Friedel-Crafts alkylation reaction carbocation rearrangements

If the carbocation formed from the alkyl halide used in a Friedel-Crafts alkylation reaction can rearrange, the major product will be the product with the rearranged alkyl group. 

Carbocations can rearrange during the Friedel-Crafts alkylation reaction, leading to the formation of unpredicted products. One example is the formation of isopropyl benzene by the reaction of propyl chloride with benzene. 

It is not possible to obtain a good yield of an alkylbenzene containing a straight-chain alkyl group via a Friedel-Crafts alkylation reaction, because the incipient primary car-bocation will rearrange to a more stable carbocation. 

The answer is D. This reaction is a Friedel-Crafts alkylation reaction. This rules out Choice A. In an alkylation reaction, there is a possibility of rearrangement. The primary alkyl halide ( -propyl chloride) can form secondary carbocations by rearrangement. To avoid this type of rearrangement, chemists often use Friedel-Crafts acylation reactions which do not involve rearrangement. The carbonyl group in the acylation product can be easily reduced to get the desired hydrocarbon. 

The reaction of benzene with a carbocation leads to an arene in what is known as Friedel-Crafts alkylation. The reaction of an alkyl halide with a strong Lewis acid gives a carbocation, which is subject to rearrangement. Friedel-Crafts alkylation reactions are subject to poly alkylation because the arene is more reactive than benzene. 

The third limitation of Friedel-Crafts alkylation reaction is the structural rearrangement of the alkyl carbocation generated from the alkyl halide. A rearrangement of the alkyl group gives a different product than the one desired. For example, the reaction with 1-chloropropane in the presence of AlCl yields a small amount of propylbenzene, but a larger amount of the isomer, isopropylbenzene. 

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

From what has been said thus far, it is evident that the electrophile in Friedel-Crafts alkylation is a carbocation, at least in most cases. This is in accord with the knowledge that carbocations rearrange in the direction primary — secondary —> tertiary (see Chapter 18). In each case, the cation is formed from the attacking reagent and the catalyst. For the three most important types of reagent these reactions are... 

A number of reactions have been explained on the basis of generation of carbocations. The examples include the Friedel-Crafts alkylation and arylation reactions. Besides pinacol-pinacolne rearrangement, Beckmann rearrangement and Wagner-Merwein rearrangement are other examples. 

This involvement of carbocations actually limits the utility of Friedel-Crafts alkylations, because, as we have already noted with carbocations, rearrangement reactions complicate the anticipated outcome (see Section 6.4.2). For instance, when 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... 

Limitation 2 Like other carbocation reactions, the Friedel-Crafts alkylation is susceptible to carbocation rearrangements. As a result, only certain alkylbenzenes can be made using the Friedel-Crafts alkylation. fm-Butylbenzene, isopropylbenzene, and ethylbenzene can be synthesized using the Friedel-Crafts alkylation because the corresponding cations are not prone to rearrangement. Consider what happens, however, when we try to make n-propylbenzene by the Friedel-Crafts alkylation. 

Well, that depends. You have now seen a few useful carbocation rearrangements that give single products in high yield. But you have also met at least one reaction that cannotbe done because of carbocation rearrangements Friedel-Crafts alkylation using primary alkyl halides. 

The Friedel-Crafts alkylation illustrates the problems of trying to use carbocation rearrangements to make single products in high yield. We can give three guidelines to spotting this type of reaction. 

Carbonium ions can be generated at a variety of oxidation levels. The alkyl carbocation can be generated from alkyl halides by reaction with a Lewis acid (RCl + AICI3) or by protonation of alcohols or alkenes. The reaction of an alkyl halide and aluminium trichloride with an aromatic ring is known as the Friedel-Crafts alkylation. The order of stability of a carbocation is tertiary > secondary > primary. Since many alkylation processes are slower than rearrangements, a secondary or tertiary carbocation may be formed before aromatic substitution occurs. Alkylation of benzene with 1-chloropropane in the presence of aluminium trichloride at 35 °C for 5 hours gave a 2 3 mixture of n- and isopropylbenzene (Scheme 4.5). Since the alkylbenzenes such as toluene and the xylenes (dimethylbenzenes) are more electron rich than benzene itself, it is difficult to prevent polysubsiitution and consequently mixtures of polyalkylated benzenes may be obtained. On the other hand, nitro compounds are sufficiently deactivated for the reaction to be unsuccessful. 

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