The El Mechanism

In an El process, there are two separate steps the leaving group first leaves, generating a carbocation intermediate, which then loses a proton in a separate step  

The first step (loss of the leaving group) is the rate-determining step, much Uke we saw for SnI processes. The base does not participate in this step, and therefore, the concentration of the base does not affect the rate. Because this step involves only one chemical entity, it is said to be uiumolecular. Unimolecular elimination reactions are called El reactions, where the 1 stands for unimolecular.  

Notice that the first step of an El process is identical to the first step of an SnI process. In each process, the first step involves loss of the leaving group to form a carbocation intermediate  

An El reaction is generally accompanied by a competing SnI reaction, and a mixture of products is generally obtained. At the end of this chapter, we will explore the main factors that affect the competition between substitution and elimination reactions. 

Sample Problem 8.1 Predict the major product in the foiiowing E2 reaction. 

The alkyl halide has two different p C atoms (labeled Pi and P2), so two different alkenes are possible one formed by removal of HCI across the a and Pi carbons, and one formed by removal of HCI across the a and P2 carbons. Using the Zaitsev rule, the major product should be A, because it has the more substituted double bond. 

Problem 8.12 What alkenes are formed from each alkyl halide by an E2 reaction Use the Zaitsev rule to predict the major product. 

The dehydrohalogenation of (CH3)3CI with H2O to form (CH3)2C=CH2 can be used to illustrate the second general mechanism of elimination, the El mechanism. 

Like the SnI mechanism, the kinetics suggest that the reaction mechanism involves more than one step, and that the slow step is unimolecular, involving only the alkyl halide. 

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FIGURE 5 12 The El mechanism for the dehydrohalogenation of 2 bromo 2 methylbutane in ethanol... 

Typically elimination by the El mechanism is observed only for tertiary and some secondary alkyl halides and then only when the base is weak or m low con centration Unlike eliminations that follow an E2 pathway and exhibit second order kinetic behavior... 

The reactivity order parallels the ease of carbocation formation Increasing rate of elimination by the El mechanism... 

We have seen this situation before m the reaction of alcohols with hydrogen halides (8ection 4 11) m the acid catalyzed dehydration of alcohols (8ection 5 12) and m the conversion of alkyl halides to alkenes by the El mechanism (8ection 5 17) As m these other reactions an electronic effect specifically the stabilization of the carbocation intermediate by alkyl substituents is the decisive factor The more stable the carbo cation the faster it is formed... 

As depicted, the E2 mechanism involves a bimolecular transition state in which removal of a proton to the leaving group is concerted with departure of the leaving group. In contrast, the rate-determining step in the El mechanism is the unimolecular ionization of... 

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

In the El mechanism, the leaving group has completely ionized before C—H bond breaking occurs. The direction of the elimination therefore depends on the structure of the carbocation and the identity of the base involved in the proton transfer that follows C—X heterolysis. Because of the relatively high energy of the carbocation intermediate, quite weak bases can effect proton removal. The solvent m often serve this function. The counterion formed in the ionization step may also act as the proton acceptor ... 

This elimination reaction is the reverse of acid-catalyzed hydration, which was discussed in Section 6.2. Because a carbocation or closely related species is the intermediate, the elimination step would be expected to favor the more substituted alkene as discussed on p. 384. The El mechanism also explains the general trends in relative reactivity. Tertiary alcohols are the most reactive, and reactivity decreases going to secondary and primary alcohols. Also in accord with the El mechanism is the fact that rearranged products are found in cases where a carbocation intermediate would be expected to rearrange ... 

FIGURE 5.6 The El mechanism for the acid-catalyzed dehydration of tert-butyl alcohol. 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting spontaneous dissociation process, l- urthermore, El reactions show- no deuterium isotope effect because rupture of the C—H (or C—D) bond occurs after the rate-limiting step rather than during it. Thus, we can t measure a rate difference between a deuterated and nondeuterated substrate. 

The El mechanism is a two-step process in which the rate-determining step is ionization of the substrate to give a carbocation that rapidly loses a P proton to a base,... 

The lUPAC designation is Dn + De (or Dn+Dh). This mechanism normally operates without an added base. Just as the E2 mechanism is analogous to and competes with the Sn2, so is the El mechanism related to the SnE In fact, the first step of the El is exactly the same as that of the SnI mechanism. The second step differs in that the solvent pulls a proton from the P carbon of the carbocation rather than attacking it at the positively charged carbon, as in the SnI process. In a pure El reaction (i.e., without ion pairs), the product should be completely nonstereospecific, since the carbocation is free to adopt its most stable conformation before giving up the proton. 

In the El mechanism, X leaves first, and then H. In the E2 mechanism, the two groups leave at the same time. There is a third possibility the H leaves first, and then the X. This is a two-step process, called the ElcB mechanism, or the carbanion mechanism, since the intermediate is a carbanion ... 

In the El mechanism, the leaving group is gone before the choice is made as to which direction the new double bond takes. Therefore the direction is determined almost entirely by the relative stabilities of the two (or three) possible alkenes. In such cases Zaitsev s rule operates. This rule states that the double bond goes mainly toward the most highly substituted carbon. That... 

The mechanism of these reactions is often El. However, in at least some cases, an E2 mechanism operates.It has been shown that stereoisomers of cyclic y-amino halides and tosylates in which the two leaving groups can assume an anti-periplanar conformation react by the E2 mechanism, while those isomers in which the groups cannot assume such a conformation either fragment by the El mechanism or do not undergo fragmentation at all, but in either case give rise to side products characteristic of carbocations. " ... 

Increasing the temperature of the reaction favors reaction by the El mechanism at the expense of the SN1 mechanism. 

An alkene can accept a proton to generate a carbocation in a process that is essentially the reverse of the deprotonation step in the El mechanism for dehydration of an alcohol. 

Just as the E2 mechanism shares features of the Sn2 mechanism, the El mechanism shares features of the Sn 1 reaction. The initial step is formation of a carbocation intermediate through loss of the leaving group. This slow step becomes the rate-determining step for the whole reaction, i.e. the El mechanism is unimolecular. In general terms, the reaction can be represented as follows. 

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