Carbocation alkyl halide reaction with Lewis

Alkyl halides react with Lewis acids to form tertiary, secondary, or primary carbocations. The tertiary cation (160, r1=r2=r3= alkyl) is sufficiently stable that good yields of alkylated products are obtained upon reaction with benzene. Formation of the primary cation (160, R1=r2=H, R = alkyl) is energetically more difficult and is, of course, subject to rearrangement primary alkyl derivatives are rarely observed under these conditions. Secondary cations (160, R =H, r2=R = alkyl) are readily formed but are also subject to rearrangement. The extent of rearrangement is dependent on the reaction conditions and the nature of the... 

An alkyl halide reacts with Lewis acid (anhy AlCl ) and forms an alkyl carbocation by acid - base equilibrium reaction... 

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... 

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. ... 

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... 

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. 

Carbocations are perhaps the most important electrophiles capable of substituting onto aromatic rings, because this substitution forms a new carbon-carbon bond. Reactions of carbocations with aromatic compounds were first studied in 1877 by the French alkaloid chemist Charles Friedel and his American partner, James Crafts. In the presence of Lewis acid catalysts such as aluminum chloride (A1C13) or ferric chloride (FeCl3), alkyl halides were found to alkylate benzene to give alkylbenzenes. This useful reaction is called the Friedel-Crafts alkylation. 

Enamines are among the most powerful neutral nucleophiles and react spontaneously with alkyl halides. Silyl enol ethers are less reactive and so require a more potent electrophile to initiate reaction. Carbocations will do, and they can be generated in situ by abstraction of a halide or other leaving group from a saturated carbon centre by a Lewis acid. 

The best alkylating agents for silyl enol ethers are tertiary alkyl halides they form stable carbocations in the presence of Lewis acids such as TiCLj or SnCLj. Most fortunately, this is just the type of compounds that is unsuitable for reaction with lithium enolates or enamines, as elimination results rather than alkylation a nice piece of complementary selectivity. 

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. 

Lewis superacid-catalyzed direct alkylation of alkanes is also possible with alkyl cations prepared from alkyl halides and SbFs in sulfuryl chloride fluoride solution. " Typical alkylation reactions are those of propane and butanes by 2-butyl and ZerZ-butyl cations. The ClfU-Sbfs and C2H5F-SbF5 complexes acting as incipient methyl and ethyl cations besides alkylation preferentially cause hydride transfer. Since intermolecular hydride transfer between different carbocations and alkanes are faster than alkylation, a complex mixture of alkylated products is usually formed. A significant amount of 2,3-dimethylbutane was, however, detected when propane was propylated with the 2-propyl cation at low temperature [Eq. (6.36)]. No 2,2-dimethylbutane, the main product of conventional acid-catalyzed alkylation, was detected, which is a clear indication of predominantly nonisomerizing reaction conditions. 

Slater determinant, 251, see also Determinantal wave function SnI mechanism, 129-130 alkyl halides, 129 carbocation intermediates in, 106 leaving group, 130 Lewis acid catalysis, 130 Sn2 mechanism, 130-136 alkyl halides, 130 carbocation intermediates in, 106 and E2, 143 gas phase, 144 geometry of approach, 131 leaving group, 130, 132 nucleophilicity, 131-132 substituent effects, 132-134 transition state, 132, 133 VBCM description, 134-135 Snoutene, 247 rearrangement, 289 Sodimn borohydride, 83, 278 Sodimn hydride, 83 Soft Electrophiles, 110 reaction with etiolate, 110 Spin function, 234... 

Alkylation of aromatic compounds (eqs. 4.10 and 4.11) is referred to as the Friedel-Crafts reaction, after Charles Friedel (French) and James Mason Crafts (American), who first discovered the reaction in 1877. The electrophile is a carbocation, which can be formed either by removing a halide ion from an alkyl halide with a Lewis acid catalyst (for example, AICI3) or by adding a proton to an alkene. For example, the synthesis of ethylbenzene can be carried out as follows ... 

Similar electronic reasons discussed for Bronsted-Lowry bases predict that an ether is a stronger Lewis base than an alcohol. Interestingly, the alkyl halide is a reasonably good Lewis base with BFg, forming 55. Although 55 is much more reactive (less stable) than 54 or 53, a species such as 55 can be generated and used in chemical reactions. In the case of an alkene and an alkyne, the carbocations 56 and 57 are quite unstable, and if formed at all will be very reactive (see Chapter 10, Section 10.6). Compared with the heteroatom functional groups, alkenes and alkynes are poor Lewis bases. Nonetheless, there are reactions in which they do react as Lewis bases (see Chapter 10, Section 10.6). 

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. 

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