Alkyl bromides rearrangements

Additional evidence for carbocation intermediates in certain nucleophilic substitutions comes from observing rearrangements of the kind normally associated with such species For example hydrolysis of the secondary alkyl bromide 2 bromo 3 methylbutane yields the rearranged tertiary alcohol 2 methyl 2 butanol as the only substitution product... 

Vinylcyclopropane reacts with HBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows, show the structure of the carbocation intermediate in brackets, and show the structure of the final product. 

The reaction of an alcohol with PBr3 does not involve the formation of a carbocation and usually occurs without rearrangement => PBr3 is often preferred for the formation of an alcohol to the corresponding alkyl bromide. 

ControUed-potential oxidations of a number of primary, secondary, and tertiary alkyl bromides at platinum electrodes in acetonitrile have been investigated [10]. For compounds such as 2-bromopropane, 2-bromobutane, tert-butyl bromide, and neopentyl bromide, a single Ai-alkylacetamide is produced. On the other hand, for 1-bromobutane, 1-bromopentane, 1-bromohexane, 1-bromo-3-methylbutane, and 3-bromohexane, a mixture of amides arises. It was proposed that one electron is removed from each molecule of starting material and that the resulting cation radical (RBr+ ) decomposes to yield a carbocation (R" "). Once formed, the carbocation can react (either directly or after rearrangement) with acetonitrile eventually to form an Al-alkylacetamide, as described above for alkyl iodides. In later work, Becker [11] studied the oxidation of 1-bromoalkanes ranging from methyl to heptyl bromide. He observed that, as the carbon-chain length is increased, the coulombic yield of amides decreases as the number of different amides increases. 

Problem 6.30 The addition of HBr to some alkenes gives a mixture of the expected alkyl bromide and an isomer formed by rearrangement. Outline the mechanism of formation and structures of products from the reaction of HBr with a) 3-methyl-1-butene, (b) 3,3-dimethy 1-1-butene. 

For this 2° alcohol, oxonium-ion formation is followed by loss of HjO to give a 2° carbocation that rearranges to a 3 carbocation. Both carbocations react by two pathways they form bonds to Br to give alkyl bromides or they lose H to yield alkenes. 

The reactions were between the alkyl bromide and OEr The rate for isopropyl bromide was actually greater than that for ethyl bromide, if the temperature difference is considered. Neopentyl bromide, the next compound in the 3-branching series, cannot be compared because it has no 3-hydrogen and cannot give an elimination product without rearrangement. 

The initially formed radical R disproportionates (path a),dimerizes (path b), reacts with active cathodes (path c), rearranges (path d), adds to double bonds (path e) or is reduced to an anion (path f). Products of radical origin (a—e) occur mainly in the reduction of alkyl iodides, benzyl halides and in some cases of alkyl bromides. 

One generally applicable method of oxidation for alkyl bromides involves the Pummerer rearrangement. This gives aldehydes upon rearrangement of the alkyl aryl sulfoxide formed by displacement of halide by thiolate followed by oxidation (equation 41)358. 

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. 

Attempted N-alkylation of 2,4,6-trichlorophenoxyacetamide 55 (Scheme 21) with various alkyl bromides in the presence of potassium hydroxide and tris(3,6-dioxaheptylamine) (TDA-1) as the phase-transfer catalyst, in toluene at reflux temperature, yielded the corresponding N-alkylated 2,4,6-trichlorophenylamine derivatives 56.2,4,6-Trichloro substitution on the phenyl ring was essential for the rearrangement [38]. 

Explain the following observation. When 3-methyl-2-butanol is treated with HBr, a single alkyl bromide is isolated, resulting from a 1,2-shift. When 2-methyl-1 -propanol is treated with HBr, no rearrangement occurs to form an alkyl bromide. 

The above-mentioned reactions for the introduction of bromine always involve an activation of the alcohol as the first step. In some cases it can be useful to isolate the activated intermediate. The reactions of alkyl mesylates or tosylates with MgBr2 give alkyl bromides under mild conditions with good yields (equation 20) Neopentyl tosylate with LiBr in HMPA gives neopentyl bromide in 67% yield (equation 21). Complete inversion with little or no rearrangement is observed in this difficult case. 

The rather unreactive arylcadmium reagents, ArCdCl, do react satisfactorily with both primary and secondary a-bromo esters and with secondary allylic bromides, but curiously they react much more poorly with allyl and benzyl bromide, and a-chloro ethers ca. 40% yields) no coupling occurs using primary alkyl bromides. Finally, the old Wurtz-Fittig mediodology, whereby an aryl bromide and an alkyl iodide are coupled in the presence of elemental sodium, can sometimes lead to acceptable yields (40-60%) no rearrangement products are formed, and it is likely that free radicals are not involved. 

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