Alkyls chloride exchange

Secondary or tertiary alkyl halides are much less reactive. For example an alkyl dichloride with a primary and a secondary chloride substituent reacts selectively by exchange of the primary chloride. The reactivity with respect to the Finkelstein reaction is thus opposite to the reactivity for the hydrolysis of alkyl chlorides. For the Finkelstein reaction on secondary and tertiary substrates Lewis acids may be used," e.g. ZnCla, FeCls or MesAl. 

I Substitution reactions occur when two reactants exchange parts to give two new products. An example is the reaction of an alkane with Cl2 in the presence of ultraviolet light to yield an alkyl chloride. A Cl atom from Cl2 substitutes for an H atom of the alkane, and two new products result. 

Halide exchange, sometimes call the Finkelstein reaction, is an equilibrium process, but it is often possible to shift the equilibrium." The reaction is most often applied to the preparation of iodides and fluorides. Iodides can be prepared from chlorides or bromides by taking advantage of the fact that sodium iodide, but not the bromide or chloride, is soluble in acetone. When an alkyl chloride or bromide is treated with a solution of sodium iodide in acetone, the equilibrium is shifted by the precipitation of sodium chloride or bromide. Since the mechanism is Sn2, the reaction is much more successful for primary halides than for secondary or tertiary halides sodium iodide in acetone can be used as a test for primary bromides or chlorides. Tertiary chlorides can be converted to iodides by treatment with excess Nal in CS2, with ZnCl2 as catalyst. " Vinylic bromides give vinylic iodides with retention of configuration when treated with KI and a nickel bromide-zinc catalyst," or with KI and Cul in hot HMPA." ... 

Zinc chloride exchanged clay catalysts have been reported to be highly active for the Friedel-Crafts alkylation and acylation reactions these are commercially sold by Contract Catalysts under the name Envirocats. These are montmorillonite catalysts modified by ZnCU and FeCli. Some of the reported examples of Friedel-Crafts reactions are given below there are claims that some of the processes are commercially practised. 

Perdeuteriated isopropyl, t-butyl, and t-pentyl fluorides were prepared from the corresponding deuteriated alkyl chlorides (bromides) by halogen exchange. The perdeuteriated alkyl fluorides were then used in the formation of the deuteriated carbonium ions under similar conditions as those for the protium complexes. The resonance spectra were obtained at 9 2 Mc/s and the data are summarized in Table 7... 

Clayden and Julia reported the 1,3-C,H insertion reaction of lithium carbenoid (69) derived from a primary alkyl chloride (68) by H-Li exchange reaction (eqnation 19). Treatment of 68 with a mixture of n-BuLi and tert-BnOK gave three prodncts. These... 

Long (81) showed that the complex from biscyclopentadienyltitanium dichloride and methylaluminum chloride or a simply derived product from it, was an active ethylene polymerization catalyst. There have been a number of attempts to determine the exact nature of initiation in polyethylene. However, by any techniques available until now, it has not been possible to determine the actual ionic nature of the active catalyst which polymerizes ethylene. Karapinka and Carrick (82) studied the polymerization of ethylene with biscyclopentadienyltitanium dichloride and various alkylaluminum compounds. They found that the alkyl group exchanged so readily between the aluminum and titanium, that the location of the initiating site could not be determined. All that could be concluded was that an ethyl group initiated the polymerization more easily than the phenyl. 

There are no more than three or four examples of successful sulfur-lithium exchange in sulfides, and, as with alkyl chlorides, the conversion of sulfides to organolithiums is much better achieved reductively (see section 4.4.1). Nucleophilic attack on sulfide S displaces an organolithium only when that organolithium is highly stabilised, and was first noted by Brandsma,148 who found that PhLi displaced the alkynyllithium 185 from 184. 

Exchange of potassium or sodium iodide with primary alkyl bromides or chlorides is one of the most thoroughly studied phase transfer reactions173,174. It is normally performed in a two-phase aqueous/organic system utilizing a soluble or a polymer-bound PTC. Starks and collaborators have shown175,176 that primary alkyl chlorides or bromides quantitatively exchange with aqueous sodium iodide in the presence of few mol% of a soluble quaternary ammonium PTC at 108 °C within a few hours (secondary alkyl halides normally yield elimination products under these conditions). The mechanism, which is typical of PTC, comprised two consecutive steps ... 

The reversible190 exchange of alkyl chlorides with metal bromides in a two-phase system, under PTC, has been reported to depend on the nature of the metal cation and the concentration of the aqueous phase. Most effective bromide donors are lithium191 and calcium bromides192 particularly when applied in the presence of a small amount of water. Interestingly, both the equilibrium composition and the rate in the reaction of various alkali and alkali earth bromides with -octyl chloride catalyzed by tetra- -hexylammonium bromide are strongly dependent on the amount of water present in the system193,194. 

Halogen Exchange Reactions The Sn2 reaction provides a useful method for synthesizing alkyl iodides and fluorides, which are more difficult to make than alkyl chlorides and bromides. Halides can be converted to other halides by halogen exchange reactions, in which one halide displaces another. 

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