Lecture 5 Friday January 15 2016 2 56 PM If in a reaction we get a less stable product as the major product we can conclude that the reaction is under kinetic control Reaction that is under thermodynamic control will always give the more stable product as the major product and can be disqualified So we can conclude that the Dials Alder reaction is under kinetic control What can we conclude from a reaction that gives a more stable product as the major product Is that reaction under kinetic or thermodynamic control We know that the reaction in this case could be under thermodynamic control What about kinetic control Unfortunately from the relative stability of the products we cannot conclude anything about the rates of their formation In other words the product that forms faster can be more stable or less stable There is no way to tell Below are the energy diagrams for two reactions in which starting material SM leads to the formation of 2 products P1 and P2 In both cases P2 is more stable However in the first case P2 forms slower and in the second faster Both are plausible Why is rate decoupled from the stability of the products Let s remember that for the rate of the reaction what matters is the activation energy the difference between the ground state of the starting material and the transitions state TS Because the SM is the same for two products we can only look at the TSs If you remember Hammond postulate it says that for exotermic reactions majority of those we care about and the majority of the ones that determine which product is formed Those are usually reactions of high energy intermediates cations radicals or anions which produce stable products TS looks more like a starting material and less like a product In other words the stability of the products is poorly reflected in the TS and therefore the rate of the product formation does not have a strong connection to the stability of the product It depends on lot of other things Overall if we get a more stable product in a reaction there is no way to tell just from that information if the reaction is under kinetic or thermodynamic control For export Page 1 Lecture 5 Friday January 15 2016 2 56 PM Guide for DA reaction Number all atoms that participate in DA reaction based on the simple template shown at the top Write the product with all atoms number again Copy substituents from starting materials to the products For export Page 2 Lecture 5 Friday January 15 2016 2 56 PM From these reaction we can conclude that three double bonds in benzene interact with each other and as a result are much more stable than 3 isolated double bonds would be By about 40 kcal mol 3X30 50 40 We can also conclude that conjugated acyclic triene triene not in a ring is less stable than the cyclic one Just making the conjugate triene cyclic makes it significantly more stable The reason for that is in the MO for the cyclic and acyclic trienes When we talked about allylic system we talked about MO of acyclic systems and said that the most stable pi orbital pi1 has zero nodes electron can be on all atoms that contribute for the pi system the next orbital higher in energy pi2 has 1 node pi3 has two nodes and so on In cyclic systems it is a bit different pi1 has 0 nodes pi2 and pi3 are of same energy that s because there are two ways to introduce a nodal plane to a molecule across the bonds or accorss atoms and the two arrangements are same energetically the fact that there are two arrangements for building orbitals above pi1 is a result of the inherent symmetry of the molecule Cyclic molecules are more symmetrical than acyclic molecules You can see that in both pi2 and pi3 electrons are shared between 3 atoms pi3 covers 2 atoms but also space towards the other two so overall effect is the same as in pi2 that contains exactly 3 atoms Names of different arrangements of two substituents around a benzene ring you should know this For export Page 3