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Organic Chemistry Reviews Chapter 13

Organic Chemistry Reviews Chapter 13. Cindy Boulton March 15, 2009. Molecular Orbitals. Sigma ( σ ) Bond Combine two S Atomic Orbitals Form a sigma bond- “in phase” at a lower energy Form a sigma (*) antibond - “out of phase” at a higher energy

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Organic Chemistry Reviews Chapter 13

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  1. Organic Chemistry ReviewsChapter 13 Cindy Boulton March 15, 2009

  2. Molecular Orbitals • Sigma (σ) Bond • Combine two S Atomic Orbitals • Form a sigma bond- “in phase” at a lower energy • Form a sigma (*) antibond- “out of phase” at a higher energy • Large energy gap between sigma bond and anitbond* • A double bond is made of 1 sigma bond and 1 pi bond • Pi (π)Bond • Both an “in phase” and “out of phase” with a pi bond and pi (*) antibond • Smaller energy gap between pi bond and antibond* • Non-bonding pairs of electrons (n) • Forbidden Transition

  3. UV/Vis Spectroscopy • Electron movement to a higher orbital is caused by absrobation of Energy • Specific wavelength of Energy required depending on the size of the gap • ε: Molar Absorbtivity • A = ε b c • Movement from HOMO to LUMO orbital • HOMO: Highest Occupied Molecular Orbital • LUMO: Lowest Unoccupied Molecular Orbital • Decrease Energy gap, Decrease frequency, Increase wavelength • E = ch or E = c/λ • Move into Visible Spectrum portion and color is observed

  4. Visible Spectrum • 400-700 nm • <400: UV • >700: IR • Seeing Color • Source emits a specific wavelength/color of light • White light shine on an object but only specific wavelenght/color is absorbed. Complement color is observed or reflected back off the object • White light: All colors/wavelengths of spectrum • Color Wheel • Red – Green, Yellow – Violet, Blue – Orange

  5. Conjugated Systems • Sp2 hybridization and p-orbitals adjacent to one another • p-orbital on an atom adjacent to a double bond • p-orbital is vacant, contains a single electron, or part of another double bond • Delocalized π bonds allows free electron movement across conjugated system • Causes π bond molecular orbital's to be stacked on top of one another, decreasing the Energy difference between π bond and antibond • If an sp3 insulator is present the sp2 conjugated systems are separated • Extended π system: π bond that encompasses more than two nuclei

  6. Chemical Reactions with Conjugated Systems • Alkene (double bond) + alkyl halide • Markovinkov: H goes to Carbon with most Hydrogens • Forms a Carbocation • Halide bonds to Carbocation • Conjugated Systems • π system + alkyl halide -> 1,2 Addition + 1,4 Addition • Markovinkov • Mechanism • Incoming Hydrogen goes to terminal, primary Carbon (Markovinkov) • Forms a carbocation intermediate resonance hybrid, double bond electrons migrate between sp2 Carbons, partial positive charge on Carbons 2,4,… for halide to bind • Mixture of product will form (1,2), (1,4),… • Not an equal ratio of products but depends upon sterics and electronics

  7. Diels-Alder Reactions • Diene (conjugated system) + Dienophile (alkene) • p-orbitals interact • Forms a 6 member ring (Adduct) by the addition product • Stereochemistry is retained • If groups attached to alkene are trans, they are attached to the 6 member ring as trans • If groups attached to alkene are cis, they are attached to the 6 member ring as cis

  8. Diels-Alder Reactions (cont.) • Reaction Rates • Electronics determine rate, not sterics • Opposite charges attract and same charges repell • With opposite charges, the reaction will occur faster • With same charges, the reaction will occur slower • Diene with electron withdrawing groups cause it to be electron poor with large δ+ • Dienophile with electron donating groups cause it to be electron rich with large δ-

  9. Diels-Alder Reactions (cont.) • Alkyne dienophile causes to be a 6 member ring with 2 double bonds • Double bonds on diene have to be on the same side (cis) in order to react • Psuedo π bond on single bond in conjugate system • Weak interaction between p-orbitals • All Carbons are sp2 hybridized • Causes restricted rotation

  10. Bicyclic Diel-Alder Reactions • Diene is a ring with the two adjacent double bonds reacting with an alkene to form a bicyclic product • Endo Product • Larger Groups (cis) on alkene will be on the inside of the bicyclic product • Kinetics Product- faster product will form • Cold temperature • Lower Activation Energy- requires less energy • Exo Product • Larger Groups (cis) on alkene will be on the outside of the bicyclic product • Thermodynamics Product- more stable product will form • Room Temperature • Higher Activation Energy- requires more energy • If high enough energy, Endo and Exo Product will be formed but the stable, exo product will be favored

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