17.1 Classes of Dienes Conjugated Dienes. There are three categories for dienes:

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1 17.1 Classes of Dienes There are three categories for dienes: Cumulated: pi bonds are adjacent. Conjugated: pi bonds are separated by exactly ONE single bond. Isolated: pi bonds are separated by any distance greater than ONE single bond Classes of Dienes There are three categories for dienes: Cumulated: pi bonds are perpendicular. Conjugated: pi bond overlap extends over the entire system. Isolated: pi bonds are separated by too great a distance to experience extra overlap Classes of Dienes This chapter focuses on conjugated systems. Heteroatoms may be involved in a CONJUGATED system. Draw a picture of the molecule shown to the right indicating where the pi bonds are and how they overlap. A sterically hindered base can be used to form dienes while avoiding the competing substitution reaction. Practice with CONCEPTUAL CHECKPOINT Practice with CONCEPTUAL CHECKPOINT Single bonds that are part of a conjugated pi system are SHORTER than typical single bonds. The more s character a carbon has, the shorter its bonds will be. WHY? What is a pm? The hybridization of a carbon affects its bond length. Practice with CONCEPTUAL CHECKPOINT

2 HOW do heats of hydrogenation provide information about stability? WHY is energy released upon hydrogenation? HOW does conjugation affect stability? Practice CONCEPTUAL CHECKPOINTs 17.4 and Rank the following compounds in order of increasing stability. In general, single bonds will freely rotate. The two most stable rotational conformations for butadiene are the s cis and s trans. What does the s of s cis and s trans stand for? Are there any other rotational conformations that you think might be possible? The s cis and s trans both allow for full pi system overlap. Why is s trans lower in energy? Other possible conformations will be higher in energy. WHY?

3 About 98% of the molecules are in the s trans form. The highest energy conformer will not be conjugated Molecular Orbital Theory 17.3 Molecular Orbital Theory Let s review molecular orbital (MO) theory: An MO forms when atomic orbitals overlap. An MO extends over the entire molecule. Recall the pi bonding and antibonding MOs for ethylene Which is more stable? WHY? What is a node? The number of MOs must be equal to the number of combined atomic orbitals (AOs). Note how the shorthand drawing matches the actual MOs Molecular Orbital Theory The four pi electrons in butadiene will occupy the lowest energy MOs. HOW will that affect stability? 17.3 Molecular Orbital Theory MO theory also explains why the central C C single bond is shorter and stronger than a typical C C single bond

4 17.3 Molecular Orbital Theory 1,3,5 hexatriene should have six pi MOs. What are HOMO and LUMO? The HOMO and LUMO are the frontier MOs Molecular Orbital Theory Reactions that molecules undergo can often be explained by studying their FRONTIER ORBITALS. Light can be used to excite an electron from the HOMO to the LUMO Molecular Orbital Theory We can measure the energy gap for the HOMO LUMO excitation. HOW? 17.4 Electrophilic Addition Recall the Markovnikov addition of H X to a C=C double bond from Section 9.3. In general, HOW does that give us information about a molecule s structure? With a conjugated diene as the substrate, two products are observed. Practice with CONCEPTUAL CHECKPOINT Electrophilic Addition The pi electrons attack the acid to give the most stable carbocation. What intermediate would result if the H were added to any of the other carbons in the molecule? 17.4 Electrophilic Addition The halide can attack the resonance stabilized carbocation at either site that is sharing the (+) charge. How is 1,2 addition different from 1,4 addition? Practice with SKILLBUILDER

5 17.4 Electrophilic Addition The addition of bromine to a diene also gives both 1,2 and 1,4 addition Thermodynamic Control vs. Kinetic Control The ratio of 1,2 vs. 1,4 addition is often temperature dependant. Predict the MAJOR products for the reaction below. Pay close attention to stereochemistry. The energy diagram must be examined to see WHY temperature affects the product distribution Thermodynamic Control vs. Kinetic Control Why are the products unequal in free energy? 17.5 Thermodynamic Control vs. Kinetic Control Draw a structure for each of the transition states of the nucleophilic attack Thermodynamic Control vs. Kinetic Control The 1,2 addition should occur more often regardless of temperature. WHY? 17.5 Thermodynamic Control vs. Kinetic Control Explain how high temperatures yield one product while low temperatures yield the other

6 17.5 Thermodynamic Control vs. Kinetic Control Predict the MAJOR product for the following reactions. H Br H Br 40 C 0C 17.5 Thermodynamic Control vs. Kinetic Control Many polymerization reactions rely on 1,4 addition. Give a reasonable mechanism for the cationic polymerization. Practice with CONCEPTUAL CHECKPOINT Introduction to Pericyclic Reactions Pericyclic reactions occur without ionic or free radical intermediates. There are three main types of pericyclic reactions: 1. CYCLOADDITION reactions: 17.6 Introduction to Pericyclic Reactions There are three main types of pericyclic reactions: 2. ELECTROCYCLIC reactions: 3. SIGMATROPIC rearrangements: How is it an ADDITION reaction? What is the difference between an ADDITION and a REARRANGEMENT? Introduction to Pericyclic Reactions Pericyclic reactions have four general features 1. The reaction mechanism is concerted. It proceeds without any intermediates. 2. The mechanism involves a ring of electrons moving around a closed loop. 3. The transition state is cyclic. 4. The polarity of the solvent generally has no effect on the reaction rate. WHY is that significant? 17.6 Introduction to Pericyclic Reactions Changes in the number of pi and sigma bonds distinguish the pericyclic reactions from one another

