Alkyl halides aromatics + Lewis acids

On the other hand, there is additional evidence (of a kind we cannot go into here) that makes it very likely that there is a second mechanism for Friedel-Crafts alkylation. In this mechanism, the electrophile is not an alkyl carbonium ion, but an acid-base complex of alkyl halide and Lewis acid, from which the alkyl group is transferred in one step from halogen to the aromatic ring. 

Aromatic rings are usually not good enough nucleophiles to react with alkyl halides. If Lewis acids are added to improve the electron sink, then electrophilic aromatic substitution occurs via the carbocation if it is more stable than secondary (an SnI viewed from the electrophile). With Lewis acids and methyl, primary, or secondary alkyl halides, electrophilic aromatic substitution usually occurs via direct displacement of the leaving group (an Sn2 viewed from the electrophile). 

Friedel-Crafts alkylations can also be done using alkenes instead of alkyl halides. A Lewis acid is not needed, but a mineral acid is required. Treatment of the alkene with the acid leads to a carbocation that can then react with an aromatic ring by the electrophilic substitution mechanism already described (Following fig.). For an alkene, this is another example of electrophilic addition where a proton is attached to one end of the double bond and a phenyl group is added to the other. 

Friedel-Crafts Alkylation (Section 22.1 C) The electrophile is a carbocation formed as an ion pair by interaction of an alkyl halide with a Lewis acid. Rearrangements from a less stable carbocation to a more stable carbocation are common. The mechanism involves an initial reaction between the alkyl halide and Lewis acid AICI3 to )deld an intermediate that can be thought of as a carbocation, AlCl " ion pair. The carbocation portion of the ion pair reacts as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the final product. Because carbocations are involved in the mechanism, rearrangements can be a problem, especially with... 

Electrophiles for aromatic substitution include the halonium ion, the nitronium ion and the carbonium ion. The latter may be generated from alkyl and acyl halides using Lewis acid catalysis in the Friedel-Crafts reactions. 

The Lewis acid may be AICI3, BF3, HF, etc. RX represents an alkyl halide, alcohol, or alkene, but not an aryl halide. The Lewis acid functions by generating a carbonium ion (R+) from RX, which subsequently attacks the electron rich aromatic (benzene) ring to give the... 

Alkyl halides by themselves are insufficiently electrophilic to react with benzene Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophihcity of the alkylating agent With tertiary and secondary alkyl halides the addition of aluminum chlonde leads to the formation of carbocations which then attack the aromatic ring... 

FRIEDEL - CRAFTS Alkylation-Acylation Alkylation or acylation ol aromatic compounds by means of alryl halides, alcohols.alkenes, acyl halides in the presence of Lewis acids... 

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. 

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. 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

The Friedel-Crafts alkylation of aromatic compounds with alkyl halides in the presence of Lewis acid is well defined in organic chemistry. However, alky-... 

The actual proportions of products obtained in many cases are not necessarily found to reflect the relative stabilities of the incipient carbocations, unrearranged and rearranged, however. This follows from the fact that their relative rates of reaction with the aromatic species almost certainly do not follow the order of their relative stabilities, and may well be diametrically opposed to it. Attack on the aromatic species by the first formed polarised complex may be faster than its rearrangement. The study of these rearrangements is also complicated by the fact that Lewis acids are found to be capable of rearranging both the original halides, and the final, alkylated end-products, e.g. ... 

Introduction of an alkyl group onto an aromatic substrate by treating the substrate with an alkylating agent such as alkyl halide, alkene, alkyne and alcohol in the presence of a Lewis acid. 

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 carbon atom of alkyl halides, R—Hal, is electrophilic, but rarely is it sufficiently so to effect the substitution of aromatic species the presence of a Lewis acid catalyst, e.g. AlHalj is also required. That alkyl halides do react with Lewis acids has been demonstrated by the exchange of radioactive bromine into EtBr from AlBr on mixing and re-isolation also the actual isolation of solid 1 1 complexes, e.g. CHjBr-AlBrj, at low temperatures (-78°). These complexes, though polar, are only faintly conducting. Where R is capable of forming a particularly stable carbocation, e.g. with McsC—Br, it is probable that the attacking electrophile in alkylation is then the actual carbocation, Me3C , as part of an ion pair ... 

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. 

Frequently substantially more than catalytic amounts of a Lewis acid metal halide are required to effect Friedel-Crafts alkylation. This is due partly to complex formation between the metal halide and the reagents or products, especially if they contain oxygen or other donor atoms. Another reason is the formation of red oils. Red oils consist of protonated (alkylated) aromatics (i.e., arenium ions) containing metal halides in the counterions or complexed with olefin oligomers. This considerable drawback, however, can be eliminated when using solid acids such as clays,97 98 zeolites (H-ZSM-5),99,100 acidic cation-exchange resins, and perfluoro-alkanesulfonic acid resins (Nafion-H).101-104... 

A considerable difference between Friedel-Crafts alkylation and acylation is the amount of the Lewis acid necessary to induce the reaction. Friedel-Crafts alkylation requires the use of only catalytic amounts of the catalyst. Lewis acids, however, form complexes with the aromatic ketones, the products in Friedel-Crafts acylations, and the catalyst is thus continuously removed from the system as the reaction proceeds. To achieve complete conversion, therefore, it is necessary to use an equimolar amount of Lewis acid catalyst when the acylating agent is an acyl halide. Optimum yields can be obtained using a 1.1 molar excess of the catalyst. With... 

Sulfonylation of aromatic hydrocarbons in the presence of a Lewis acid and the reaction of sodium benzenesulfinate with alkyl halides proved to be particularly easy and useful to prepare starting materials for the Julia olefination procedure (see Section 4.3.2). 

---