Normal E2 mechanism

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism.66 McLennan has suggested that the transition state is that shown as 18.w An ion-pair mechanism has also been proposed.70 Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases.71 These reactions also have the following general characteris-... 

If l-chloro-1, 1-dideuteriobutane is treated with a very strong base such as phenyl sodium, then, even though some of the resultant alkene contains two deuterium atoms on the terminal carbon atom, a higher percentage of the 1-alkene contains only one deuterium atom. This deuterium atom is located on the terminal carbon atom. The expected product, with two deuterium atoms, which is formed in smaller amounts, results from the normal E2 mechanism. Suggest a mechanism that accounts for the formation of the major product. 

Like alcohol dehydrations, El reactions of alkyl halides can be accompanied by carbocation reariangements. Eliminations by the E2 mechanism, on the other hand, normally proceed without reanangement. Consequently, if one wishes to prepare an alkene from an alkyl halide, conditions favorable to E2 elimination should be chosen. In practice this simply means cariying out the reaction in the presence of a strong 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 an E2 mechanism one normally expects second-order kinetics however, Harlow73 observed that the decomposition approached first-order kinetics, in agreement with the assumption of an ion pair, R4N + OH, as an intermediate. 

Alkyl halides normally undergo elimination with hard nucleophiles. Elimination usually occurs from the conformer in which the leaving group and H are anti to one another. The product is Z-PhC(Me)=C(Me)Ph by the E2 mechanism. 

In order for an E2 mechanism to take place a base must approach the proton marked. In C this proton is shielded on both sides by R and R. In D the shielding is on only one side. Therefore, when anti elimination does take place in such systems, it should give more cis product than trans. Also, when the normal anti elimination pathway is hindered sufficiently to allow the syn pathway to compete, the anti — trans route should be diminished more than the anti — cis route. When syn elimination begins to appear, it seems clear that E, which is less eclipsed than F, should be the favored pathway and syn elimination should generally give the trans isomer. In general, deviations from the syn-anti dichotomy are greater on the trans side than on the cis. Thus, trans olefins are formed partly or mainly by syn elimination, but cis olefins are formed entirely by anti elimination. Predominant syn... 

Effect on El versus E2 versus ElcB. In the El mechanism, an external base is generally not required The solvent acts as the base. Hence, when external bases are added, the mechanism is shifted toward E2. Stronger bases and higher base concentrations cause the mechanism to move toward the ElcB end of the El-E2-ElcB spectmm."° However, weak bases in polar aprotic solvents can also be effective in elimination reactions with certain substrates (the E2C reaction). Normal E2 elimination has been accomplished with the following bases " HjO, NR3, OH, OAc, OR, OAr, NHj, COa ,... 

The hydroxide ion removes the (P-hydrogen in a normal anti-E2 mechanism. 

An example of an E2 mechanism that is paenecarbanionic is the base elimination of water from a 3-hydroxycarbonyl derivative. Normally, water is eliminated under acidic conditions in which the hydroxyl group is protonated to form a substituent, namely -OH2+, that in turn would give rise to a more favourable leaving group, i.e. H20. When the first stage of the aldol reaction is performed... 

When we looked at anti-E2 bimolecular eliminations, we saw that the Hofmann exhaustive methylation followed that pathway. There are occasions, however, when it proceeds via different route. Normally, the hydroxide anion removes the (P-hydrogen, which then initiates the E2 mechanism. If, however, the quaternary ammonium derivative is so highly hindered that this is not possible, then the hydroxide anion removes a proton from one of the methyl... 

All of the 1,2-elimination mechanisms discussed here have assumed that, at some point, a base abstracts a proton jS to the leaving group by directly attacking that proton. Some authors have distinguished between the E2H ("normal E2") pathway and an E2C pathway, in which the base interacts with the a-carbon atom attached to the leaving group prior to removal of the -hydrogen... 

The secondary a- and perdeutero-deuterium KIEs E2 S 2 ratios reaction efficiencies transition-state looseness parameters and the relative basicities of the nucleophiles in the gas-phase reactions between methyl, ethyl, i-propyl, and f-butyl iodides and SH , Cr, and CN have been used to show that (i) the ethyl and i-propyl iodides react mainly by an Sf 2 mechanism, while the i-butyl iodide reacts by an E2 mechanism, (ii) that the E2 barrier is more sensitive to basicity of the nucleophile than the 5 2 barrier, and (iii) that stronger bases promote more 2 elimination. The KIEs in the ethyl iodide reactions indicate that the transition state becomes looser as the nucleophile changes from Cl to CN to SH. Larger (more normal) deuterium KIEs and looser S 2 transition states are found when more alkyl groups are added to the C of the substrate. 

Section 8 13 When nucleophilic substitution is used for synthesis the competition between substitution and elimination must be favorable However the normal reaction of a secondary alkyl halide with a base as strong or stronger than hydroxide is elimination (E2) Substitution by the Sn2 mechanism predominates only when the base is weaker than hydroxide or the alkyl halide is primary Elimination predominates when tertiary alkyl halides react with any anion... 

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