7 Diels Alder reactions can be very useful. The arrows could be drawn in a clockwise or counterclockwise direction. They allow a synthetic chemist to quickly build molecular complexity. What is meant by [4+2] cycloaddition? Like all pericyclic reactions, the mechanism is concerted. Can you draw a reasonable transition state? Why do the products generally have lower free energy? Most Diels Alder reactions are thermodynamically favored at low and moderate temperatures. At temperatures above 200 C, the retro Diels Alder can predominate Will the reaction probably be favored or disfavored by entropy? The pi sigma conversion provides a ( )ΔH. ΔS should be ( ): Two molecules combine to form ONE. A ring (with limited rotational freedom) forms. What will the sign be (+/ ) for the TΔS term?

8 Given the signs for ΔH and TΔS, how should temperature affect reaction spontaneity (favorability of reactant vs. product)? In the Diels Alder reaction, the reactants are generally classified as either the DIENE or DIENOPHILE. If a dienophile is not substituted with an electron withdrawing group, it will not be kinetically favored (a lot of activation energy or high temperature is required). Are there any disadvantages if the temperature is too low? Think kinetics However, high temperatures do not favor the products thermodynamically When an electron withdrawing group is attached to the dienophile, the reaction is generally spontaneous. Show how the groups highlighted in red are electron withdrawing using resonance and induction where appropriate. Diels Alder reactions are stereospecific depending on whether the (E) or (Z) dienophile is used. Which alkene is (E) and which is (Z)? We will investigate the stereochemical outcome later in this chapter A C C triple bond can also act as a dienophile. Predict products for the following reactions. Practice with SKILLBUILDER

9 Diels Alder reactions can also be affected by DIENE structure. Recall that many dienes can exist in an s cis or an s trans rotational conformation. Which conformation is generally more stable? WHY? Diels Alder reactions can ONLY proceed when the diene adopts the s cis conformation Dienes that can only exist in an s trans conformation cannot undergo Diels Alder reactions because carbons 1 and 4 are too far apart. 4 1 Dienes that are locked into the s cis conformation undergo Diels Alder reactions readily. Cyclopentadiene is so reactive, that at room temperature, two molecule will react together. Show the reaction and products Draw four potential bicyclic Diels Alder products for the reaction below. When bicyclic systems form, the terms ENDO and EXO are used to describe functional group positioning. Two of the potential stereoisomers are impossible. WHICH ones? WHY? The electron withdrawing groups attached to dienophiles tend to occupy the ENDO position. The Diels Alder transition state that produces the ENDO product benefits from favorable pi system interactions. Major product Minor Product Is this a kinetic or thermodynamic argument? Draw an appropriate energy diagram. Practice with CONCEPTUAL CHECKPOINT

10 17.8 MO Description of Cycloadditions Note the MO descriptions below MO Description of Cycloadditions In the Diels Alder, the HOMO of one compound must interact with the LUMO of the other MO Description of Cycloadditions With an electron withdrawing group, the dienophile s LUMO will accept electrons from the diene s HOMO MO Description of Cycloadditions The phases of the MOs align to allow for orbital overlap. If there is conservation of ORBITAL SYMMETRY, the process is SYMMETRY ALLOWED. Note the carbons that change their hybridization from sp 2 to sp MO Description of Cycloadditions Similar to a Diels Alder ([4+2] cycloaddition), the reaction below is a [2+2] cycloaddition MO Description of Cycloadditions The LUMO of one reactant must overlap with the HOMO of the other. WHY can t both of the HOMOs interact together? Draw a reasonable concerted mechanism. Unless the reaction is symmetry allowed, the process will not occur, so let s analyze the MOs

11 17.8 MO Description of Cycloadditions The phases of the HOMO and LUMO cannot line up to give effective overlap, so the reaction is SYMMETRY FORBIDEN MO Description of Cycloadditions [2+2] cycloadditions are only possible when light is used to excite an electron Draw a picture that shows how the reaction is now symmetry allowed. Practice with CONCEPTUAL CHECKPOINT Determine how the number of σ and π bonds change for the representative electrocyclic reactions below. When substituents are present on the terminal carbons, stereoisomers are possible. Explain why the equilibriums favor either products or reactants in the examples above Note that the use of light vs. heat gives different products Often the energy present at room temperature is sufficient to promote thermal electrocyclic reactions. The symmetry of the HOMO determines the outcome. Note that with the use of heat, the configuration of the reactant determines the product formed. The terminal carbons rotate as they become sp 3 hybridized and lobes that are in phase overlap

