Friedel-Crafts alkylation reactions electrophiles

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

Carbonyl compounds, particularly aromatic aldehydes, when activated with electrophilic catalysts, can also react with aromatics.241 The process is often called condensation or reductive alkylation, but it is actually a multistep Friedel-Crafts alkylation reaction. 

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,... 

Phenols are such good nucleophiles that protonated carbonyl compounds functionalize two phenol molecules. The first phenol molecule reacts in an Ar-SE reaction by the carboxonium ion formed in an equilibrium reaction. Subsequently, the second equivalent of phenol becomes the substrate of a Friedel-Crafts alkylation. The electrophile is the benzyl cation that is formed from the initially obtained benzyl alcohol and the acid. Protonated acetone is only a weak electrophile for electronic and steric reasons it contains two electron-donating and relatively large... 

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. 

O The electrophilic carbon of a protonated formaldehyde molecule adds to phenol in a Friedel-Crafts alkylation reaction. The hydroxy... 

The Friedel-Crafts alkylation reaction is an electrophilic aromitk substitution in which the electrophile is a carbocation, R Aluminum chh> ride catalyzes the reaction by helping the a Ikyl haUde to ionize in muck th-same way that PeBr., catalyzes aromatic brooiinations by potarizu B iSecrion loss of a proton than completes the reaction, as shows <... 

The alkyl carbocations formed from alkyl halides, aikenes and alcohols act as an electrophile in Friedel-Crafts alkylation reactions. ... 

Friedel-Crafts Alkylation Reactions. The activation of glyoxylate esters,trifluoromethyl pyruvate esters, and unsaturated a-ketoesters by catalyst 2 converts these materials into effective electrophiles for asymmetric Friedel-Crafts alkylation reactions with activated arenes (eqs 16 and 17). In fact, bis(triflate) (2) is far superior to tbe bis(hexafluoroantimonate) complex at catalyzing the enantioselective alkylation of benzene derivatives. Aniline and anisole derivatives both give the reaction, as do heterocyclic aromatic compounds such as indole and furan. 

Mechanism of the Friedel-Crafts alkylation reaction. The electrophile is a carbocation generated by AlCij-assisted ionization of an aikyi halide. 

In Section 16.3, we will see that an alkyl-substituted benzene is more reactive than benzene. Therefore, to prevent further alkylation of the alkyl-substituted benzene, a large excess of benzene is used in Friedel-Crafts alkylation reactions. This approach ensures that the electrophile is more likely to encounter a molecule of benzene than a molecule of alkyl-substituted benzene. 

In 1877, Charles Friedel and James M. Crafts discovered new methods for the preparation of alkylbenzenes, known as Friedel-Crafts alkylation reactions. The mechanism includes an electrophilic aromatic substitution whereby a carbocation is generated as the electrophile in the presence of a Lewis acid catalyst. The general scheme of F-C alkylation reaction is (16) as follows ... 

Like Friedel-Crafts alkylation reactions, the corresponding acylation reactions require the presence of Lewis or Bronsted acids. Unlike the alkylation process, acylations usually require more than stoichiometric quantities of the acid and the electrophilic reaction partner is usually an expensive and hydrolytically unstable acid chloride. As a consequence, these Lewis and Bronsted acids require neutralization and subsequent expensive disposal. For instance, the conventional acylation of phenol with acetyl chloride consumes more than stoichiometric quantities of AICI3. Neutralizing this AICI3 requires land-filhng several pounds of... 

Ethylbenzene is also made from ethylene. This is a Friedel-Crafts alkylation reaction, a type of electrophilic aromatic substitution. 

Friedel-Crafts alkylation an electrophilic substitution reaction that puts an alkyl group on a benzene ring. 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

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)... 

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