Electrophilic aromatic substitution reactions Friedel-Crafts alkylation

Electrophilic Aromatic Substitution Reactions. Friedel-Crafts alkylation, acylation, and the Vilsmeier-Haack formylation, shown below, are excellent reactions for the synthesis of substituted aromatic compounds. 

The Friedel-Crafts alkylation is a classic illustration of the general class of electrophilic aromatic substitutions. Traditionally, Friedel-Crafts reactions require an alkyl halide as the electrophile source and at least a molar equivalent of aluminum chloride, a hygroscopic and caustic powder that is rather problematic to use in the introductory lab. An easy variant of this procedure utilizes the reactive substrate 1,4-dimethoxybenzene, /er butyl alcohol as the electrophile precursor, and sulfuric acid as the catalyst (72). We run this reaction on a microscale, and use commercial rubbing alcohol (70% aqueous 2-propanol) in place of methanol as the recystallization solvent. The product, l,4-di-7er/-butyl-2,5-dimethoxybenzene, exhibits simple and NMR, and... 

Other typical electrophilic aromatic substitution reactions—nitration (second entry) sul fonation (fourth entry) and Friedel-Crafts alkylation and acylation (fifth and sixth entnes)—take place readily and are synthetically useful Phenols also undergo elec trophilic substitution reactions that are limited to only the most active aromatic com pounds these include mtrosation (third entry) and coupling with diazomum salts (sev enth entry)... 

The synthesis of an alkylated aromatic compound 3 by reaction of an aromatic substrate 1 with an alkyl halide 2, catalyzed by a Lewis acid, is called the Friedel-Crafts alkylation This method is closely related to the Friedel-Crafts acylation. Instead of the alkyl halide, an alcohol or alkene can be used as reactant for the aromatic substrate under Friedel-Crafts conditions. The general principle is the intermediate formation of a carbenium ion species, which is capable of reacting as the electrophile in an electrophilic aromatic substitution reaction. 

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... 

Friedel-Crafts reaction (Section 16.3) An electrophilic aromatic substitution reaction to alkylate or acylate an aromatic ring. 

First introduced by Charles Friedel and James Crafts in 1877, the FC alkylation is an electrophilic aromatic substitution reaction where the electrophile is a carbocation, R. This carhocation is generated hy AICI3-catalysed ionization of alkyl halide. For example, benzene reacts with isopropylchloride in the presence of Lewis acid to produce isopropylbenzene. 

An alkyl group can be added to a benzene molecule by an electrophile aromatic substitution reaction called the Friedel-Crafts alkylation reaction. One example is the addition of a methyl group to a benzene ring. 

The Friedel-Crafts acylation reaction, another example of an electrophilic aromatic substitution reaction, is similar to the Friedel-Crafts alkylation reaction except that the substance that reacts with benzene is an acyl halide,... 

Friedel-Crafts type reactions of strongly deactivated arenes have been the subject of several recent studies indicating involvement of superelectrophilic intermediates. Numerous electrophilic aromatic substitution reactions only work with activated or electron-rich arenes, such as phenols, alkylated arenes, or aryl ethers.5 Since these reactions involve weak electrophiles, aromatic compounds such as benzene, chlorobenzene, or nitrobenzene, either do not react, or give only low yields of products. For example, electrophilic alkylthioalkylation generally works well only with phenolic substrates.6 This can be understood by considering the resonance stabilization of the involved thioalkylcarbenium ion and the delocalization of the electrophilic center (eq 4). With the use of excess Fewis acid, however, the electrophilic reactivity of the alkylthiocarbenium ion can be... 

Know the meaning of electrophilic aromatic substitution, halogenation, nitration, sulfonation, alkylation, acylation, Friedel-Crafts reaction. 

Two of the reactions that are used in the industrial preparation of detergents are electrophilic aromatic substitution reactions. First, a large hydrocarbon group is attached to a benzene ring by a Friedel-Crafts alkylation reaction employing tetrapropene as the source of the carbocation electrophile. The resulting alkylbenzene is then sulfonated by reaction with sulfuric acid. Deprotonation of the sulfonic acid with sodium hydroxide produces the detergent. 

Friedel-Crafts alkylation (Section 18.5A) An electrophilic aromatic substitution reaction in which benzene reacts with an aUcyl halide in the presence of a Lewis acid to give an alkyl benzene. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

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. 

