Bimolecular 1,2-Elimination

The reaction of ethoxide ion with tort-butyl bromide in ethanol as solvent results in the formation of the elimination product 2-methylpropene. 

This reaction follows the second-order rate law  

The rate law is consistent with a concerted or one-step mechanism. Because two species, ethoxide ion and tort-butyl bromide, react in this step, the reaction is described as a bimolecular elimination or an E2 reaction. This reaction is quite common when alkyl halides are treated with strong bases. Because many nucleophiles are also quite basic, the E2 reaction often competes with the SN2 reaction. 

What evidence is available to support the mechanism shown for the E2 reaction The experimental rate law tells us that both the base and the alkyl halide are present in the transition state or in some step prior to the transition state. Many other experimental techniques can be used to test whether a mechanism that has been proposed for a reaction is the one that is most plausible. Several of these employ the substitution of a less common isotope for one or more of the atoms of the compound. For example, a normal hydrogen atom ( H) can be replaced with a deuterium atom (2H or D) or a tritium (j H orT) atom. Or a normal carbon ( 62C) atom can be replaced with a C or C atom. Because isotopic substitution has only a very small effect on the chemical behavior of a compound, the iso-topically modified compound undergoes the same reactions and follows the same mechanisms as its unmodified counterpart. In one type of experiment, the isotope is used to trace the fate of the labeled atom as the reactant is converted to the product. 

As an example, comparison of the experimental rate of this elimination reaction 

A process that often competes with Sn2 displacements for organic systems is E2 (which stands for elimination, bimolecular ). When there is one or more hydrogens P with respect to the leaving group (see reaction 1.8) and the incoming nucleophile is a strong enough base, bimolecular elimination occurs, often in competition with nucleophilic displacement, 

An important point is that, although an E2 reaction involves movement of three electron pairs, it all happens as a concerted one-step process. 

A sterically hindered carbon center, such as the tertiary carbon in a t-butyl halide, is generally not conducive to an S 2 displacement. 

In addition to Sn2, SnI, and El reactions, there is a fourth pathway by which haloalkanes may react with nucleophiles that are also strong bases elimination by a bimolecular mechanism. This method is the one employed when aUcene formation is the desired outcome. 

The preceding section taught us that unimolecular elimination may compete with substitution. However, a dramatic change of the kinetics is observed at higher concentrations of strong base. The rate of alkene formation becomes proportional to the concentrations of both the starting halide and the base The kinetics of elimination are now second order and the process is called bimolecular elimination, abbreviated E2. 

What products do you expect from the reaction of bromocyclohexane with hydroxide ion  

Give the products (if any) of the E2 reaction of the following substrates CH3CH2I CH3I (CH3)3CC1 (CH3)3CCH2l. 

Rehybridization of the reacting carbon centers from sp to sp to furnish the two p orbitals of the emerging double bond 

Using the Hammet postulate, the transition state for this reaction is product-like (a late transition state), so that factors stabilizing the alkene product will have a strong influence on the transition state. Alkyl groups that are attached to an sp carbon release electrons to the Jt bond and enhance the strength of the Jt bond. A more highly substituted alkene is therefore more stable and a relative order of alkene stability is i  

Chapter 2. Acids, Bases, Functional Group Exchanges 

Although DBN and DBU are very selective, the most common bases used in E2 reactions are hydroxide (in water) or alkoxides in alcohol solvent (sodium methoxide in methanol, sodium ethoxide in ethanol, potassium tert-butoxide in tert-butanol). Amide bases such as sodium amide or lithium diethylamide can be used in ammonia or amine solvents. In these latter cases, the bases are also good nucleophiles and in reactions with 

It is clear that a variety of substrates with different leaving groups can be converted to alkenes via elimination. -Elimination of sulfones is also known.A general transform for the important E2 reaction is 

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Because the rate determining step involves two molecules—the alkyloxonium ion and water—the overall reaction is classified as a bimolecular elimination and given the sym bol E2... 

E2 reaction (Section 11.8) A bimolecular elimination reaction in which both the hydrogen and the leaving group are lost in the same step. 

Notice that there is only one mechanistic step (no intermediates are formed), and that step involves both the substrate and the base. Because that step involves two chemical entities, it is said to be bimolecular. Bimolecular elimination reactions are called E2 reactions, where the 2 stands for bimolecular. ... 

