Chapter 15: Conjugated Systems, Orbital Symmetry, and UV Spectroscopy

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1 Chapter 15: Conjugated Systems, Orbital Symmetry, and UV Spectroscopy Conjugated unsaturated systems have a p orbital on a carbon adjacent to a double bond The p orbital can come from another double (e.g. 1,3 butadiene) or triple bond The p orbital may be the empty p orbital of a carbocation (e.g. allylic carbocation) or a p orbital with a single electron in it (e.g. allylic radical) Conjugation affords special stability to the molecule and they can be detected using UV spectroscopy. Conjugated dienes contain alternate Single and Double bonds e.g. : 1,3-butadiene, 1,3- pentadiene. The conjugated diene has properties that are very different from those of the nonconjugated diene, 1,4-pentadiene Me conjugated diene, 1,3-butadiene conjugated diene, 1,3-pentadiene nonconjugated or isolated diene, 1,4-pentadiene Preparation and Stability of Conjugated Dienes Elimination of an allylic halide, leads to a conjugated diene Alkoxide Br Bromo butene 1,3 butadiene Conjugated dienes are more stable than nonconjugated based on heats of hydrogenation Hydrogenating 1,3-diene takes up 16 kj/mol more heat than 1,4 diene Heat of hydrogenation for 1,4-pentadiene is 5 kj/mol, about twice that of 1-pentene ΔH for 1-pentene is -16 kj/mol and for trans--pentene is -116 kj/mol, so expect -4 kj/mol for trans-1,3-pentadiene. Actual ΔH is -5 kj/mol, so the conjugated diene is more stable. Difference, -4 kj (-5 kj) = 17 kj/mol, is the Resonance energy

2 Molecular Orbital Description of 1,3-Butadiene π molecular orbitals are the sideways overlap of p orbitals. p orbitals have lobes. Plus (+) and minus (-) simply indicate the opposite phases of the wave function, not electrical charge. The Plus (+) and minus (-) sign can be replaced with different colors or lighter /darker shades. When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed. When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. The single bond between the conjugated double bonds is shorter and stronger than sp 3. π 1 Lowest energy. 3 bonding interactions. Electrons delocalized over four nuclei. π Bonding and 1 antibonding overlap A Bonding MO. π 3 1 Bonding and antibonding overlap Antibonding MO. π 4 Highest energy. 3 Antibonding MO. Empty at ground state. The bonding π-orbitals are made from 4 p orbitals that provide greater delocalization and lower energy than in isolated C=C. The 4 molecular orbitals include fewer total nodes than in the isolated case.

3 In addition, the single bond between the two double bonds is strengthened by overlap of p orbitals. In summary, we say electrons in 1,3-butadiene are delocalized over the π bond system. Delocalization leads to stabilization The MO DIAGRAM of 1,3 Butadiene and Ethylene above shows the location of the π electrons of each molecule with respect to their lowest and highest MO Electrophilic Additions to Conjugated Dienes via an intermediate Allylic Carbocation The Allyl Cation is an example of a conjugated system where the empty p orbital of a carbocation is in conjugation with the p orbitals of a ouble bond. A o allylic carbocation is more stable than a 3 o carbocation while a 1 o allyl cation is intermediate in stability between a 3 o and o carbocation

4 H + H + Cl 1 o Carbocation not formed 3 o Carbocation formed "Review: addition of electrophile to C=C Markovnikov regiochemistry via more stable carbocation Resonance theory predicts that the allyl cation is a hybrid of equivalent structures D and E Both molecular orbital theory and resonance theory suggest that structure C (below) is the best representation for the allyl cation 1 1/ H C H C CH H C CH A B C 1/ Electrophilic Additions to Conjugated Dienes Addition of HBr or HCl to 1,3 butadiene leads to the protonation of the terminal alkene which causes the formation of a delocalized secondary allylic carbocation. H + HBr Br Br - CH 3 H 3 C Br 1, adduct 1,4 adduct Nucleophile Br - can add to either cationic site of the hybrid carbocation. Kinetic vs. Thermodynamic Control of Reactions

