride (Scheme 3.1). 

One of the most important modifications of the Friedel-Crafts reaction involves the use of acid chlorides rather than alkyl halides. An acyl group, RCO—, becomes attached to the aromatic ring, thus forming a ketone the process is called acylation. As usual for the Friedel-Crafts reaction (Sec. 12.8), the aromatic ring undergoing substitution must be at least as reactive as that of a halobenzene catalysis by aluminum chloride or another Lewis acid is required. 

Benzene rings may also be alkylated, thereby adding a carbon side chain to the ring. This is the Friedel-Crafts reaction, and is commonly performed by reacting an alkyl halide with the aromatic compound in the presence of a Lewis acid, such as A1C13. Suggest how the electrophile is formed. 

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