Predicting and Exploiting Steric Effects

In the previous sections, we learned the skULs that we need in order to predict the products of an electrophilic aromatic substitution. We saw many cases where there is more than one product For example, if the ring is activated, then we expect ortho and para products. In this section, we will see that it is possible to predict which product will be the major product and which will be the minor product (ortho vs. para). It is even possible to control the ratio of products (ortho vs. para). This is VERY important for synthesis problems, which wiU be the next (and final) section of this chapter. 

Consider an electrophilic aromatic substitution with propyl benzene. The propyl group is a weak activator, and therefore, we expect the directing effects to be ortho-para  

There are two products here. But let s try to figure out which one of these products is the major product ortho or paral At first, we might be tempted to say that the ortho product should be major. 

Let s see why. The propyl group is an ortho-para director, so there should be a total of three positions that can get attacked (two ortho positions and one para position)  

Therefore, the chances of attacking an ortho position should be two-thirds (or 67%), while the chances of attacking the para position should be one-third (33%). From a purely statistical point of view, we should therefore expect our product distribution to be 67% ortho and 33% para. But the product ratio is different from what we might expect, because of steric considerations. Specifically, the propyl group is fairly large, and it partially blocks the ortho positions. We do still observe ortho products, but less than 67%. In fact, the para product is the major product in this case  

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The facility and accuracy of empirical calculations for carbon have been exploited through commercial computer programs for general predictions of C chemical shifts. As with calculations (Section 3-2e), the results are only as good as the data set used in their creation. The programs assume that the effects of multiple substitution are additive, unless specific corrections have been incorporated. Unconsidered or nonadditive phenomena, such as conformational and other steric effects, can cause unexpected deviations between observed and calculated chemical shifts. 

Steric effects provide examples of hard cases with respect to predicting reactivities. The same might be said to be true of solvent effects for reactions of n-nucleophiles or carbanions. However, while values of N may vary with solvent the differences can be exploited, for example, in promoting a desired reaction in synthesis. Moreover, in attempting to interpret solvent effects, it is possible that comparing measurements of reaction rates and (preferably)... 

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