12 The terminal carbons rotate as they become sp 3 hybridized and lobes that are in phase overlap. Use MO theory to explain the products for the reactions below. DISROTATORY rotation (one rotates clockwise and the other counterclockwise) gives the cis product. Will DISROTATORY or CONROTATORY rotation be necessary? Practice with CONCEPTUAL CHECKPOINT Predict the major product for the reaction below. Pay close attention to stereochemistry. Under photochemical conditions, light energy excites an electron from the HOMO to the LUMO. What was the LUMO becomes the new HOMO. Heat Will the new excited HOMO react via a disrotatory or conrotatory mode? Make the same MO analysis to predict the symmetryallowed product for the reaction below. Pay close attention to stereochemistry. Light Draw the expected product

13 The disrotatory nature of the photochemical [2+2] electrocyclic ring closing is difficult to observe directly because the thermodynamics favor ring opening. The ring opening reaction gives products that result from disrotatory rotation. Predict products below. The Woodward Hoffmann rules for thermal and photochemical electrocyclic reactions are found in Table Practice with SKILLBUILDER Sigmatropic Rearrangements SIGMATROPIC REARRANGEMENT is a pericyclic reaction in which one sigma bond is replaced with another. Note that the pi bonds switch locations Sigmatropic Rearrangements The notation for sigmatropic rearrangements is different from reactions we have seen so far. Count the number of atoms on each side of the sigma bonds that are breaking and forming. This is a [3,3] sigmatropic rearrangement Sigmatropic Rearrangements The reaction below is a [1,5] sigmatropic rearrangement Sigmatropic Rearrangements The Cope rearrangement is a [3,3] sigmatropic reaction in which all six atoms in the cyclic transition state are CARBONS. Practice with CONCEPTUAL CHECKPOINT In general, what factors affect the spontaneity of the reaction (product favored vs. reactant favored)? Practice with CONCEPTUAL CHECKPOINT

14 17.10 Sigmatropic Rearrangements Sigmatropic Rearrangements The Claisen rearrangement is a [3,3] sigmatropic reaction in which one of the six atoms in the cyclic transition state is an OXYGEN. In general, what factors affect the spontaneity of the reaction? Practice with CONCEPTUAL CHECKPOINTs and Two pericyclic reactions occur in the biosynthesis of vitamin D UV Vis Spectroscopy If light with the NECESSARY ENERGY strikes a compound with pi bonds, an electron will be excited from the HOMO to the LUMO. Light energy is converted into potential energy UV Vis Spectroscopy In general, the NECESSARY ENERGY to excite an electron from π π* (HOMO to LUMO) is either in the UV or visible region of the spectrum. What might happen after the electron is excited? UV Vis Spectroscopy UV Visible (UV Vis) spectroscopy gives structural information about molecules: A beam of light ( nm) is split in two. Half of the beam travels through a cuvette with the analyte in solution. The other half of the beam travels through a cuvette with just the solvent (used as a negative control). The intensities of light that pass through the cuvettes are compared to determine how much light is absorbed by the analyte UV Vis Spectroscopy UV Visible (UV Vis) spectroscopy gives structural information about molecules: The resulting data is plotted to give a UV Vis absorption spectrum. Compounds require specific wavelengths of energy to excite. WHY?

15 17.11 UV Vis Spectroscopy More conjugation gives a smaller π π* energy gap. The smaller the energy gap, the greater the lambda max (λ max ) UV Vis Spectroscopy The group of atoms responsible for absorbing UV Vis light is known as the chromophore. Woodward and Fieser developed rules to predict λ max for chromophore starting with butadiene as the base UV Vis Spectroscopy Woodward and Fieser developed rules to predict λ max for chromophore starting with butadiene as the base UV Vis Spectroscopy Practice with SKILLBUILDER The Woodward Fieser rules are a guide to ESTIMATE λ max Color The visible region of the spectrum ( nm) is lower energy than UV radiation. Lycopene is responsible for the red color of tomatoes Color The color observed by your eyes will be the opposite of what is required to cause the π π* excitation. WHY? β carotene is responsible for the orange color of carrots. Practice with CONCEPTUAL CHECKPOINT

16 Bleaching agents generally work by breaking up conjugation through an addition reaction. Destroying long range conjugation destroys the ability to absorb colored light. WHY? Does bleach actually remove stains? Color Chemistry of Vision Rods and cones are photosensitive cells: Rods are the dominant receptor in dim lighting. Owls have only rods allowing them to see well at night. Cones are responsible for detection of color. Pigeons have only cones providing sensitive daytime vision. Rhodopsin is the light sensitive compound in rods Chemistry of Vision Sources of 11 cis retinal include vitamin A and β carotene Chemistry of Vision When rhodopsin is excited photochemically, a change in shape occurs that causes a release of Ca 2+ ions. The Ca 2+ ions block channels through which billions of Na + ions generally travel each second. The decrease of Na + ion flow culminates in a nerve impulse to the brain. Our eyes are extremely sensitive. Just a few photons can cause a nerve impulse