ORGANIC - BRUICE 8E CH.8 - DELOCALIZED ELECTRONS AND THEIR EFFECT

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2 CONCEPT: RESONANCE STRUCTURES Resonance theory is used to represent all the different ways that the same molecule can distribute its electrons. Atoms move! The only thing that moves is of these contributing structures will be a realistic representation of what the molecule actually looks like Rules: Use curved arrows to represent electron movement Use double-sided arrows and to link related structures to each other Arrows always travel from region of electron density to electron density The net charge of each structure must be EXAMPLE: Common forms of resonance Page 2

3 PRACTICE: Draw all of the contributing structures for the following molecules a. b. c. Page 3

4 CONCEPT: RESONANCE HYBRIDS The resonance hybrid represents the mathematical combination of all the contributing structures It indicates where the resonating electrons within the molecule are to reside EXAMPLE: Isocyanate Resonance Hybrid CONCEPT: MAJOR CONTRIBUTORS Often one of the resonance structures will be more so it will contribute to the more than the others. Major contributors will often have the following characteristics: structures are almost always more stable than charged ones If possible, every atom should fill its Use electronegativity trends to determine best placement of charges EXAMPLE: Isocyanate major contributor Page 4

5 PRACTICE: Draw all of the contributing structures for the following molecules. Label the major contributor if applicable and draw the resonance hybrid. a. b. Page 5

6 CONCEPT: INTRODUCTION TO CONJUGATION Conjugation exists when three or more atoms with the ability to resonate are adjacent to each other (overlapping). Conjugation provides an electron highway to from one side of the molecule to the other. Conjugated molecules display unique chemical reactivity. The higher the conjugation, the the UV wavelength. EXAMPLE: Which of the following molecules exists in a conjugated state? Allylic carbocations, carbanions, and radicals are unusually stable due to Page 6

7 CONCEPT: STABILITY OF CONJUGATED INTERMEDIATES Regardless of the type of reactive intermediate, conjugation increases the stability. Carbocations Radicals Due to the stability of the allylic position, radical and carbocation intermediated allylic reactions are common. Page 7

8 CONCEPT: ALLYLIC HALOGENTION GENERAL MECHANISM Recall the reaction of diatomic halogen with a double bond. This reaction proceeds through a bridged-ion intermediate. However, in the presence of a radical initiator, radical intermediates will predominate, changing the site of reaction. General Mechanism: Initiation: Propagation: Termination: Page 8

9 CONCEPT: ALLYLIC HALOGENATION SPECIFIC REACTIONS Allylic Chlorination: Allylic Bromination: Page 9

10 CONCEPT: CONJUGATED HYDROHALOGENATION Recall the addition of a strong halohydric acid on a double bond. This reaction is called hydrohalogenation. Carbocation rearrangements are possible Conjugated hydrohalogenation, also known as hydrohalogenation of dienes, or 1,2 vs. 1,4 addition to dienes, is the same reaction, except with a possibility of multiple products due to the presence of a conjugated intermediate. This reaction undergoes kinetic vs. thermodynamic control. Temperatures above 40 C favor the, also called the thermodynamic product. Temperatures below 0 C favor the, also called the kinetic product. EXAMPLE: Products of conjugated hydrohalogenation at different temperatures. Page 10

11 CONCEPT: CONJUGATED HYDROHALOGENATION KINETIC VS THERMODYNAMIC CONTROL Conjugated hydrohalogenation is one of the reactions that undergoes kinetic vs. thermodynamic control. Hot reaction conditions favor the thermodynamic product Cold reaction conditions favor the kinetic product EXAMPLE: Simplified Conjugated Hydrohalogenation Energy Diagram Summarizing Temperature Control: The kinetic pathway has a more stable intermediate but less stable product The thermo pathway has a less stable intermediate but more stable product Page 11

