Transition state 2 is structurally most likely as :

Options

A. intermediate 1
B. transition state 3
C. intermediate 2
D. product

Answer

C

Explanation

Knowing something about the structure of a transition state is important when you are trying to predict the products of a reaction. The structure of the transition state lies between the structure ofthe reactants and the structure ofthe products. Butwhat do we mean by"between"? Does the structure ofthe transition state lie exactlyhalfwaybetween the structures of the reactants and products (as in 11, below), or does it resemble the reactants more closely than it resembles the products (as in I), or is it more like the products than the reactants (as in Ill)?

According to the Hammond postulate, the transition statewill be more similarinstructure to the species that it is more similar to in energy. In the case ofan exergonic reaction, the transition state is more similar in energy to the reactant than to the product. (curve I). Therefore, the structure ofthe transition state will more closely resemble the structure of the reactantthanthatofthe transition statewill more closelyresemble the structure ofthe reactantthanthatofthe product. In anendergonicreaction (curve III), thetransitionstate is more similar in energy to the product, so the structure ofthe transition state will more closely resemble the structure of the product. Only when the reactant and the product have identical energies (curve Il) would we expect the structure ofthe transition state to be halfway between the structures ofthe reactant and the product.

Nowwe can understand whythe tert-butyl cation is formed faster than the isobutyl cation when 2-methylpropene reacts with HC!. Of the two possible products, the tert-butyl cation (a tertiary carbocation) is more stable than the isobutyl cation (a primary carbocation). Because formation ofa carbocation is an endergonic reaction, the structure ofthe transition state will resemble the structure ofthe carbocation product. This means thatthe transition statewill have a significant amountofpositive charge on a carbon. The same factors that stabilize the positively charged product will stabilize the partially positively charged transition state. Therefore, the transition state leading to the tert-butyl cation is more stable than the transition state leading to the isobutyl cation, so the tert-butyl cation will be formed faster. It is important to keep in mind thatthe amount of positive charge inthe transitionstateis notas greatas the amountofpositive charge inthe transitionstateis notas great as the amountofpositive charge inthe carbocationproduct, so the difference in the stabilities of the two transition states is not as great as the difference in the stabilities ofthe two carbocation products.