Alkyl chlorides rearrangements

The similar yields of the two alkyl chloride products indicate that the rate of attack by chloride on the secondary carbocation and the rate of rearrangement must be very similar... 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

Thionyl chloride (SOCl2) converts 1° and 2° alcohols to alkyl chlorides (usually without rearrangement) ... 

The numerous straightforward examples of internal displacement reactions leading to isolable cyclic products will not be discussed here, but only, for the most part, those ionization reactions in which a cyclic intermediate or transition state is deduced from the rearranged structure of the product. A well-known example is mustard gas and other alkyl chlorides with sulfur on the /3-carbon atom. Although mustard gas is a primary and saturated alkyl chloride, its behavior is like that of a typical tertiary alkyl chloride. It reacts so fast by a first order ionization that the rate of the usual second order displacement reaction of primary alkyl halides is not measureable. Only the ultimate product, not the rate, is determined by the added reagent.228 Since the effect of the sulfur is too large to be explicable in terms of a carbon sulfur dipole or similar explanation, a cyclic sulfonium ion has been proposed as an... 

The alkyl halides react with cyanide to produce alkyl cyanides. But this reaction has rarely been employed to obtain the increased length of the chain because of the long reaction times and poor yields. However, the use of DMSO as a solvent has simplified the procedure and improved the yields for the conversion of primary and secondary alkyl chlorides into cyanides, without any rearrangement. 

These results are consistent with ionization of the cyclopropylcarbinyl chloride on the zeolite, with formation of the C4H7+ cation. Attack of the chloride ion (internal return) might then occur at the three possible positions, giving the rearranged alkyl chlorides. This hypothesis was supported by the data obtained with impregnation of the NaBr on the NaY zeolite. The observation of the three alkylbromides is consistent with a mechanism involving ionization and attack of the external bromide nucleophile. 

The results of cyclopropylcarbinyl chloride rearrangement over NaY impregnated with NaBr suggest that there is an equilibrium between the bicyclobutonium cation and the alkyl-aluminumsilyl oxonium ion, explaining the preferred formation of the allylcarbinyl bromide in the rearranged products. It also suggests that zeolites may act as solid solvents, providing unsymmetrical solvation for the ions inside the cavities. 

The prominent role of alkyl halides in formation of carbon-carbon bonds by nucleophilic substitution was evident in Chapter 1. The most common precursors for alkyl halides are the corresponding alcohols, and a variety of procedures have been developed for this transformation. The choice of an appropriate reagent is usually dictated by the sensitivity of the alcohol and any other functional groups present in the molecule. Unsubstituted primary alcohols can be converted to bromides with hot concentrated hydrobromic acid.4 Alkyl chlorides can be prepared by reaction of primary alcohols with hydrochloric acid-zinc chloride.5 These reactions proceed by an SN2 mechanism, and elimination and rearrangements are not a problem for primary alcohols. Reactions with tertiary alcohols proceed by an SN1 mechanism so these reactions are preparatively useful only when the carbocation intermediate is unlikely to give rise to rearranged product.6 Because of the harsh conditions, these procedures are only applicable to very acid-stable molecules. 

Primary alkyl chlorides give the corresponding perfluoroalkylchlorides (Fig. 10) [58] in good yield demonstrating that the carbon-chlorine bond resists the fluorination process. However, secondary alkyl chlorides are susceptible to rearrangement processes (Fig. 11). 

Perfluorination of tertiary alkyl chlorides gives products arising solely from rearrangement processes (Fig. 13) [60]. Rearrangement of the initially formed radical species 10 by a 1,2-chlorine shift to a more stable tertiary radical 11 in the early stages of the reaction (Fig. 14), accounts for these findings. In other... 

Detailed wide-ranging studies are available on the addition of HC1 and HBr to alkenes. The most useful procedure is to react dry HC1 gas and the alkene neat or in an inert organic solvent. Water or acetic acid may also be used. Alkenes yielding tertiary or benzylic alkyl chlorides react most readily. Styrene, however, adds HC1 only at — 80°C to give a-chloroethylbenzene without polymerization.101 At more elevated (room) temperature polymerization prevails. HBr adds to alkenes in an exothermic process more rapidly than does HC1. Rearrangements may occur during addition indicating the involvement of a carbocation intermediate 102... 

Keywords alkyl chloride, trialkyl phosphite, alumina, microwave irradiation, Arbuzov rearrangement, dialkyl alkylphosphonate... 

Rearrangement may be largely (but not entirely) suppressed by preparing the alkyl chloride from a reaction of the alcohol with thionyl chloride, either (i) alone, or (ii) in the presence of catalytic or equimolar proportions of pyridine, or (iii) in the presence of dimethylformamide.85 In (i) a chlorosulphite is first formed which decomposes via two sequentially formed ion-pair species the second ion pair collapses to yield the alkyl chloride. 

Notice that these reactions take place with allylic chlorides. We should not expect an alkyl chloride to be particularly good at Sn2 reactions as chloride ion is only a moderate leaving group and we should normally prefer alkyl bromides or iodides. AUylic chlorides are more reactive because of the alkene. Even if the reaction occurs by a simple Sn2 mechanism without rearrangement, the alkene is still making the molecule more electrophilic. 

Rearrangements can occur even when no free carbocation is formed initially. For example, the 1° alkyl chloride in Equation [2] forms a complex with AICI3, which does not decompose to an unstable 1° carbocation, as shown in Mechanism 18.9. Instead, a 1,2-hydride shift forms a 2° carbocation, which then serves as the electrophile in the two-step mechanism for electrophilic aromatic substitution. 

I Mechanism 18.9 A Rearrangement Reaction Beginning with a 1° Alkyl Chloride... 

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