Mechanisms elimination

Substitution can take place by the S l or the 8 2 mechanism elimination by El or E2 How can we predict whether substitution or elimination will be the principal reac tion observed with a particular combination of reactants The two most important fac tors are the structure of the alkyl halide and the basicity of the anion It is useful to approach the question from the premise that the characteristic reaction of alkyl halides with Lewis bases is elimination and that substitution predominates only under certain special circumstances In a typical reaction a typical secondary alkyl halide such as iso propyl bromide reacts with a typical Lewis base such as sodium ethoxide mainly by elimination... 

In the E2 mechanism (elimination, bimolecular), the two groups depart simultaneously, with the proton being pulled off by a base ... 

It has been shown e that two mechanisms, elimination-addition (benzyne) and SN2 displacement, are operative in the liquid-phase hydrolysis of halogenatcd aromatic compounds. The formation of isomeric phenols as a result of the availability of the benzyne route makes the reaction of limited synthetic value. The incorporation of the copper-cuprous oxide system suppresses reaction via the benzyne route, so that the present method has general utility for the preparation of isomer-free phenols. For example, >-cresol is the only cresol formed from -bromotoluene under the conditions of this preparation. 

The 2-methylenecyclopentanone initially formed presumably rearranges into 2-methyl-2-cyclopentenone under the reaction conditions. The final step of the mechanism, elimination of the cobalt carbonyl group, is not well understood but the same kind of elimination and reduction reactions occur with known 3-ketocobalt complexes. As mentioned above, crotonaldehyde, acrolein (27), and glyddaldehyde (38) react rapidly with cobalt hydrocarbonvl under similar conditions to give reduction products, rather than forming stable alkyl- or acyl-cobalt tetracarbonyl derivatives. 

The benzyne mechanism (elimination-addition) is likely when the ring has no strong electron-withdrawing groups. It usually requires a powerful base or high temperatures. 

There is another, complementary version of the Peterson reaction that uses base to promote the elimination. The starting materials are the same as for the acid-promoted Peterson reaction. When base (such as sodium hydride or potassium hydride) is added, the hydroxyl group is deprotonated, and the oxyanion attacks the silicon atom intramolecularly. Elimination takes place this time via a syn-periplanar transition state—it has to because the oxygen and the silicon are now bonded together, and it is the strength of this bond that drives the elimination forward, this diastereoisomer via this mechanism eliminates to give this alkene... 

The ElcB Mechanism (Elimination, Monomolecular, Conjugate Base) 5.1,2.3 The E2 Mechanism (Elimination, Bimolecular)... 

The ElcB Mechanism (Elimination, Monomolecular, Conjugate Base)... 

Energy Surface Organizes Three Routes E1cB (Via Anionic Intermediate), E1 (Via Cationic Intermediate), E2 (Concerted) E1 in Detail E2 in Detail ElcB in Detail E1/E2/E1cB Spectrum Eliminations in Cyclic Systems Eliminations Yielding Carbonyls Approaches to Elimination Mechanisms Elimination Flowchart... 

Another example of a short residence time PFR is that of New York University [25]. It has a different design in the reactor segment in that a twin-screw extruder is used as a moving-bed mechanism eliminating the external slurry pump. Although the detailed design in acid injection and temperature... 

Mechanical treatments As an additional approach, mechanical elimination of the N-region from RNi is promising, since the N-region is... 

Dehydrogenation of the cyclohexane ring after the edge mechanism (elimination of the first two atoms of... 

---