Rearrangements 2-bromo-2-methylbutane

Why does the carbocation intermediate in the hydrolysis of 2 bromo 3 methylbutane rearrange by way of a hydride shift rather than a methyl shift ... 

An example of a reaction with rearrangement is the SnI reaction of 2-bromo-3-methylbutane in boiling ethanol. The product is a mixture of 2-ethoxy-3-methylbutane (not rearranged) and 2-ethoxy-2-methylbutane (rearranged). 

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... 

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. 

Neopentyl alcohol, (CH3)3CCH2OH, reacts with concentrated HBr to give 2-bromo-2-methylbutane, a rearranged product. Propose a mechanism for the formation of this product. 

A carbocation intermediate is formed in an SnI reaction. In Section 4.6, we saw that a carbocation will rearrange if it becomes more stable in the process. If the carbocation formed in an SnI reaction can rearrange, SnI and Sn2 reactions of the same alkyl halide can produce different constitutional isomers as products, since a carbocation is not formed in an Sn2 reaction and therefore the carbon skeleton cannot rearrange. For example, the product obtained when HO is substituted for Br in 2-bromo-3-methylbutane by an SnI reaction is different from the product obtained by an Sn2 reaction. When the reaction is carried out under conditions that favor an SnI reaction, the initially formed secondary carbocation undergoes a 1,2-hydride shift to form a more stable tertiary carbocation. 

Because the reaction of a secondary or a tertiary alcohol with a hydrogen halide is an SnI reaction, a carbocation is formed as an intermediate. Therefore, we must check for the possibility of a carbocation rearrangement when predicting the product of the substitution reaction. Remember that a carbocation rearrangement will occur if it leads to formation of a more stable carbocation (Section 4.6). For example, the major product of the reaction of 3-methyl-2-butanol with HBr is 2-bromo-2-methylbutane, because a 1,2-hydride shift converts the initially formed secondary carbocation into a more stable tertiary carbocation. 

The NMR of the product closely resembles that of the tertiary alcohol 2-methyl-2-butanol (Problem 47, spectrum C), but the signal for the OH group is missing, and the molecular formula has a Br instead of an OH. The product is 2-bromo-2-methylbutane, and the signals in the spectrum are assigned similarly. How does it form Rearrangements ... 

For example, the products obtained from the solvolysis of 2-bromo-2-methylbutane are the same as those from the solvolysis of l-bromo-2,2-dimethylpropane. When a driving force for cation rearrangement exists, migration of an alkyl group almost always takes place faster than trapping of a less stable cation by solvent or another nucleophile (Scheme 2.23). 

Carbocation Rearrangement in the SnI Hydrolysis of 2-Bromo-3-methylbutane THE OVERALL REACTION ... 

The solvolysis of 2-bromo-3-methylbutane potentially can give several products, including both El and products from both the unrearranged carbocation and the rearranged carbocation. Mechanism Boxes 6-6 (page 247) and 6-9 (above) show the products from the rearranged carbocation. Summarize all the possible products, showing which carbocation they come from and whether they are the products of El or reactions. 

One way to test for the intermediacy of carbocations in reaction mechanisms is to look for rearrangements, for example, from a 2° carbocation to a 3° carbocation. In the addition of bromine to 3,3-dimethyl-l-butene (7) in methanol, however, the only products observed were l,2-dibromo-3,3-di-methylbutane (8), 45%, and 2-bromo-l-methoxy-3,3-dimethylbutane (9), 44%. There was no evidence for products such as 10, which might have been expected if a free 2° carbocation were formed and then rmderwent a methyl shift to yield a 3° carbocation. Therefore, the intermediate in the addition of bromine to alkyl-substituted alkenes appears not to behave like a carbocation. 

Sterically hindered alcohols react with phosphorus tribromide, but tend to give large quantities of rearranged product. The product mixture obtained from 2,2 dimethyl-l-propanol (neopentyl alcohol) contains 63% l-bromo-2,2-dimethylpropane, 26% 2-bromo-2-methylbutane, and 11 % 2-bromo-3-methylbutane. Explain the origin of the products. Why are sterically hindered alcohols more prone to give rearranged products ... 

Another complication that arises with S l and El reactions is the rearrangement of the carbocation intermediate to form a more stable intermediate before the S l or El product is formed. Such a situation arises, for example, when 2-bromo-3-methylbutane is heated with water. The major S l product is not 3-methylbutan-2-ol but rather 2-methylbutan-2-ol. 

The major product is produced following the rearrangement of the 2° carbocation that is formed when the carbon-bromine bond in 2-bromo-3-methylbutane breaks to release a Br ion. As suggested below, two possible rearrangements, labeled (a) and (b), can occur. Both rearrangements involve the movement of a hydrogen atom. These rearrangements are examples of a hydride shift. 

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