PHYS 201 1st Edition Lecture 3Outline of Last Lecture I. Kinetic and thermodynamic products and Diels Alder ReactionsOutline of Current Lecture II. Stereochemistry and Diels AlderIII. BenzeneIV. Aromatic RingsA. AnnulenesB. HeteroaromaticsC. Aromatic Hydrocarbon ionsCurrent LectureStereochemistry of Diels Alder reactions:Remember that Diels Alder reactions involve a diene and a dienophile and the diene must be cis to react.However, if the dienophile is cis or trans, it is important that the product of the reaction be stereochemically accurate; if you have a cis substituted dienophile, you should have a cis product, if you have a trans substituted dienophile, you should have a trans product.Example:EBenzene: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.Benzene is a six membered ring containing three π bonds. The ring is made up of alternating single and double bonds. Because all resonance structures of the benzene ring are equal, the bonds are all in equilibrium.The double bonds move around the ring so fast that all carbon-carbon bonds are equivalent. Every carbon on the benzene ring has a p orbital that is perpendicular to the ring with six electrons shared between the six p orbitals because of conjugation and resonance. Therefore, energy is lowered.Aromatic rings: Aromatic rings are special because they are lower in energy than other alkenes. For a molecule to be considered an aromatic ring, it must meet all the conditions of Huckel’s Criteria.Criteria 1: The molecule must be cyclic. This can be any kind of closed rind, from three bonds to a hundred. Criteria 2: The molecule must be planar. However, this is beyond our class so for now just assume that allmolecules meet this criteria.Criteria 3: There must be a p orbital on each atom within the ring. However, groups outside of the ring need not have a p orbital.Criteria 4: Molecules must satisfy the π electron rule of 4n+2, where n must be a positive integer.Example: This ring has 6 π electrons. 6=4n+2 n=1 which is an integer. Therefore, this structure is an aromatic ringThis ring has 4 π electrons. 4=4n+2 n=1/2 which is not an integer. Therefore, this structure is not and aromatic ring.Annulenes:Annulenes are cyclic hydrocarbons with continuous alternating of single and double bonds. However, this doesn’t necessarily mean that they are aromatic.Heteroaromatic: If you replace one or more carbons with a hetero atom (O, S, N) you get a heterocyclic compound. These can also be aromatic rings, as long as they are in sp2 orbitals. Remember that for a lone pair to be able toresonate, it has to be in a p orbital. Aromatic hydrocarbon ions:Certain molecules that are not aromatic can become aromatic rings by deprotonating or by removing a hydride.Example:This is not an aromatic ring as the bond at the top is sp3. However, if we remove a hydride, the bond at the top of the ring has a positive charge. This means that there is a p orbital, but not a π bond, so the structure only has 2 π electrons. If we check the 4n+2 rule, n is 0, which counts as a positive integer.Deprotonation can also make a structure aromaticExample: Na+OH- - Deprotonation makes the ring aromatic nowNaNH2, LDA, and nBuLi are also bases that can work for
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