12 CONCEPT: DIELS-ALDER REACTION GENERAL FEATURES The Diels-Alder reaction is a heat-catalyzed, reversible pericyclic reaction between a conjugated 1,3-diene and dienophile. Diels-Alder reactions always yield 6-membered rings as products. The stereochemistry of all substituents must be Page 12

13 CONCEPT: DIELS-ALDER BRIDGED PRODUCTS Bicyclic bridged products are obtained when s-cis-1,3-diene is. EXAMPLE: Cyclopentadiene Dimerization Exo/Endo Stereochemistry: When a bridged product is made, substituents must face in the direction, away from the bridge. Page 13

14 CONCEPT: DIELS-ALDER RETROSYNTHESIS You may be given an end product and asked to provide the original diene and dienophile that were required to cyclize. EXAMPLE: Which diene and dienophile would you choose to synthesize the following compound? 1. Find the 2. Cross out the new 3. Isolate the Answer: EXAMPLE: Which diene and dienophile would you choose to synthesize the following compounds? a. b. Page 14

15 CONCEPT: BASICS OF MOLECULAR ORBITAL THEORY As previously discussed, non-bonding orbitals have the unique ability to conjugate with adjacent non-bonding orbitals. Bonding/non-bonding takes place in the outermost shell. Let s review atomic orbitals of valence electrons: When adjacent non-bonded atomic orbitals overlap, they create more favorable molecular orbitals. We can use a linear combination of atomic orbitals (LCAO) to visualize the resultant molecular orbitals EXAMPLE: Simplified LCAO Model of Ethene. Page 15

16 CONCEPT: DRAWING ATOMIC ORBITALS Transforming a conjugated molecule into atomic orbitals requires two rules: EXAMPLE: Provide the correct atomic orbitals for the following conjugated molecules. a. b. c. Page 16

17 CONCEPT: DRAWING MOLECULAR ORBITALS Rules for drawing conjugated molecular orbitals: 1. # molecular orbitals = # atomic orbitals 2. One orbital must never change phases (1 st is preferred) 3. Last orbital must always change phases 4. Number of nodes must begin = 0 and increase by 1 with each increasing energy level 5. Nodes must be symmetrical as possible. If in doubt, draw sin wave from fake atom [0] to [n + 1]. 6. If a node passes through an orbital, delete that orbital. 7. Fill molecular orbitals according to rules of electron configuration (Aufbau, Pauli, Hund s) EXAMPLE: Provide the molecular orbitals of 1,3-butadiene. Page 17

18 PRACTICE: Propose reasonable molecular orbitals for the following conjugated atomic orbitals. Page 18

19 CONCEPT: FRONTIER MOLECULAR ORBITAL THEORY FINDING HOMO/LUMO Frontier orbital interactions are the driving force behind many reactions in organic chemistry FMOT is based on being able to identify/understand HOMO and LUMO HOMO = Highest Occupied Molecular Orbital LUMO = Lowest Unoccupied Molecular Orbital EXAMPLE: Frontier Orbitals of Ethene PRACTICE: Consider the Molecular Orbitals (MO s) of the allyl anion. Which are the HOMO and LUMO? 1) HOMO = B, LUMO = C 2) HOMO = B, LUMO = A 3) HOMO = C, LUMO = A 4) HOMO = A, LUMO = C 5) HOMO = C, LUMO = B Page 19

20 CONCEPT: ORBITAL DIAGRAMS: 3-ATOM ALLYLIC IONS Allyl positions are famous for their unique ability to resonate, reacting in multiple locations. Regardless to the identity of the ion, this reactivity can be explained through allylic molecular orbitals. EXAMPLE: Simplified LCAO Model of Propenyl Ions EXAMPLE: Use both resonance theory and MO theory to predict the reactive sites of the following radical. Page 20

21 PRACTICE: Predict the molecular orbitals and identify the HOMO and LUMO orbitals of 1-propenyl cation (allyl cation). Page 21

