Crowded eclipsed conformation

As we discussed previously, in Mechanism 1 the steric effects determined the relative reaction rates during the 5yn-elimination step a more crowded eclipsed conformation leads to a slower elimination reaction. Thus, one would expect that 1,2-dibromodecane 12 should be a very reactive substrate, (minimal steric interactions in the eclipsed conformation of the salt 14). Instead, 11 reacts 330 times faster, despite the more crowded eclipsed conformation of the salt 13 during the elimination step (Scheme 29.8). 

If A and B are large bulky groups they will bump together, attainment of the eclipsed conformation will be almost impossible, and rotation will be severely restricted. Even if A and B are hydrogen atoms (ethane), there will be a rotational barrier in the eclipsed conformation which amounts to 12 kj (3 kcal) per mole because of the crowding of the hydrogen atoms as they pass each other.5 9 This can be appreciated readily by examination of space-filling molecular models. 

Staggered conformations require sufficient energy for the chain to overcome the high barrier represented by crowded eclipsed forms, and polyisobutene does not retain its elastic character at temperatures as low as those at which more resilient rubbers can be used. 

A concerted syn-elimination requires the salt to adopt an eclipsed conformation that should be very sensitive to the steric interactions between the R groups. Then we should expect that a more crowded conformation (as in 7) would lead to a slower elimination process. This is consistent with the experimental observation that eryt/zro-dibromo derivatives react much faster than their threo-aaaXogs. In addition, the increasing ring strain for syn-eliminations in cyclic molecules could justify the slow debrominations observed for trans-l,2-dibromocycloalkanes. 

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