Elimination of HX X F, Cl, Br, I from Alkyl and Alkenyl Halides

P-elimination of hydrogen halides from the corresponding alkyl or alkenyl halide frequently accompanies nucleophilic subsitution. When a nucleophile is strongly basic, elimination may be made the major reaction pathway.  

As early as 1935, Ingold (vide supra) and coworkers recognized and codified two, idealized, limiting elimination pathways. By analogy to the nucleophilic substitution processes, SnI and Sn2 (discussed above), these were called El and E2 (for 

Finally, given the similarities between the SnI and El reactions, the choice as to which might preferentially occur frequently rests on (1) the temperature at which the reaction is carried out and (2) the basicity of the nucleophile. 

Problem 7.9. Examine the equations (Equations 7.29-7.42) describing the kinetics associated with the SnI reaction and, either de novo or by suitable modification, write a set that is in concert with what might be expected for the El reaction. 

Interestingly, as shown in Table 7.9, the ratio of more highly substituted (more stable) alkene to less highly substituted (less stable) alkene produced in alcohol-water solvent mixture at 25°C appears to also be a function of the steric bulk of the attached substituents. 

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