22 CONCEPT: ORBITAL DIAGRAMS: 4-ATOM 1,3-BUTADIENE Conjugated polyenes are famous for their unique ability to resonate, reacting in multiple locations. They can participate in many types of reactions due to the symmetry of their molecular orbitals. EXAMPLE: Predict the LCAO Model of 1,3-butadiene. Identify the HOMO and LUMO Orbitals. Note: You may see these orbitals generated through the addition and subtraction of π-orbitals. Which orbitals would we need to sum to produce the above pattern? Page 22

23 CONCEPT: ORBITAL DIAGRAMS: 5-ATOM ALLYLIC IONS Like propenyl ions, 5-atom allylic systems have the ability to resonate, reacting in multiple locations. Regardless to the identity of the ion, this reactivity can be explained through allylic molecular orbitals. EXAMPLE: Predict the LCAO Model of 5-carbon allylic system. Identify bonding, non-bonding and antibonding orbitals. Page 23

24 PRACTICE: Predict the molecular orbitals and identify the HOMO and LUMO orbitals of the following cation. Page 24

25 CONCEPT: ORBITAL DIAGRAMS: 6-ATOM 1,3,5-HEXATRIENE Conjugated polyenes are famous for their unique ability to resonate, reacting in multiple locations. They can participate in many types of reactions due to the symmetry of their molecular orbitals. EXAMPLE: Predict the LCAO Model of 6-carbon 1,3,5-hexatriene. Identify bonding, non-bonding and antibonding orbitals. Determine the HOMO and LUMO orbitals. Page 25

26 CONCEPT: ORBITAL DIAGRAMS: EXCITED STATES Conjugated polyenes have the ability to absorb light energy and kick electrons up to a higher energy state. When this happens, the identity of HOMO/LUMO orbitals change, impacting their reactivity (more later). EXAMPLE: 1,3-butadiene is irradiated with photons, exciting an electron up to a higher energy molecular orbital. Predict the identity of the HOMO and LUMO orbitals after irradiation. PRACTICE: 4-Methylbenzylidene camphor (4-MBC) is used by the cosmetic industry for its ability to protect the skin against UV-B radiation. Circle the part of the molecule that you theorize is responsible for its effects on UV light. Page 26

27 CONCEPT: INTRO TO PERICYCLIC REACTIONS Conjugated polyenes have the ability to react in non-ionic, concerted, cyclization reactions called pericyclic reactions. All pericyclic reactions share the following properties, regardless of the type: Non-ionic. Solvents have no effect on them since there are partial charges. Concerted. All bonds are created and destroyed simultaneously. There are no intermediates. Cyclizations. Mechanisms involve a ring of electrons around a closed loop with transition states. Reversible. Also known as the principle of microscopic reversibility. All can occur either thermally or photochemically. Pericyclic reactions can be easily categorized by the number of that are destroyed after a cyclic mechanism. Cycloadditions: Pericyclic reactions in which π-bonds are destroyed after a cyclic mechanism. Electrocyclic Reactions: Pericyclic reactions in which π-bonds are destroyed after a cyclic mechanism. Sigmatropic Shifts: Pericyclic reactions in which π-bonds are destroyed after a cyclic mechanism. Page 27

28 PRACTICE: Determine if the following reactions are cycloadditions, electrocyclic reactions or sigmatropic shifts. a. b. c. Page 28

29 CONCEPT: THERMAL ELECTROCYCLIC REACTIONS Pericyclic reactions in which π-bond is destroyed after a -activated cyclic mechanism Always intramolecular All conjugated polyenes are capable of intramolecular electrocyclic reactions, however stereochemistry is variable. The HOMO orbital is capable of cyclizing in either a or fashion When substituents are involved in cyclization, stereochemistry is dependent on rotation type. EXAMPLE: Predict the product in the following electrocyclic reaction. Label the reaction as either conrotatory or disrotatory. Page 29

