Alkyl halide/Lewis acid

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... 

III. Alkyl Halide-Lewis Acid Halide Systems. 307... 

The transitory existence of alkylcarbonium ions in alkyl halide-Lewis acid halide systems has been inferred from a variety of observations, sueh as vapour-pressure depressions of OHsCl and C2H5CI in the presence of gallium chloride (Brown et al., 1950), the electric conductivities of aluminium chloride in ethyl chloride (Wertyporoch and Firla, 1933) and of alkyl fluorides in boron trifluoride (Olah et al., 1957), as well as the... 

The reaction of dialkylditellurides with pressurized acetylenes in the presence of an electrophilic reagent (alkyl halide, Lewis acid), in the system KOH/crown ether/ben-zene, gives rise to alkylethynyl tellurides and 1,2-bis-alkyltelluroacetylenes in high yields. ... 

Scheme 9 Mechanism of alkyl halide/Lewis acid-initiated polymerizations of alkenes. (From Ref. 57.)...

Kinetic studies have been reported only for a few controlled/living carbocationic systems initiated with alkyl halide/Lewis acid combinations (cf.,... 

In other words, of the initial charge of isopropyl chloride, 34% of the carbon is converted to propane under these conditions. A comparable sample of 1-chloropropane under identical reaction conditions, of excess alkyl halide Lewis acid, was converted after five minutes to the extent of 18% to propane. After 15 minutes the relative amount of "propane" had decreased as a result of further acid catalyzed polycondensation reactions. Similarly, isopropyl chloride reacts in HBr-AlBr3 at room temperature to give a gas product which is entirely propane after 2 min. In this system we again begin to see a buildup of heavier hydrocarbon species with time. The fact that the reaction proceeds so rapidly here can probably be attributed to a homogeneous hydrocarbon/acid liquid phase. This last result and additional experiments are summarized in Table IV. 

The addition of a third component to the metal-alkyl is a widespread practice with MgCI2 catalysts in order to improve their performance and to control the polymer molecular structure. In ethylene polymerization the addition of modifiers (alkyl-halides, Lewis acids such as A1C13, halogens such as I2, and others) is rather limited and is principally used to modify the MWD (see Table 11 in Ref.53)). On the other hand, the addition of modifiers is almost indispensable to obtain satisfactorily stereoregular propylene polymers. The additives used for this purpose are generally electron donor compounds (Lewis bases) and a wide variety has been described in patent and scientific literature. 

Because the.se substitution reactions involve electron-rich nucleophiles, they are called nucleophilic substitution reactions. Examples are shown in Equations [l]-[3]. Nucleophilic substitutions are Lewis acid—base reactions. The nucleophile donates its electron pair, the alkyl halide (Lewis acid) accepts it, and the C-X bond is heterolytically cleaved. Curved arrow notation can be used to show the movement of electron pairs, as shown in Equation [3]. 

Besides the alkyl halide-Lewis acid combination, two other routes to carbonium ions are quite widely used in synthesis. Alcohols can serve as sources of carbonium ions in strongly acidic media such as sulfuric acid and phosphoric acid. The alkylation of aromatic rings by alcohols is also catalyzed by BF3 and AlQs. ... 

Cationic polymerization of cyclic trivalent phosphorus compounds, initiated by alkyl halides, Lewis acids, Friedel-Crafts catalysts, and/or esters, was reported for various cyclic phosphites (34), " " phosphonites (35), " phosphoramidites (36), cyclic phos-phanols (37), " and cyclic phosphanethiols (38). °° Mel, PhCH2Br, and CF3S03Me were mainly used as initiators. The resulting polymers had molar mass not... 

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