Elimination reaction, second-order competing reactions

The refers to a nucleophilic substitution process where some nucleophile attacks an electrophile and substitutes for some part of the electrophile. The E refers to an elimination process where the nucleophile attacks an electrophile and causes the elimination of something. The 1 and 2 refer to the order of the reaction. A 1 (first order) means only one molecule determines the rate of the reaction, whereas a 2 (second order) means that a combination of two molecules determines the rate of the reaction. In many cases, two or more of these mechanisms are competing and more than one product may result. 

Elimination to give propene competes with substitution to give ethyl isopropyl ether. Furthermore, the rate of elimination, like the rate of substitution, is proportional to the concentrations of 2-bromopropane and ethoxide ion. Thus elimination here is a second-order reaction (it may be helpful to review Section 8-4 at this point) ... 

The mechanisms of the reductive eliminations in Scheme 5 were studied [49,83], and potential pathways for these reactions are shown in Scheme 6. The reductive eliminations from the monomeric diarylamido aryl complex 20 illustrate two important points in the elimination reactions. First, these reactions were first order, demonstrating that the actual C-N bond formation occurred from a monomeric complex. Second, the observed rate constant for the elimination reaction contained two terms (Eq. (49)). One of these terms was inverse first order in PPh3 concentration, and the other was zero order in PPh3. These results were consistent with two competing mechanisms, Path B and Path C in Scheme 6, occurring simultaneously. One of these mechanisms involves initial, reversible phosphine dissociation followed by C-N bond formation in the resulting 14-electron, three-coordinate intermediate. The second mechanism involves reductive elimination from a 16-electron four-coordinate intermediate, presumably after trans-to-cis isomerization. 

This is a typical SN2 reaction (1) kinetics usually display clean second order (b) when the reagent is chiral, inversion of configuration occurs at the asymmetric carbon (71 ACS 18) when the alkylating reagent is tertiary (t-butyl iodide) or even secondary (i-Pr iodide), E2 elimination competes with aliphatic substitution (55JA1715 76AJC1745). 

Dehydrohalogenation is the elimination of a hydrogen and a halogen from an alkyl halide to form an alkene. In Sections 6-17 through 6-21 we saw how dehydrohalogenation can take place by the El and E2 mechanisms. The second-order elimination (E2) is usually better for synthetic purposes because the El has more competing reactions. 

The crucial step in this mechanism is the second one in which the a-haloether moiety 115 solvolyzes to give an oxocarbocation. In order to obtain the ionic bicyclobutane this step has to compete effectively with both protonation of the carbanion and the 1,3-elimination reaction. By using oxygen nucleophiles, e.g. MeO", EtO" and CF3CH2O", in pro tic solvents, the rate of these three reactions was found to decrease in the order solvolysis > elimination > protonation. Although an apolar medium is expected to enhance the elimination reaction and to slow down the solvolytic step, it was shown that for MeO" in THF, the ionic bicyclobutane route still prevails. ... 

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