One rather unfortunate aspect of the M + hydrocarbon (and M + OX) reactions mentioned thus far is that the products of the reactions were not detected directly, but were instead inferred via the pressure and temperature dependencies of the measured rate constants for metal reactant consumption and by comparison to ab initio calculations. Exceptions are the reactions of Y, Zr + C2H4 and C3H6, for which the Weisshaar group employed the 157 nm photoionization/mass spectrometry technique to identify the products of the reaction as those resulting from bimolecular elimination of H2.45 47 95... 

Depending on the relative timing of the bond breaking and bond formation, different pathways are possible El reaction or unimolecular elimination and E2 reaction or bimolecular elimination. 

More practical methods involve the dehydrohalogenation [100], dehydro-cyanation [101], or stereospecific bimolecular -elimination of mesitoate esters using potassium t-butoxide in dimethyl sulfoxide [102] as described in Eqs. (40H42). 

With tertiary halides, bimolecular elimination usually occurs if isomeric alkenes can result, the proportions formed depend on the steric requirements of the pyridine because formation of the more substituted alkene (Saytsev Rule) is more sensitive to steric hindrance than formation of the less substituted alkene (Hofmann Rule). Pyridine and r-amyl bromide give 25% of 2-methylbut-l-ene (less... 

When hard bases are the catalysts, the rate of elimination of a compound depends on the proton basicity of the catalyst as shown in Equation 7.34 (where kB is the rate constant for bimolecular elimination) 89... 

An alternative bimolecular elimination process involves the thermal decomposition in an atmosphere of nitrogen of a quaternary ammonium hydroxide (Hofmann exhaustive methylation procedure). 

The oxidant may aid the elimination in a concerted or E2 type of mechanism, as illustrated in Eq. (7) for such examples, the oxidant is not bonded to the substrate, except possibly in the transition state. Other oxidants, for example chromic acid, have been shown to form intermediate esters such as 1 (although other mechanisms have been proposed7), which subsequently decompose by a related, bimolecular elimination [Eq. (2)] here the leaving group is the reduced form of the oxidant, and the C-H bond must necessarily break with the liberation of a proton. As in Eq. (7), the capture of electrons by the oxidant is the driving force of the reaction, so that the breaking of the C-H bond occurs simultaneously in the rate-determining step (Scheme 1). 

Hofmann s Rule is valid for all intramolecular eliminations and for the Hofmann Elimination. Most bimolecular eliminations will follow Saytzeff s Rule. 

Data assembled by Parker (201 demonstrate these effects for bimolecular reactions involving sulfur nucleophiles and haloaliphatic substrates. As an illustration for the case of Reactions 4, the S 2 displacement of iodide from CH3I by SCN at 25°C is accelerated relative to its rate in water tty 0.2 log units in methanol, by 1.1 log units in 10% aqueous dimethyl sulfoxide (v/v), and tty approximately 2.4 log units in dimethyl formamide (DMF). Furthermore, the rates of bimolecular elimination and substitution of cyclohexyl bromide in the presence of thiophenolate at 25°C both increase by 2.7 log units when the solvent is changed from ethanol to dimethylfonnamide (20). 

An ab initio study of the. S N2 and E2 mechanisms has been performed for the reaction between the cyanide ion and ethyl chloride in dimethyl sulfoxide solution.5 Theoretical calculations have predicted a free energy barrier for nitrile formation of 24.1 kcal mol-1, close to the experimental value of 22.6 kcal mol-1 (Scheme 3). It has also been predicted that the isonitrile formation is less favorable by 4.7 kcal mol-1, while the elimination mechanism is less favorable by more than 10 kcal mol-1. These results indicated that isonitrile formation and bimolecular elimination are not significant side-reactions for primary alkyl chloride reactions. 

The influence and impact of these semi-empirical calculations and absolute reaction rate theory on the thinking of physical organic chemists was profound. It makes clear, for example, the electronic basis for some of Ingold s broad generalizations, e.g. In bimolecular eliminations, E2, in systems H—Cp—Ca—X, where X may be neutral or charged, the ]8-CH electrons, independently of the electrostatic situation, enter the Ca octet on the side remote from X, because repulsive energy between electron-pairs in the transition state can thus be minimized the result is anti-elimination, independently of the structural details of the system (Ingold, 1953). 

Bimolecular elimination of suitably labeled ea o-norbornyl tosylate... 

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