Alkyl halides Friedel-Crafts reactions

The Friedel-Crafts Reaction, in which an aromatic hydrocarbon reacts with an alkyl halide under the influence of aluminium chloride ... 

Alkylation of benzene with alkyl halides m the presence of aluminum chloride was discovered by Charles Friedel and James M Crafts m 1877 Crafts who later became president of the Massachusetts Institute of Technology collaborated with Friedel at the Sorbonne m Pans and together they developed what we now call the Friedel-Crafts reaction into one of the most useful synthetic methods m organic chemistry... 

Another version of the Friedel-Crafts reaction uses acyl halides instead of alkyl halides and yields aryl ketones... 

Vinylic halides and aryl halides do not form carbocations under conditions of the Friedel-Crafts reaction and so cannot be used in place of an alkyl halide or an acyl halide... 

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. 

Carboncations also form from an alkyl halide when a Lewis acid catalyst is used. Aluminum chloride is the commonly used Friedel-Crafts alkylation catalyst. Friedel-Crafts alkylation reactions have been reviewed by Roberts and Khalaf ... 

Yet a final limitation to the Friedel-Crafts reaction is that a skeletal rearrangement of the alkyl carbocation electrophile sometimes occurs during reaction, particularly when a primary alkyl halide is used. Treatment of benzene with 1-chlorobutane at 0 °C, for instance, gives an approximately 2 1 ratio of rearranged (sec-butyl) to unrearranged (butyl) products. 

Problem 16.4 Which of the following alkyl halides would you expect to undergo Friedel-Crafts reaction without rearrangement Explain. 

To optimize the alkylation conditions, ferrocene was reacted with allyldimethyl-chlorosilane (2) in the presence of various Lewis acids such as aluminum halides and Group lO metal chlorides. Saturated hydrocarbons and polychloromethanes such as hexane and methylene chloride or chloroform were used as solvents because of the stability of the compounds in the Lewis acid catalyzed Friedel-Crafts reactions. The results obtained from various reaction conditions are summarized in Table IV. 

Apart from the alkyl halide-Lewis acid combination, two other sources of carbo-cations are often used in Friedel-Crafts reactions. Alcohols can serve as carbocation precursors in strong acids such as sulfuric or phosphoric acid. Alkylation can also be effected by alcohols in combination with BF3 or A1C13.37 Alkenes can serve as alkylating agents when a protic acid, especially H2S04, H3P04, and HF, or a Lewis acid, such as BF3 and A1C13, is used as a catalyst.38... 

Many important reactions involve catalysis by Lewis acids or bases. One of the most important of these is the type of reaction carried out by Charles Friedel and James Crafts. These reactions, known as the Friedel-Crafts reactions, actually involve several types of important processes. One of these is alkylation, which is illustrated by the reaction of benzene with an alkyl halide in the presence of A1C13, a strong Lewis acid. 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

The opposite of the stabilisation of an ester is its activation. If we include in the concept ester the alkyl halides, their Friedel-Crafts reactions provide familiar examples of this phenomenon. An unusual example especially relevant to our present considerations is provided by some results made available to me in advance of publication by Giusti and Andruzzi. Their results [38] on the polymerisation of styrene by iodine and hydrogen iodide can be interpreted in terms of an organic iodide derived from styrene, probably 1-phenylethyl iodide, being activated by the co-ordination of one or two molecules of iodine. This process appears to polarise the C—I bond to such an extent that the normally stable ester becomes activated to a chain-propagating species and induces a pseudocationic polymerisation ... 

Friedeb Crafts reaction Anisole undergoes Friedel-Crafts reaction, i.e., the all rl and acyl groups are introduced at ortho and para positions by reaction with alkyl halide and acyl halide in the presence of anhydrous alurntnlurn chloride (a Lewis acid) as catalyst. 

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... 

Friedel-Crafts reactions are electrophilic substitution reactions in which the electrophile is a Ccirbocation or an acylium ion. The removal of a halide ion from an alkyl halide is the means of generating the ceirbocation. An acylium ion is created by removing a chloride ion from an acid chloride (R-CO-Cl). Both of these processes require a Lewis acid as a catalyst. The most commonly used Lewis acid is aluminum chloride. 

Aromatic compounds are usually readily alkylated or acylated by a Friedel-Crafts reaction.150 The combination of reagents used most commonly for aromatic alkylation is an alkyl halide with a strong Lewis acid (Equation 7.65). However, alkenes, alcohols, mercaptans, and a number of other types of organic... 

By this strategy, reactive carbon electrophiles can be generated for successful reaction with a variety of weak carbon nucleophiles. More important examples include the Friedel-Crafts reaction in which aromatic compounds (nucleophiles) react with alkyl and acyl halides (electrophiles in the presence of Lewis acids). 

Benzylation of toluene with benzyl chloride, which is a typical example of Friedel-Crafts alkylation, is known to be catalyzed by Lewis-type superacids such as A1C13 and BF3. This type of catalyst has been mostly used for the Friedel-Crafts reaction, which is one of the most studied of organic reactions. This reaction was performed over several metal oxides and sulfates, and iron sulfates showed an unexpected effectiveness for the reaction (102-104). The catalytic activities of FeS04 and Fe2(S04)3 for the reaction were examined in detail the activities were remarkably dependent on calcination temperature, the maximum activity being observed with calcination at 700°C (105-107). Catalytic actions analogous to the above case were also observed with other Friedel-Crafts reactions, the benzoyl-ation of toluene with benzoyl chloride (108), the isopropylation of toluene with isopropyl halides (109), and the polycondensation of benzyl chloride UIO). 

The Friedel-Crafts reaction is polar (ionic) alkylation or acylation of electron-rich aromatics by alkyl cation or acyl cation species, derived from the reactions of alkyl halides or acyl halides with A1C13. Therefore, electron-rich aromatics such as anisole are very reactive, but electron-deficient aromatics such as pyridine are inert. 

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