5 At completion, all reactions are at equilibrium and the relative concentrations are controlled by the differences in free energies of reactants and products (Thermodynamic Control) If a reaction is irreversible or if a reaction is far from equilibrium, then the relative concentrations of products depends on how fast each forms, which is controlled by the relative free energies of the transition states leading to each (Kinetic Control) Addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1, adduct predominates over the 1,4 adduct At higher temperature, product ratio changes and 1,4 adduct predominates Allylic radicals are conjugated unsaturated systems that have a p orbital (with a single electron in it ) on a carbon adjacent to a double bond. Allylic radicals form readily because they are more stable than ordinary primary, secondary, tertiary, or vinyl radicals Both molecular orbital theory and resonance theory can explain the stability of allyl radicals Molecular Orbital Description of the Allyl Radical When an allylic hydrogen is abstracted to form an allyl radical, the developing p orbital on the sp carbon overlaps with the p orbitals of the alkene. The new p orbital is conjugated with the double bond p orbitals. The radical electron and the p electrons of the alkene are delocalized over the entire conjugated system. Delocalization of charge and electron density leads to increased stability

6 The three p orbitals of the allylic system combine to form three molecular orbitals The bonding molecular orbital contains two spin-paired electrons and this orbital increases bonding between the carbons The nonbonding orbital contains a lone electron which is located at carbons 1 and 3 only Resonance Description of the Allyl Radical: The allyl radical has two contributing resonance forms. These resonance forms A, B can be interconverted using single-barbed arrows. The resonance structures are equivalent. Recall that equivalent resonance structures lead to much greater stability of the molecule than either structure alone would suggest. The true structure of the allyl radical as suggested by resonance theory is C H C CH H C H C CH 1/ 1/ A B C

7 The Diels-Alder Cycloaddition Reaction Discovered by Otto Paul Hermann Diels and Kurt Alder in Germany in the 1930 s Conjugate dienes can combine with alkenes to form six membered cyclic compounds The formation of the ring involves no intermediate (concerted formation of two bonds). The alkene component is called a dienophile. To proceed in good yield and at low temperature the dienophile should have electron withdrawing groups. It also helps if the diene has electron releasing groups. Dienes with electron donating groups and dienophiles with electron withdrawing group can also react well together Some Diels Alder Dieneophiles Stereochemical issues of the reaction. I. Conformations of Dienes in the Diels-Alder Reaction. The relative positions of the two double bonds in the diene are the cis or trans two each other about the single bond (being in a plane maximizes overlap) These conformations are called s-cis and s-trans ( s stands for single bond ) Dienes react in the s-cis conformation in the Diels-Alder reaction

8 s-trans conformation would lead to formation of a highly unstable trans bond in a 6- membered ring. Cyclic dienes which have to be in the s-cis conformation ( to avoid ring strain) are highly reactive. II. The Diels-Alder reaction is stereospecific The relative relationships of the groups on reactants are maintained in the product. The reaction is a syn addition, and the configuration of the dienophile is retained in the product. Groups that are cis on the dienophile will be cis in the product; groups that are trans on the dienophile will be trans in the product III. Regiochemistry of the Diels-Alder Reaction: III A. Exo vs Endo Reactants align to produce endo (rather than exo) product endo and exo indicate relative stereochemistry in bicyclic structures. Substituent on one bridge is exo if it is anti (trans) to the larger of the other two bridges and endo if it is syn (cis) to the larger of the other two bridges

9 Endo Rule: The p orbitals of the electron-withdrawing groups on the dienophile have a secondary overlap with the p orbitals of C and C3 in the diene. Stereochemical preferences for electron-withdrawing substituent to appear in endo position (inside the pocket) is called endo rule. III B. Diels-Alder Reactions Using Unsymmetrical Reagents The 6-membered ring product of the Diels-Alder reaction will have electron-donating and electron-withdrawing groups at 1, or 1,4 but not 1,3 positions. The regiochemistry of the Diels-Alder reaction is determined by the position of the electron donating groups of the diene. It may be easier to explain by simply looking at the resonance structures of the diene and dienophile. Typical electron donating groups on the diene are ethers, amines and sulfide; all have a nonbondingpair of electrons to donate.

10 The regiochemistry of this Diels-Alder reaction is explained by looking at the dipolar resonance structures. The electron-rich carbon of the diene forms a bond with the electron-poor carbon of the dienophile. When the electron donating groups is at the -position of the diene: In this case the Diels-Alder reaction usually gives a single product (or a major product) rather than a random mixture. When the diene has a substituent on C and the dienophile is substituted, the major product of the reaction will have the electron-donating group of the diene in a 1,4- relationship with the electron-withdrawing substituent on the dienophile.

11 When the diene has a substituent on C1 and the dienophile is substituted, the major product of the reaction will have the electron-donating group of the diene in a 1, - relationship with the electron-withdrawing substituent on the dienophile.

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