Transition states bimolecular elimination

Thermal organic reactions are often classified in terms of the molecular and electronic structure of their transition state or reactive intermediate (which is often taken as a model of the transition state). Thus, for instance, one has the Sn2 transition state for concerted bimolecular nucleophilic substitution reactions one has the E2 and Ei transition states in elimination reactions, etc. Given the transient nature of the transition states, the use of quantum chemical methodologies is essential for the determination of their detailed geometrical and electronic structure. Furthermore, the computation of the associated transition vectors provides information on the reactive mode... 

Elimination bimolecular (E2) mechanism (Section 5.15) Mechanism for elimination of alkyl halides characterized by a transition state in which the attacking base removes a proton at the same time that the bond to the halide leaving group is broken. 

Conversely, when soft bases are used, elimination rates, as would be expected from transition state 43, show no such correlation. Instead there is a relationship between the rate of elimination and the rate of Sw2 substitution by the base as shown in Equation 7.35 (where ks is the rate constant for bimolecular substitution and X is a constant) ... 

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). 

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). 

A different effect of these product-like transition states is seen in the fact that the overall debromination of df-stilbene dibromide with lithium bromide may proceed in the syn sense. That is, the rate ratio at 59° k(meso)/k(dl) = 50 is composite. In the dl compound, electronic factors which favor anti elimination collide with steric or conformational factors, which favor either bimolecular syn eliminations or some other path to frans-stilbene. The rate ratio for production of trans-stilbene k meso)jk dl) 60, while that for anti elimination is k(meso)lk(dl) 310. 

The gas-phase base-induced elimination reaction of halonium ions was thoroughly investigated in radiolytic experiments22. Radiolytically generated acids C/JH5+ (n = 1,2) were allowed to react at 760 Torr with selected 2,3-dihalobutanes to form the halonium intermediates which, in the presence of trimethylamine, undergo base-induced bimolecu-lar elimination as shown in Scheme 6. This elimination reaction occurs in competition with unimolecular nucleophilic displacement to the cyclic halonium ion and subsequent rearrangement. Isolation and identification of the neutral haloalkenes formed and kinetic treatment of the experimental results indicated that 3-halo-1 -butene is formed preferentially with respect to the isomeric 2-halo-2-butenes and that the bimolecular elimination process occurs predominantly via a transition state with an anti configuration22. 

The rate of this elimination is proportional to the concentrations of both the alkyl halide and the base, giving a second-order rate equation. This is a bimolecular process, with both the base and the alkyl halide participating in the transition state, so this mechanism is abbreviated E2 for Elimination, bimolecular. 

Geometric features of transition states for addition reactions of olefins (Chapter 3), bimolecular substitutions (Si f3 reactions p. 36), bimolecular eliminations (E2 reactions p. loi) and many other reactions have been defined from studies of a variety of steroid systems (e.g. Fig. la). Reactions proceeding through carboniumions (e.g. Fig.Tb seep. 228) are sensitive to... 

E2 mechanism (Sections 8.3, 8.4) An elimination mechanism that goes by a one-step concerted process, in which both reactants are involved in the transition state. E2 is an abbreviation for Elimination Bimolecular. ... 

Dehydrobromnation. )8-Eliminations of HX to form olefins have generally been believed to proceed through an anti transition state. A recent review of bimolecular... 

As this example and many others show, the bimolecular reaction of alkyl halides involves OAi/i-elimination in the transition state the hydrogen and the leaving group are located as far apart as possible, in the anti relationship (Sec. 3.3) as opposed to gauche or eclipsed (see Fig. 14.4, p. 482). 

Bimolecular nucleophilic substitution at tetracoordinate phosphorus (Eq. (8)) may proceed by either direct (Sjq2) substitution or by an addition-elimination mechanism In the former, 16 represents a transition state, while in the... 

Three-center SN2 displacement and anti-eliminations from unsaturates are obvious examples of the coplanarity principle. DePuy et al. (1965) noted that when anti eliminations cannot have coplanar reacting centers the syn coplanar-transition state may become more favorable. Syn bimolecular eliminations had been noted in various systems previously, e.g. haloethenes (Miller, 1961), but these were generally slower than anti eliminations. There were, however, syn bimolecular eliminations whose rates approached that of the anti form or exceeded it. The relative rates of elimination of 2-phenylcyclopentyl and cyclohexyl tosylates with t-butoxide in t-butyl alcohol at 50° are as follows syn-cyclopentyl, 3 awii-cyclopentyl, 26 syw-cyclohexyl, 0 and anii -cyclohexyl, 2. In the cyclopentyl system in which the torsional angle (r) between the leaving groups approaches zero the syn rate is close to the anti rate. In the cyclohexyl system in which t of the stable form is ca. 60° the syn rate is 0. LeBel et al. (1964), report that in the reactions of t-butoxide with the 2,3-dihalobornanes, 92-95,... 

Elimination reactions of the 20-hydroxycholanic acid derivative (108) probably illustrate the differing character of the transition state, depending upon the reagent. Thionyl chloride in pyridine gave the 17(20)-enes (109 cis -I- trans, 80% total), as expected from thermodynamic control when a bimolecular elimination has considerable carbonium ion character ( l-like) in the transition state. The 20(22)-enes (110) cis + trans) were major products when phosphoryl... 

In reactions of tertiary halides, unimolecular processes dominate in protic solvents (especially water and aqueous solvent mixtures) where substitution is usually faster than elimination. Substitution involves nucleophilic attack directly at a cationic center, whereas elimination involves removal of an acidic hydrogen two bonds removed from the cationic atom, explaining the difference. Clearly, a nucleophilic species is strongly attracted to the most positive center and that should lead to the major product. In aprotic solvents, bimolecular substitution is not observed for tertiary halide due to the high energy required to form the pentacoordinate transition state (sec. 2.7.A.i). Under conditions that favor bimolecular reactions and in the presence of a suitable base, elimination is the dominant process. 

Clear evidence that the geometry of the transition state is not constant has been provided by studies of eclipsing effects . Assuming anti stereospecificity, the threo and erythro isomers of the 1,2-diphenyl-l-propyl-X systems undergo bimolecular elimination to give the trans- and cw-a-methyl stilbenes respectively (75). 

Such a mechanism requires that the major product, 1,2,4-trichlorobenzene should contain deuterium when the reaction is performed in a deuterated medium , but the amount of label found was no greater than predicted from the rate of exchange of the product under the reaction conditions . Presumably fairly extensive double-bond character is exhibited in the transition state for bimolecular elimination from the benzene hexachlorides and the abnormally high activation energy for the beta isomer is most plausibly attributed to the lack of coplanarity in the iy -c//nfl/-elimination or the high energy and low population of the boat conformer required for jy/i-elimination. Alternatively, a sparsely populated conformation for a concerted syn-clinal-... 

Although certain experimental observations are more easily interpreted by the merged mechanism than the more conventional transition states of bimolecular elimination and substitution, they do not require it. The unusual effectiveness of weak bases to promote elimination reactions is unconvincing, and Bunnett has stated that equilibrium and kinetic basicity need not parallel... 

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