Electrophilic aromatic substitution reactions are a very important class of chemical reactions that allow the introduction of substituents on to arenes by replacing a hydrogen atom covalently bonded to the aromatic ring structure by an electrophile. The most common reactions of this type are aromatic nitrations, halogenations, Friedel-Crafts alkylations and acylations, formylations, sulfonations, azo couplings and carboxylations - to name just a few. 

The Friedel-Crafts reaction involves an electrophilic aromatic substitution that facilitates the alkylation or acylation of arenes (135) and heterocyclic compounds catalyzed by acidic catalysts. Zinc oxide has been found to be an effective catalyst for the Friedel-Crafts acylation of activated and nonactivated aromatic compounds (135) (Hosseini-Sarvari and Sharghi 2004) under solvent-free and room temperature conditions (Scheme 9.44). The catalyst provides a large surface area for the reaction. This Friedel-Crafts reaction is a safe and environmentally benign method which requires simple workup, mild reaction conditions and a short reaction time. 

Friedel - Crafts alkylation is an electrophilic aromatic substitution reaction, in which an alkyl group is introduced in benzene ring, in the presence of anhydrous aluminium chloride... 

Reaction of an aromatic compound with a carbocation (called the Friedel-Crafts alkylation) affixes an alkyl group onto the aromatic ring. In this electrophilic aromatic substitution reaction, there are several methods for generating the carbocation, including from alkyl halides (RX plus FeXs, or AICI3), alcohols (ROH, BF3) and alkenes (alkene, acid). 

The Friedel—Crafts alkjiation reaction is an electrophilic aromatic substitution that attaches an alkyl group to the aromatic ring. It is named for Charles Friedel (1832-1899) and James M. Crafts (1839—1917), who discovered it by acddent when they tried to synthesize pentyl chloride from pentyl iodide through reaction with aluminum chloride in an aromatic solvent. Instead of the hoped-for chloride, substituted aromatic hydrocarbons appeared. The Friedel-Crafts reaction is closely related to bromination and chlorination of benzene. If we treat benzene with isopropyl bromide in the hope that a nucleophilic attack of benzene on isopropyl bromide will produce isopropylbenzene (cumene), we are sure to be disappointed (Fig. 14.34). Benzene is by no means a strong nucleophile and isopropyl bromide can scarcely be described as a powerful electrophile. The proposed reaction is utterly hopeless, and ftuls to give product. 

A Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction that attaches a carbon-carbon bond to the ring.The electrophile is R, which will add to the aromatic ring to produce a cyclohexadienyl cation. Aromaticity is regained when that intermediate cyclohexadienyl cation is deprotonated. That s all there is to it—all the rest is details. Remember to watch out for rearrangements, because this Friedel-Crafts alkylation is especially prone to them. In the Friedel-Crafts acylation, an acid chloride is used to generate the acylium ion which is the reactive electrophile. No rearrangements are observed in Friedel-Crafts acylation. 

Representative Electrophilic Aromatic Substitution Reactions of Benzene 457 Mechanistic Principles of Electrophilic Aromatic Substitution 458 Nitration of Benzene 459 Sulfonation of Benzene 461 Halogenation of Benzene 462 Biosynthetic Halogenation 464 Friedel-Crafts Alkylation of Benzene Friedel-Crafts Acylation of Benzene Synthesis of Alkylbenzenes by Acylation-Reduction 469 Rate and Regioselectivity in Electrophilic Aromatic Substitution 470 Rate and Regioselectivity in the Nitration ofToluene 472... 

The chemistry of aromatic compounds is dominated hy electrophilic aromatic substitution reactions, both in the laboratory and in biological pathways. Many variations of the reaction can be carried out, including halogenation, nitration, sulfonation, and hydroxylation. Friedel-Crafts alkylation and acylation, which involve reaction of an aromatic ring with carho-cation electrophiles, are particularly useful. 

Friedel-Crafts reactions are the slowest of the electrophilic aromatic substitution reactions. Therefore, if a benzene ring has been moderately or strongly deactivated— that is, if it has a meta-directing substituent—it will be too unreactive to undergo either Friedel-Crafts acylation or Friedel-Crafts alkylation. In fact, nitrobenzene is so unreactive that it is often used as a solvent for Friedel-Crafts reactions. 

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