Second order elimination

On the very safe assumption that an atom of tin scatters X-rays more strongly than an atom of magnesium, the value of 5 for the third order is less than that for the second order, eliminating these two arrangements. 

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... 

This hypothesis has been criticized by Busvine (2,3), Domenjoz (10), Muller (18), and Cahn (4). Domenjoz and Muller have shown that there is no direct correlation between activity toward a variety of insects in a number of compounds of the type Ar2CHCCl3 and the amount of hydrogen chloride liberated under standard conditions. Busvine attempte( 1 a correlation for similar compounds between activity toward lice and bedbugs and this author s reaction-rate constants (2, 5) for second-order elimination with ethanolic alkali and found that no statistically significant correlation exists. 

In the course of work on the mechanism of elimination reactions, the author and his co-workers have measured reaction-rate constants for the second-order elimination of hydrogen chloride from six dichloroethyl compounds of type Ar2CHCHCl2 and three monochloroethyl compounds of type Ar2CHCH2Cl (7). Samples of each of these materials were furnished to the Bureau of Entomology and Plant Quarantine for insecticidal testing, and the author is indebted to C. C. Deonier and I. H. Gilbert for permission to use certain of their data in this paper. The rate constants and larvicidal results are given in Table I. 

Cristol, S. J. (1947) The kinetics of the alkaline dehydrochlorination of the benzene hexachloride isomers. The mechanisms of second-order elimination reactions. Journal of the American Chemical Society, 69, 338-342. 

In the remaining sections of this chapter we will discuss further examples of kinetic isotope effects. The first considers a system in which there is a competition between two mechanisms, Sn2 and E2 and returns to reaction 10.15. (By E2 we refer to a second order elimination reaction, see Fig. 10.6). In Equation 10.15 the hypochlorite... 

Roberts et al. (1993) have provided an in-depth analysis of QSARs for dehydrohalogenation reactions of polychlorinated and polybrominated alkanes. The QSARs were developed based on a dataset of 28 polychlorinated and polybrominated compounds in aqueous solution at 25° C. The QSARs are for the OH- and water-mediated second-order elimination reactions (E2), as Equation (25) shows. The first QSAR is for base-promoted dehydrochlorination and is based on the inductive parameter o, (Equations (26) and (27)) ... 

First-Order Elimination The E1 Reaction 258 Key Mechanism 6-8 The E1 Reaction 258 Mechanism 6-9 Rearrangement in an E1 Reaction 261 Summary Carbocation Reactions 262 6-18 Positional Orientation of Elimination Zaitsev s Rule 263 6-19 Second-Order Elimination The E2 Reaction 265 Key Mechanism 6-10 The E2 Reaction 266 6-20 Stereochemistry of the E2 Reaction 267... 

Second-order elimination is a reliable synthetic reaction, especially if the alkyl halide is a poor Sn2 substrate. E2 dehydrohalogenation takes place in one step, in which a strong base abstracts a proton from one carbon atom as the leaving group leaves the adjacent carbon. 

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 E2 dehydrohalogenation gives excellent yields with bulky secondary and tertiary alkyl halides, such as ferf-butyl bromide in the preceding example. A strong base forces second-order elimination (E2) by abstracting a proton. The molecule s bulkiness hinders second-order substitution (SN2), and a relatively pure elimination product results. Tertiary halides are the best E2 substrates because they are prone to elimination and cannot undergo Sn2 substitution. 

Young s modulus standard electrode potential standard electromotive force half-wave potential first-order elimination second-order elimination for example... 

Facts (a), (b), and (c) are, of course, exactly what we would expect for the E2 mechanism. The rate-determining step (the only step) involves reaction between a molecule of alkyl halide and a molecule of base, in which a carbon-hydrogen bond is broken, and in which there is no opportunity for rearrangement. In particular, these three facts rule out a carbonium ion (El) mechanism for second-order elimination. 

When this reaction was interrupted, and unconsumed jS-phenylethyl bromide recovered, it was found by mass spectrometric analysis to contain no deuterium. Similar experiments with other systems have given similar results in typical second-order elimination reactions there is no hydrogen-deuterium exchange. 

Product distributions from second-order eliminations from neo-menthyl chloride (16) and neo-menthyltrimethylammonium hydroxide (19). 

Hughes, E. D. Wilby, J. /. CItem. Soc. 1960,4094. The product distributions reflect second-order elimination. Some first-order elimination accompanied the reaction. 

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