Lecture 4 Monday January 11 2016 11 53 AM Reaction diagram Furthermore if we expose the 1 2 addition product to HBr small amount we get again the mixture of products reversible reactions thermodynamic control more stable product dominates Irreversible reactions kinetic control faster to form dominates We can compare the two transition states TS if we remember that the structure of the TS will be more similar to the starting material when starting material is higher in energy greater H than the product exotermic reaction And conversely the structure of the TS will be more similar to the products if products are higher in energy greater H than the starting material endothermic reaction This is given by the Hammond postulate page 166 in your book So in our case the starting material the carbocation is higher in energy than the products so it will be closer in energy to the TSs and the TSs will look more like the carbocations involved So now we just compare the two carbocations involved and the one that gives the 1 2 addition product is secondary and allylic the other one is primary and allylic So based on that we can conclude that the TS leading to the formation of the 1 2 addition product is more stable than the other transition state Finally from that we can conclude that the 1 2 addition product is easier to form and will be favored when the reaction is under kinetic control Finally our reaction seems to be reversible under the reaction conditions at 25 C but at 80 C Explanation for that is that at 80 C there is not enough energy to go from really stable products back to the starting materials While at 25 C there is enough energy for that In general reactions tend to be more reversible at higher temperatures For export Page 1 Lecture 4 Monday January 11 2016 11 53 AM Diels Alder reaction Diels Alder reaction This is the most important reaction of dienes It is a reaction between dienes and alkene or in this context dienophiles and produces a cyclohexene There are no reagents just heat There are no intermediates formed There is a cyclic transition state in which there is a continuous orbital overlap that allows redistribution of electrons There is whole class that proceed by simoltaneous redistribution of electrons through a cyclic transition state in which there is a continuous orbital overlap and they are called pericyclic reactions Fukui Japanese chemist suggested that we can analyze most reactions in terms of the interaction between the donor orbital HOMO of one molecule and the acceptor orbital LUMO of another molecule This approach is called frontier molecular orbital theory FMO and for it Fukui was awarded a Nobel Prize in chemistry Using this approach we can see how Diels Alder reaction proceeds If we look at the HOMO of the diene and the LUMO of the alkene we can see that the orbitals can interact with each other They have lobes that are of the same phase red or blue and can overlap at the same time The orbital overlap is shown by the doubleheaded arrows This orbital overlap is ultimately what allows the redistribution of the electrons and what leads to the stabilization of the transition state that leads to the products We chose HOMO of a diene because diene is a better electron donor while the alkene is a better acceptor Why does not reaction like this happen with all unsaturated compounds What is so special about a combination of a diene and an alkene Take the follwong reaction in consideration For export Page 2 Lecture 4 Monday January 11 2016 4 52 PM FMO theory provide an explanation for why this reaction does not occur Because of the difference in phase it is not possible to achieve productive orbital overlap on both ends of the pi system the way we could achieve that in the diene alkene case Here on one end the orbitals are in phase and on the other end they are out of phase This prevents the reorganization of electrons and the formation of new bonds So because of the mismatch between the HOMO and the LUMO we say that the reaction is symmetry forbidden Electronic effects in Diels Alder reaction For a given diene which acts essentially as an electron donor the reaction will be faster if we have a dienophile that is e lectron poor if it is a better electron acceptor So more electronwithdrawing groups on a dienophile the faster the reaction slower than Conformational effect in Diels Alder reactions The diene has to be in a s cis conformation the orbital interaction we talked about is possible only in that case The easier it is for diene to adopt s cis conformation foster the reaction The Z alkene makes it harder to addopt s cis conformation and the Z isomer reacts much slower in this reaciton Cyclopentadiene reacts much faster than the acyclic diene because it is always in s cis conformation For export Page 3 Lecture 4 Monday January 11 2016 5 18 PM Stereochemistry of Diels Alder reaction From these reactions we can conclude that the Diels Alder D A reaction is stereospecific If we change the stereochemistry of the dienophile the stereochemistry of the product also changes The relative orientation of the substituents in the starting dienophile cis or trans is preserved in the D A product We did not show it but the same is true for the diene From the second reaction we can also conclude that the D A reaction of stereoselective We get the endo product preferentially over the exo product In the endo product the electron withdrawing group is in the enod position on the bicyclic product closer to the alkene while in the exo product the electron withdrawing groups are away from the alkene Question for thenext lecture We know that the exo product is more thermodynamically stable than the endo product Based on that information is the D A reactrion under kinetic or thermodynamic control For export Page 4