Neopentyl carbocations

The low Sn2 reactivity of 1°-alkyl bromide, 2,2-dimethyl-1-bromopropane (neopentyl bromide, 2.5), is explained by steric hindrance to the required 180° alignment of reacting orbitals. However, under Sn 1 conditions, neopentyl bromide (2.5) reacts at roughly the same rate as other 1°-alkyl halides such as ethyl bromide. Ionization of alkyl halides to carbocation in SnI is the rate-determining step. Although the product from ethyl bromide is ethanol as expected, neopentyl bromide (2.5) yields 2-methyl-2-butanol (2.6) instead of the expected 2,2-dimethyl-1-propanol (neopentyl alcohol) (2.7). This is because once formed the ethyl carbocation can only be transformed by a substitution or elimination process. In the case of the neopentyl carbocation, however, the initially formed l°-carbocation may be converted... 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

For some tertiary substrates, the rate of SnI reactions is greatly increased by the relief of B strain in the formation of the carbocation (see p. 366). Except where B strain is involved, P branching has little effect on the SnI mechanism, except that carbocations with P branching undergo rearrangements readily. Of course, isobutyl and neopentyl are primary substrates, and for this reason they react very slowly by the SnI mechanism, but not more slowly than the corresponding ethyl or propyl compounds. 

The first step is not a free-radical process, and its actual mechanism is not known. Compound 30 is an acyl hypohalite and is presumed to be an intermediate, though it has never been isolated from the reaction mixture. Among the evidence for the mechanism is that optical activity at R is lost (except when a neighboring bromine atom is present, see p. 899) if R is neopentyl, there is no rearrangement, which would certainly happen with a carbocation and the side products, notably RR, are consistent with a free-radical mechanism. There is evidence that the Simonini... 

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. 

The SNl-type process occurs mostly when B is a tertiary atom or has one aryl group and at least one other alkyl or aryl group. In other cases, the SN2-type process is more likely. Inversion of configuration (indicating an SN2-type process) has been shown for a neopentyl substrate by the use of the chiral neopentyl-l-d alcohol.18 On the other hand, there is other evidence that neopentyl systems undergo rearrangement by a carbocation (SNl-type) mechanism.19... 

Another feature of El reactions (and also of SN1 reactions) is the tendency of the initially formed carbocation to rearrange, especially if a more stable car-bocation is formed thereby. For example, the very slow SN1 solvolysis of neopentyl iodide in methanoic acid leads predominantly to 2-methyl-2-butene ... 

Like the previous example, this is a solvolysis reaction. Initial protonation of the alcohol followed by water leaving generates a primary carbocation. The bromide can then add to this carbocation generating neopentyl bromide. Since, for this carbocation, 1,2-hydride shifts cannot occur, a 1,2-alkyl shift generates a more stable tertiary carbocation. This new carbocation is not subject to possible 1,2-hydride shifts because any such transformation would generate either a less stable... 

When neopentyl bromide is boiled in ethanol, it gives only a rearranged substitution product. This product results from a methyl shift (represented by the symbol CH ), the migration of a methyl group together with its pair of electrons. Without rearrangement, ionization of neopentyl bromide would give a very unstable primary carbocation. 

Clearly, both spectra are of the tertiary 2-methylbutyl cation and the neopentyl cation never saw the light of day. The reaction is the same rearrangement that you saw in the substitution reaction of neopentyl iodide, but here the rate of rearrangement can be measured and it is extremely fast. Ncopentyl tosylate reacts to form a cation under these conditions about 104 times as fast as ethyl tosylate, even though both tosylates are primary. This massive rate difference shows that if migration of an alkyl group can allow rearrangement to a more stable carbocation, it will happen, and happen rapidly. 

Answer The neopentyl system would migrate a methyl to give a tertiary carbocation. Isobutyl systems migrate an H to give the fert-butyl carbocation. The strained ring compound produces the 1-methylcyclopentyl carbocation. (Relief of severe... 

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