2- Bromobutane elimination reaction

Another example is provided by the elimination reaction of 2-bromobutane using ethoxide ion as the base. The major product is the more highly substituted alkene. 

REACTION PROGRESS DIAGRAM FOR THE ELIMINATION REACTION OF 2-BROMOBUTANE. 

The E2 Elimination Reaction Dehydrohalogenation of 2-Bromobutane to Yield 1-Butene, trans-2-Butene, and c s-2-Butene... 

Finally, as foreshadowed in Chapter 7 (Scheme 7.24 ft, the Hofmann elimination) and Chapter 8, Part I, Section F, A. W. Hofmann (1818-1895) pointed out that elimination from ammonium ions yielded the least highly substituted alkene rather than the most highly substituted alkenes (the Saytzeff elimination). Thus, as shown in Scheme 10.47, when 2-aminobutane is exhaustively methylated with iodomethane (ICH3) and treated with base, there is more ehmination to yield 1-butene (about 60%) than when, for example, the same elimination reaction is effected on 2-bromobutane (about 20%). It has been argued that this result comes about because the base is closely associated with the positively charged ion and the hindered environment favors loss of the proton from the primary carbon. 

Fig. 4.14 Ratei of elimination reactions of 2-bromobutan (O) and 2,3-dibromobutane (9) over metal-ion exchanged silica gels at 433 K See note for electronegativity of metal ion. )...

The product formed when 1-bromobutane undergoes elimination is 1-butene, a simple monosubstituted alkene. When 4-bromo-1-butene undergoes elimination, the product is 1,3-butadiene, a conjugated diene, and therefore, a more stable product. The transition states leading to the products reflect the relative stabilities of the products. Since the transition state leading to 1,3-butadiene has the lower free energy of activation of the two, the elimination reaction of 4-bromo-1-butene will occur more rapidly. 

In the elimination reactions that we have studied so far, only one elimination product was possible. However, it is often the case that more than one elimination product can be produced. For example, in the reaction of 2-bromobutane with sodium methoxide in methanol, three E2 products are possible ... 

Direct experimental evidence for the trans elimination in E2 reactions has been obtained from deuterated -2-bromobutane (the deuterium atom occupies position 3). This compound on dehydrobromination forms trans and cis butene-2 in the ratio of 6 1... 

Further information about the direction of dehydrohalogenation was obtained with threo- and eryt/iro-2-deutero-3-bromobutanes on Si02 and Si02—KOH catalysts [193,195]. The determination of deuterium content in the butenes formed allowed the estimation of the extent of the syn-and anti-eliminations. The values of the deuterium kinetic isotope effect showed that the C —H (or C —D) bond is split in the rate-determining step. Over KOH—Si02, the anti-elimination was preferred, but at 300°C, syn-elimination was the peferential reaction mode. With Si02, syn-elimination was favoured under all conditions. 

Thus it is found that the relative amounts of olefinic products which result from reactions of 2-bromobutane with MeOK/MeOH, EtOK/EtOH and t-BuOK/DMSO are insensitive to changes in the base concentration (Bartsch et al., 1973a). In contrast, for eliminations induced by t-BuOK/t-BuOH, both positional and geometrical orientations are base concentration dependent. As the concentration of t-BuOK increases, a relatively higher proportion of 1-butene is... 

In contrast, 2-bromobutane has two structurally different /8-carbons from which a proton can be removed. So when 2-bromobutane reacts with a base, two elimination products are formed 2-butene and 1-butene. This E2 reaction is regioselective because more of one constitutional isomer is formed than the other. 

Chapter6 Alkyl Halides Nucleophilic Substitution and Elimination EXAMPLE Reaction of 1-bromobutane with sodium methoxide gives 1-methoxybutane. 

During dehydrohalogenation reaction halogen from one carbon atom and hydrogen from the adjacent (not from the same) carbon atom are eliminated in the form of HX. If the dehydrohalogenation of alkyl halides can yield more than one alkene, then the product is the most highly substituted alkene (Saytzeff rule). For example, 2 - bromobutane, when boiled with alcoholic KOH, yields 1- and 2-butenes in which the highly substituted 2-butene is the main product. 

At the other extreme of elimination mechanisms is a concerted process. In an E2 reaction (here illustrated by the reaction of 2-bromobutane with sodium ethoxide) proton transfer to the base, formation of the carbon-carbon double bond, and ejection of the bromide ion occur simultaneously all bond-breaking and bond-forming steps are concerted. Because the base removes a j8-hydrogen at the same time that the C— Br bond is broken to form a halide ion, the transition state has considerable double-bond character (Figure 9.7). 

A remarkable effect of alkali cation exchange of silica gel on the acid-base properties and the selectivity of dehydrobromination of 2-bromobutane has been observed.The reaction proceeds with anti-elimination over Cs- and K-exchanged silica gel in which basic sites play a primary role. On the other hand, the reaction proceeds with syn elimination over silica gel and Na- and Li-exchanged silica gels, all of which are weakly acidic. The observed dependence of the steric courses of the reaction on the acid-base properties was explained by the variation of the reaction mechanism from a concerted elimination over basic surfaces to a carbenium-ion like mechanism over acidic surfaces (Section 4.8). In conformity with this conclusion, the dehydrobromination proceeds with syn elimination over silica gels exchanged with Ba(II), Ca(II), Mg(II) and Ni(II) ions." ... 

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