Neopentyl bromide

It occasionally happens that a reaction proceeds much faster or much slower than expected on the basis of electrical effects alone. In these cases, it can often be shown that steric effects are influencing the rate. For example, Table 9.2 lists relative rates for the Sn2 ethanolysis of certain alkyl halides (see p. 390). All these compounds are primary bromides the branching is on the second carbon, so that field-effect differences should be small. As Table 9.2 shows, the rate decreases with increasing P branching and reaches a very low value for neopentyl bromide. This reaction is known to involve an attack by the nucleophile from a position opposite to that of the bromine (see p. 390). The great decrease in rate can be attributed to steric hindrance, a sheer physical blockage to the attack of the nucleophile. Another example of steric hindrance is found in 2,6-disubstituted benzoic acids, which are difficult to esterify no matter what the resonance or field effects of the groups in the 2 or the 6 position. Similarly, once 2,6-disubstituted benzoic acids are esterified, the esters are difficult to hydrolyze. 

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

We have already noticed (p. 86) that the SN2 hydrolysis of 1-bromo-2,2-dimethylpropane (neopentyl bromide, 24) is slow due to steric hindrance. Carrying out the reaction under conditions favouring the Sjyl mode can result in an increased reaction rate but the product alcohol is found to be 2-methylbutan-2-ol (26) and not the expected... 

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. 

Perfluorination of neopentyl bromide, on the other hand, gave a number of products, none of which contained bromine (Fig. 17) [58]. In this case, neopentyl bromide difluoride 12 or tetrafluoride 13 is first formed and these lose BrF2" to give carbocationic intermediates 14 and 15. Familiar carbocationic rearrangements and further fluorination yield the major isolated product, perfluoro-isopentane (Fig. 18) [58]. 

Thioethers (sulfides) can be prepared by treatment of alkyl halides with salts of thiols (thiolate ions).7S2 R may be alkyl or aryl. As in 0-35, RX cannot be a tertiary halide, and sulfuric and sulfonic esters can be used instead of halides. As in the Williamson reaction (0-12), yields are improved by phase-transfer catalysis.753 Instead of RS ions, thiols themselves can be used, if the reaction is run in benzene in the presence of DBU (p. 1023).754 Neopentyl bromide was converted to Me3CCH2SPh in good yield by treatment with PhS in liquid NH3 at -33°C under the influence of light.755 This probably takes place by an SrnI mechanism (see p. 648). Vinylic sulfides can be prepared by treating vinylic bromides with PhS in the presence of a nickel complex,756 and with R3SnPh in the presence of Pd(PPh3)4.757 R can be tertiary if an alcohol is the substrate, e.g,758... 

The nucleophilic displacement of halogens by fluorine can be successfully carried out by reaction with potassium fluoride under several conditions. Primary and secondary alkyl halides 1 are converted to the corresponding fluorides 2 by heating with a saturated aqueous solution of potassium fluoride in the presence of catalytic amounts of hexadecyltributylphosphonium bromide.56 Small amounts of the corresponding alcohol and the olefinic elimination product are formed as side products. While neopentyl bromide does not react at all, polyhaloalkanes give only products of elimination or hydrolysis. Chlorocyclohexane gives only the elimination product.56... 

The dihydrido complex [RhH2(ri5-C5Me5)(PMe3)] forms C—H insertion products when irradiated in the presence of alkanes (ethane, propane).227,228 Reaction with CHBr3 leads to bromoalkylrhodium complexes, which on treatment with bromine give ethyl bromide or 1-bromopropane in 70-85% yield. The less stable iridium complex formed with neopentane could not be converted directly to neopentyl bromide.229 It gave, however, a mercury derivative that yielded the bromide after treatment with bromine. 

Dostrovsky, I. Hughes, E. D. Mechanism of substitution at a saturated carbon atom. Part XXVI. The role of steric hindrance. (SectionA) Introductory remarks, and a kinetic study of the reactions of methyl, ethyl, fa-propyl, isobutyl, and neopentyl bromides with NaOEt in dry EtOHJ. Chem. Soc. 1946, 157-161. 

The nature of the products evidently suggests a Wagner-Merweein rearrangement. Moreover, preliminary examination in a single-pulse shock tube was reported to confirm the molecular nature of the reaction and the rearrangement of neopentyl bromide to 2-methyl-1-butene and 2-methyl-2-butene. When this bromide was thermally decomposed in the absence of the chain inhibitor cyclochexene, the reaction proceeded in a radical process. 

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