Chem 334 1nd Edition Lecture 2 Outline of Last Lecture I. Dienes and Conjugated CompoundsOutline of Current Lecture II. NBSIII. Kinetic Vs. Thermodynamic ProductA. Temperature and ProductsB. Energy DiagramIV. Diels Alder ReactionsCurrent LectureNBS : -N-bromosuccinimideNBS is a radical bromine source. It does many of the same reactions as bromine. Attached to electron-withdrawing nitrogen of succinimide, the bromine has a partial positive charge and is therefore electrophilic. The most common use of NBS is allylic bromination. Allylic bromination is the replacement of a hydrogenon a carbon adjacent to a double bond. NBS is used as a substitute for Br2 because Br2 tends to react with double bonds to form dibromides. The advantage of NBS is that it provides a low concentration of Br2.// \ // \ /BrAllylic substitution happens because of radical stability. Resonance allows for added stability.// \ > These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. Br2hv or ΔNBShv or ΔBrKinetic Vs. Thermodynamic ProductWhen a strong acid is added to certain dienes, two products are possible; a 1,2 product and a 1,4 product. Example:// \ / \ \ / \\ / Cl (1, 4-addition product)(1,2-addition product)As you can see, the Cl of the 1, 2-addition product is on the 2nd carbon on the chain, and the Cl on the 1, 4-addition product is attached to the 4th carbon in the chain. At low temperatures, 1, 2-addition product dominates. At higher temperatures, 1,4 addition product dominates. If there is kinetic control of a reaction, the reaction rate determines product. If there is thermodynamic control of reaction, stability of product determines the product ratio. // \+/ Cl - / \ +/ // \ /// \ // HCl Cl- Cl +// \ / Cl - +/ \ / \ / \\ / Cl-1. Carbocation (resonance structures are drawn)2. Lowest transition energy state. The positive charge is largely on the most substituted carbon.3. Highest energy transition state. The positive charge is on the least substituted carbon.4. This is the kinetic product (the 1, 2-addition product). This product is formed faster, but is less stable compared to the thermodynamic product.5. This is the thermodynamic product (1, 4-addition product). This product is formed slower but is more stable than the kinetic product. This is the dominant product at higher temperatures. Cl421Cl53Energy Diagram of reactions:E Intermediate A prod. B prod.In the energy diagram above, B is the more stable product. So, if the reaction is under thermodynamic control, B is the thermodynamic product. If the reaction is under cold temperatures, the reaction is very fast and there is no equilibrium. In this case, the product that forms goes over the lowest energy barrier, which in the diagram above would be A, and is the kinetic product. In a high temperature reaction, there is more energy to cross a higher barrier. In this case the product gets stuck in the lowest product, and themore stable product forms.The marble scenario;The easiest way to think about this is to imagine that there’s a bunch of marbles in the well of the intermediate. At a cold temperature there is low energy, so you shake the diagram gently. In this case, most marbles will go to A because there is a smaller barrier for the marbles to roll over, and not much energy to roll them back out. This means that A is the kinetic product. At a high temperature, you shake it more vigorously. In this case, the marbles will roll in all directions. However, the marbles that roll into Awill eventually roll back out, since the barrier keeping them in A is not that high. But, once they roll into B, they can’t come back out again. So at high temperatures all of the marbles will eventually be stuck in B, making B the thermodynamic product.Diels Alder Reactions:Diels Alder reactions refers to the [4+2]-cycloaddition of a conjugated diene and a dienophile. Specifically, the reaction involves the 4 π-electrons of the diene and 2 π-electrons of the dienophile. These reactions are important, as they result in the formation of new σ-bonds, which are more stable than the π-bonds. In these reactions, all electrons move at once. Three π bonds are broken in order to make two single bonds and one π bond. This is a really efficient way to form 6-membered rings and form carbon-carbon bonds. Dienes usually have an electron with a drawing group attached. oo ooooThe diene must be in the S-Cis configuration to react. As long as dienes are in S-cis configurations, cyclic structures can be used as well. This means that cyclopentadiene can be used in Diels Alder reactions;Dienophiles can be either alkenes or alkynes. o o