Product-determining step for El elimination

In the Elcb mechanism, the direction of elimination is governed by the kinetic acidity of the available protons, which, in turn, is determined by the inductive and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from unhindered positions leads to the formation of less-substituted alkenes. 

Comp irison of the data for methoxide with t-butoxide in Table 6.3 illustrates the second general trend Stronger bases favor formation of the less-substituted 

The direction of elimination is also affected by steric effects. Highly hindered bases shift the orientation in dehydrohalogenations toward more Hofmann rule elimination. This effect can be reasonably attributed to the fact that the internal hydrogen that must be removed for Saytzeff rule elimination becomes inaccessible to very bulky bases, and abstraction of less-hindered protons is favored  

Two elements of stereochemistry enter into determining the ratio of isomeric olefins formed in E2 reactions. First, elimination may proceed in a syn or anti fashion  

Second, in many cases, the product olefin may be a mixture of the cis- and trans-isomers. The product ratio therefore depends on these details of 

---

Fig. 5.14. Potential energy profile for product-determining step for El elimination.

As anticipated on the basis of the idea that the rate-determining step in the El reaction is the same as that in the SnI reaction, representations such as that of Figures 7.17 and 7.18, shown for the path of El elimination from a 2-halo-2-methyl substrate (L = halogen) to the corresponding alkene product (i.e., 2-methylpropene [(CH3)2C=CH2]), will be strikingly similar to that seen for an SnI reaction on the same substrate. Thus, except for the product-determining step, it would be anticipated that the paths seen in Figures 7.6 (or 7.7) and 7.17 (or 7.18) would be the same. 

As indicated in Table 13.7,1,2-dibromoethane (BrCH2-CH2Br) and 1,1,1-trichloro-ethane (CH3-CC13) are examples in which both hydrolysis and elimination are important. If in such cases the reactions occur by SN2 and E2 mechanisms, respectively, the ratio of the hydrolysis versus elimination products should vary with varying pH and temperature, since the two competing reactions likely exhibit different pH and temperature dependencies. On the other hand, if the reaction mechanisms were more SN1- and El-like, a much less pronounced effect of temperature or pH on product formation would be expected, since the rate-determining step in aqueous solution may be considered to be identical for both reactions ... 

---