Butyl bromide stability

The assumption that tertiary alkyl cations are not stable in solvents other than super-acids is widespread and was apparently well founded on many experiments by different workers over many years [20, 24]. For this reason the stability of our polymerised solutions was astonishing and it seemed at first unlikely that the cation of the electrolyte could be a simple tertiary ion the tert-butyl cation in the experiment with tert-butyl bromide and the ions 2-4 in the polymerised solutions. This was because we did not know then that Cesca,... 

It lies completely on the side of the lithio-aromatic compound and n- or /erf-butyl bromide. This is because the C atom of the C—Li bond is -hybridized in the lithio-aromatic compound and therefore more electronegative than the -hybridized C atom of the C—Li bond in n- or ferf-BuLi. More electronegative C atoms stabilize C—Li bonds because these are very electron-rich. Think of it as a carbanion. There is more s character in an sp2-hybridized orbital and a pair of electrons in such an orbital should be lower in energy than in an sp3-hybridized orbital. 

It was first suggested200,201 that the stabilized structure could be dependent upon the nature of the matrix, but irradiation of isobutyl bromide and /-butyl bromide in TMS or adamantane solutions at 77 K together with the temperature dependence of the resulting spectra led Symons and Smith202 to identify the species with the small Br coupling as the /-butyl radical bromide ion adduct Me3C —Br". 

If 14 or more carbons are present, the product may be diamantane or a substituted diamantane. " These reactions are successful because of the high thermodynamic stability of adamantane, diamantane, and similar diamond-like molecules. The most stable of a set of C H isomers (called the stabilomer) will be the end product if the reaction reaches equilibrium. Best yields are obtained by the use of sludge catalysts (i.e., a mixture of AIX3 and ferf-butyl bromide or iec-butyl bromide).Though it is certain that these adamantane-forming reactions take place by nucleophilic 1,2-shifts, the exact pathways are not easy to unravel... 

A triple anion complex containing enolate, amide, and halide functionalities can be isolated from the mixture of n-butyl bromide, hexamethyldisilazane, TMEDA, Bu Li and pinacolone (Bu COMe). The resulting solution of LiBr, LiN(SiMc3)2, LiOC(Bu )=CH2, and TMEDA produces crystals of Li4(/.t4-Br)( u-OC(Bu )=CH2)2(M-N(SiMe3)2)(TMEDA)2, which, instead of forming a ladder-type structure, consists of a planar butterfly of four lithium atoms bonded to a //4-Br the stability of this arrangement has been studied with semi-empirical (PM3) and ab initio HE/ LANL2DZ computations. ... 

In contrast to Sfj2 reactions, rates of reactions involving TSs with cationic character increase with substitution. The relative rates of formolysis of alkyl bromides at 100°C are methyl, 0.58 ethyl, 1.00 i-propyl, 26.1 and r-butyl 10. This order is clearly dominated by carbocation stability. The effect of substituting a methyl group for hydrogen depends on the extent of nucleophilic participation in the TS. A high CHj/H rate ratio is expected if nucleophilic participation is weak and stabilization of the cationic nature of the TS is important. A low ratio is expected when nucleophilic participation is strong. The relative rate of acetolysis of r-butyl bromide to... 

As shown qualitatively in Rgure 7.8, since a tertiary (3°) carbocation is more stable than its secondary (2°) carbocation counterpart, the transition state leading to it should be lower. Furthermore, since the ionization process in which the leaving group leaves is rate determining, it is reasonable that the relative rates of SnI reactions should reflect the stability of the carbocations Thus, it is not surprising to find that in water, at 50°C, 2-bromo-2-methylpropane (t-butyl bromide [(CH3)3C-Br])... 

The difference between 8 2 and 8 1 reactions can be rationalised by the reaction energy profiles shown in Figure 11.1. The nucleophilic substitutions of bromomethane and of tertiary butyl bromide in aqueous solution illustrate some of the factors that determine the size of the energy barriers and the stability of the intermediate in these mechanisms. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Proton nmr halide anion titrations reveal that the ethyl- [79], propyl-[80] and butyl- [81] linked derivatives (Fig. 43) form complexes of 1 1 stoichiometry in acetonitrile solution. Stability constant determinations suggest that the ethyl derivative [79] exhibits selectivity for the chloride anion in preference to bromide or iodide. As the chain length increases, so the selectivity for chloride decreases and also the magnitude of the stability constant which is evidence for an anionic chelate effect with the chloride anion. Receptors containing larger aryl [81], [83], [84] and alkylamino spacers [85] (Fig. 43) form complexes of 2 1 halide anion receptor stoichiometry. 

The transfer constant for f-butylbenzene is low, since there are no benzylic C—H bonds present. Primary halides such as n-butyl chloride and bromide behave similar to aliphatics with low transfer constants, corresponding to a combination of either aliphatic C—H bond breakage or the low stability of a primary alkyl radical on abstraction of Cl or Br. The iodide,... 

The intriguing combination of an a-silyl and a /1-silyl substituent in the disilanyl group (—SiR2SiR3) prompted Shimizu, Tsuno and coworkers to study the effect of a /3-Si—Si bond on the stability of carbocations86. They studied the solvolysis of the benzylic bromides 213-215 and found that the /i-SiMe3 substituent in 213 increases the solvolysis rate by a factor of 1.07 x 105 compared with the f-butyl-substituted reference compound 214 (Table 7). 

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