Friedel-Crafts acid

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Friedel-Craft reaction Friedel-Crafts Friedel-Crafts acids Friedel-Crafts acylation... 

Friedel-Crafts acids such as AlCl, AIBr. (molten), or AIBr. (ia CS2 or low boiling hydrocarbon solvents) were found to be useful ia the... 

Brmnsted-Lewis Superacids. Conjugate Friedel-Crafts acids prepared from ptotic and Lewis acids, such as HCl—AlCl and HCl—GaCl ate, indeed, supetacids with an estimated value of —15 to —16 and ate effective catalysts in hydrocarbon transformation (217). 

In place of a proton source, ie, a Briimsted acid, a cation source such as an alkyl haUde, ester, or ether can be used in conjunction with a Friedel-Crafts acid. Initiation with the ether-based initiating systems in most cases involves the haUde derivative which arises upon fast haUdation by the Friedel-Crafts acid, MX (2). 

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

Initiation. A Friedel-Craft acid (hydrochloric acid, water, phenol) is used as initiator together with a proton source ( co-initiator , BF3 or AICI3 are the most common). The mixture produces a catiogen which is the true initiating species. 

For systems which have been established by rigorous high vacuum experiments not to require the separate addition of cationogen, the use of Friedel-Crafts acid alone is recommended, ie EtA1C12/t-C Hg16, HCU/K Hg30. ... 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

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. 

The activity of an initiating system is also affected by the nature of the Friedel-Crafts acid. The following Friedel-Crafts acidity scale can he established BF, < AlCI, < TiCI4 < BCI, - SbFx < SbCL . BBr,. The advantage of the TiCL and the aluminum-based systems is their relative insensivity toward solvent polarity. The activity or the BCIt- or BBrt-based system is greatly solvent-dependent, i.e.. sufficient activity only occurs in polar solvent. 

Although it Wits long believed that most Friedel-Crafts acids, particularly halides of boron, titanium, and tin. require an additional cation source to initiate polymerization, recent results show that in many systems Friedel-Crafts acids alone are able lo initiate cationic polymerization. 

Evidence supporting the formation of 1 1 addition compounds is further substantiated by the actual isolation of stable acyl cation salts (Chapter 3). Therefore, it is highly desirable to develop methods in which only a catalytic amount of Friedel-Crafts acid catalyst may be used for effective conversion. 

REACTIONS IN STRONG FRIEDEL-CRAFTS ACIDS PROCEED VIA DIFFERENT STABILIZED SPECIES... 

Although the formalism of cationation by alkyl halides was in principle correct, the significance of this mechanism for head group control could not be exploited because the conventional Friedel-Crafts acids... 

Scheme 1 shows the model and the corresponding table the definition of the symbols. This model is comprehensive as it encompasses two formally different but fundamentally similar mechanisms polymerizations induced by a purposely-added initiator (RX) shown on the left side of Scheme 1 and polymerizations induced by adventitious protic impurities HX on the right side of Scheme 1. The two sides of Scheme 1 are connected by two routes due to chain transfer to monomer these will be discussed below. The model is valid for Friedel-Crafts acid (MtX ) coinitiated polymerizations, including all kinds of conventional and living olefin and alkyl vinyl ether polymerizations.

The reader is reminded that in essentially all carbocationic polymerization induced by Friedel-Crafts acids (MtXn) described to date, the MtXn is always in stoichiometric excess relative to that erf initiator or protic impurities, and that the [MtX ]/[RX + HX ] ratio is usually in the five to 20 range. Some of the consequences of this circumstance have been discussed (1). 

The two large split loops originating from HM + MtX +1 and ascending respectively to H + MtX +1 and HM + MtXn+1, indicate the two posable chain transfer steps (the subscripts n, for example in HM and MtXn, are integers 1,2,. n, and express the degree of polymerization (as in DP ) when they are subscripts to M, or the number of halogens connected to the metal in the Friedel-Crafts acid when they are subscripts to MtX) the longer of the two... 

Irreversible termination is in principle absent in carbocationic polymerization coinitiated by perhalogenated Friedel-Crafts acids MtX . Reversible termination, however, exists in both impurity-induced or purposely-induced processes [43, 46, 47], Indeed, because of the recognition that irreversible termination does not occur in MtX -coinitiated carbocationic polymerizations, we have developed the concept of quasiliving polymerizations the focal point of which is reversible termination [1], The fundamentals of reversible termination and its consequences have been treated recently in great detail [18, 48, 49], and the reader is referred to these sources for further discussion on this subject. 

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