CHEM 3410 1st Edition Lecture 4Outline of Current LectureI. Diels-Alder StereochemistryII. Diels-Alder Cyclic ReactionsA. Diels-Alder reactions beginning with a Cyclic DieneIII. Benzene and AromaticityA.Definition of AromaticityB.Physical CharacteristicsC.Substitution patterns ( Para, Meta, Ortho)IV.Common Names of Aromatic ringsOutline of Last LectureV. Introduction to Cumulated DienesVI. Kinetic vs Thermodynamic Control PracticeVII. Diels-Alder Reactions or “Dies-Alder Cycloadditions”Current LectureI. Diels-Alder Stereochemistry STEREOCHEMSITRY OF REACTANTS IS PRESERVED IN PRODUCTSThe stereochemistry of your dienophile is preserved in the product. That dienophile must be an alkene to have stereochemistry. Dienophiles that are alkynes do not have stereochemistry because they are sp hybridized (LINEAR), and undefined. 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.The orientation of the diene is also preserved in the products. In/Out leads to anti-addition (Trans). Out/Out leads to synaddition (cis). In/In cannot happen due to steric hindrance.He then did several problems from the Diels-Alder Practice workshop. I will not do these for you all because he has already posted the answer key on D2L/Bright space.II. Diels-Alder Cyclic ReactionsEverything is the same. You still make a six membered ring, you still see the movement of electrons…but there is a new element to this concept. Notice the carbon marked “bridge” on the diene and the outward connecting carbons of the product. This is a bridging bicyclic. There is something more about the stereochemistry that you must note. The bridge is in the up position and the carbonyls are in the down positions. When discussing diels-alder cyclic products, you refer to this phenomenon as the “endo product” (down). You will only ever see endo products*! Recall hindrance! If the carbonyls were in the up position, they would be an “exo product” Bridge UP, Groups DOWN = Endo ; Bridge DOWN, Groups UP = Exo You can easily mess up a quiz or exam if you are not careful drawing the six membered, 2D ring. Practice drawing the 3D chairs instead to help yourself understand and to show Dr. Harden you really understand when he is grading your papers.*If you start with a trans-dienophile, you will get an exo product. You will just have to remember that the endo is more favorable.Of course, you will also put your groups in the down/ endo position on their respective carbons as well if necessary. III. Benzene and AromaticityA. Definition – Aromaticity is when you have complete delocalization of bonding electrons around a ring. These have special characteristics including unusual stability, planar geometry, and a tendency towards substitution rather than addition.Reminder: The delocalization of electrons due to p orbital overlap makes conjugated dienes more stable. Aromatization is a specialized delocalization. Aromatics are absurdly stable!Benzene is extra special because it has resonance as a neutral molecule.Remember that we are not simply rotating the pi bonds, WE ARE MOVING ELECTRONS.Every carbon has pi overlap with every other carbon in the molecule. 6 pi electrons shared by 6 carbons…thus all 6 of the pi electrons are delocalized around the ring!B. Characteristics(1) All bonds in benzene are the same length. - Somewhere between what a single bond and a double bond would be. (less than 154 pm, more than 134 pm)(2) All bonds in benzene are the same strength.- Somewhere between what a single bond and a double bond would be ( less than 612 kj/mol, more than 347 kj/mol)(3) Planar geometry. Cyclohexane is buckled and twisted, but cyclic hexane with conjugation is aromatic and planar.(4) Ridiculously high stability. All 6 pi electrons are delocalized. (5) Tendency towards substitution rather than addition. In addition reactions, you remove a double bond. The aromatic ring would lose its aromaticity with the breaking of a double bond, reducing its stability, so it would not naturally want to do that. Instead, the ring prefers substitution. Common Benzene based aromatics:Classic Harden digression that will probably appear as a multiple choice on the test: The moreconjugated the system, the more light it absorbs!B. Substitution patternsWhen naming benzenes, it is important to remember the ortho, meta, and para patterns. The best way to remember how this works is to think of a coordinate system of x, y. X is your R group (like the methyl on toluene). Y is your group. If a group falls at (1,2), the group is ortho. You can have 2 ortho products. If it falls at (1,3), your group is meta. You can have two meta products. If the group fall directly across from your R, at (1,4), it is para. (para – opposite.) All other rules of nomenclature are the same as last semester. (1) Find longest parent chain.(2) Number giving the lowest possible number(3) Write substituents alphabetically before the parent name.Here you have a toluene parent with a nitrite substituent at 1,3. The is meta-Nitrotoluene…or m-Nitrotoluene. Naming Examples: I have also included some of the common aromatic compounds on the next page. There may be more in the chapter. Memorize!Common Aromatics (MEMORIZE. There may be others in Chapter 15 as