For ElcB

Still another method measures volumes of activation. These are negative for E2 and positive for ElcB mechanisms. Measurement of the activation volume therefore provides a continuous scale for deciding just where a reaction lies on the spectmm. 

There was no evidence of a second-order term in amine, nor did amine self-association account for the non-linear behaviour. Hammett p values (for variation of RNHSO2) determined for formation of the complex [S.amine] (p = 1.64) and for expulsion of the anion ( ONp) (pacyi = -1-78) are consistent with an E cB process and uncomplicated by any steric effects of bound amine in the complex. The value of Pacyi is identical with that reported previously for ElcB reaction of the same esters in 50% acetonitrile-water and much greater than for their 2-type reactions in chloroform. Consequently, an ElcB mechanism involving extensive S-O bond cleavage with the formation of a A(-sulfonylamine, ArN=S02, is supported. 

The carbanion can be destroyed in two ways, k2 or k, and two limiting types of behavior are expected for ElcB mechanisms. If k2 3> fe-i[BH], then the carbanion always decomposes to the alkene product and the rate law simplifies to feobs = fei. In other words, the rate is only dependent on carbanion formation and the rate law has a form that is identical to what would be obtained in a concerted E2 reaction. This has been referred to as an EIcBirr mechanism, where IRR indicates that deprotonation is irreversible. On the other hand, if k2 fc i[BH], then carbanion formation rarely leads to the product and the rate law simplifies to the following ... 

H(ElcB) cannot be distinguished from E2 by this means, because it has the identical rate law Rate = k substrateIIB ] The rate law for (ElcB)R is different Rate = lr substrate IIB ]/ BH). but this is often not useful because the only difference is that the rate is also dependent (inversely) on the concentration of the conjugate acid of the base, and this is usually the solvent, so that changes in its concentration cannot be measured. 

Fig. 4.4. Energy profile of the C=C-forming step of the four mechanisms according to which the /3-eliminations of Figure 4.3 can take place in principle as a function of the chemical nature of the substituent Het and the reaction conditions. The conceivable starting materials for this step are, depending on the mechanism, the four species depicted on the left, where k is for E2 elimination,2> is for /3-elimination via a cyclic transition state,3> is for El elimination, and4> is for Elcb elimination.

The /ifeq of the proton transfer is the limiting factor for the ElcB. The ElcB is most often seen when an electron-withdrawing group increases the acidity of the C-H bond. A useful iiTeq of the proton transfer for ElcB is greater than... 

The lack of stereochemical evidence for ElcB reactions renders comments of dubious value. Presumably ion pairing and neighbouring group interactions will exert complicating factors in an analogous manner to those observed in El reactions. 

Rg. 6.4. Three-dimensional (More O Fenall) diagrams depicting transition-state locations for El, Elcb, and E2 mechanisms. 

Comparison of the data for methoxide with those for t-butoxide in Table 6.4 illustrates a second general trend Stronger bases favor formation of the less substituted alkene. " A stronger base leads to an increase in the carbanion character at the transition state and thus shifts the transition state in the Elcb direction. A linear correlation between the strength of the base and the difference in AG for the formation of 1-butene versus 2-butene has been established. Some of the data are given in Table 6.5. 

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

For compounds in which hydrogen is sufficiently acidified by enough fluorines in (3 positions, the ElcB mechanism may be operating [II]... 

These reactions are often promoted by a strong base, which assists the departure of the proton. X is the leaving group. Both El and E2 mechanisms are known, as is a variant designated Elcb, for unimolecular elimination from the conjugate base of the substrate. ... 

It has been noticed that the reverse reaction of Eq. (5) is a particular type of the Hofmann elimination reaction (26) via either an E2 or an ElcB mechanism. An E2 mechanism seems to be more obvious for this reaction than an ElcB mechanism, however. 

In Europe, interest has centered particularly on polyhydroxybutyrate, which can be made into films for packaging as well as into molded items. The polymer degrades within 4 weeks in landfills, both by ester hydrolysis and by an ElcB elimination reaction of the oxygen atom p to the carbonyl group. The use of polyhydroxybutyrate is limited at present by its cost—about four times that of polypropylene. 

The fact that the rate law of hydrogen bromide elimination is first order with respect to the base may be interpreted by an E2 mechanism. The antiperiplanar position of the hydrogen and the bromine atoms in Ih also makes this mechanism very likely. Earlier the same mechanism was proposed for the elimination reaction of some tertiary a-halo ketones (ref. 19). Other mechanism, such as ElcB or El, seems to be very improbable considering the lack of any deuteration at C-2 or the lack of any rearrangement and the fact that the generation of a-keto cations requires acidic conditions (ref. 20). 

Among the evidence for the existence of the E2 mechanism are (1) the reaction displays the proper second-order kinetics (2) when the hydrogen is replaced by deuterium in second-order eliminations, there is an isotope effect of from 3 to 8, consistent with breaking of this bond in the rate-determining step. However, neither of these results alone could prove an E2 mechanism, since both are compatible with other mechanisms also (e.g., see ElcB p. 1308). The most compelling evidence for the E2 mechanism is found in stereochemical smdies. As will be illustrated in the examples below, the E2 mechanism is stereospecific the five atoms involved (including the base) in the transition state must be in one plane. There are two ways for this to happen. The H and X may be trans to one another (A) with a dihedral angle... 

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