30 CONCEPT: PHOTOCHEMICAL ELECTROCYCLIC REACTIONS Intramolecular pericyclic reactions in which π-bond is destroyed after a -activated cyclic mechanism All conjugated polyenes are capable of intramolecular electrocyclic reactions, however stereochemistry is variable. Light excites ground-state electrons to a energy state (ψ à ψ*). HOMO / LUMO orbitals change. When substituents are involved in cyclization, stereochemistry is dependent on rotation type. EXAMPLE: Predict the product in the following electrocyclic reaction. Label the reaction as either conrotatory or disrotatory. Page 30

31 CONCEPT: CUMULATIVE ELECTROCYCLIC REACTIONS Step 1: Determine ROTATION (conrotatory vs. disrotatory) a. Obtain HOMO through combination of drawing molecular orbitals + activation type OR b. Use Electrocyclic Rotation Summary Chart: Step 2: Determine STEREOCHEMISTRY a. Obtain final structure by drawing 3D-representation + ROTATION OR b. Use Electrocyclic Stereochemistry Summary Chart Page 31

32 PRACTICE: Use the summary charts to predict the product of the following reactions. If there is more than one isomer possible, draw them. a. b. PRACTICE: Electrocyclic reactions are not limited to neutral conjugated polyenes, but are also applicable to ionic conjugated systems. Propose a mechanism and product for the following reaction. Page 32

33 CONCEPT: THERMAL CYCLOADDITION REACTIONS Pericyclic reactions in which π-bonds are destroyed after -activated cyclic mechanism The Diels-Alder reaction is an example of thermal cycloaddition In cycloaddition, HOMOA must fill LUMOB. According to FMOT, bonding interaction is strongest when orbital symmetry and energy closely. 1. Reaction must be symmetry-allowed vs. symmetry-disallowed 2. Reaction must minimize HOMO-LUMO Gap EXAMPLE: Predict the favorability of a bonding interaction between HOMOB and LUMOA Page 33

34 PRACTICE: Use FMOT to predict the mechanism and products for the following cycloadditions. If no product is favored, write symmetry-disallowed in place of the product. a. 2π + 2π cycloaddition b. 4π + 4π cycloaddition Page 34

35 CONCEPT: PHOTOCHEMICAL CYCLOADDITION REACTIONS Pericyclic reactions in which π -bonds are destroyed after a -activated cyclic mechanism In cycloaddition, HOMOA must fill LUMOB. According to FMOT, bonding interaction is strongest when orbital symmetry and energy match closely. Light excites ground-state electrons to a energy state (ψ à ψ*). HOMO / LUMO orbitals change. Cycloadditions Summary: Assuming only suprafacial interactions (antrafacial not possible on small rings): Page 35

36 PRACTICE: a. Use FMOT to predict the mechanism and products for the following cycloaddition. If no product is favored, write symmetry-disallowed in place of the product. 2π + 2π cycloaddition (thymine dimerization) b. Use the cycloaddition summary rules to verify that you have come to the correct conclusion. Page 36

37 CONCEPT: INTRODUCTION TO SIGMATROPIC SHIFTS Intramolecular pericyclic reactions in which π-bonds are destroyed after a cyclic mechanism Involve the of 1 σ bond and the of 1 σ bond Take the form of numerous rearrangements. Products are typically constitutional isomers of the reactant Common examples are the Cope and Claisen Rearrangements Naming Convention: Always described as [x,y]-sigmatropic shifts. σ bond broken = Atom 1 σ bond created = Atoms [x,y] EXAMPLE: Provide the correct names and mechanisms for the following sigmatropic shifts Page 37

38 CONCEPT: COPE REARRANGEMENT A -activated [3,3]-sigmatropic shift that involves only. Can be differentiated from other pericyclic reactions due to lack of conjugation Molecule may require rotation to visualize the 3,3-location EXAMPLE: Provide the mechanism and final product for the following reaction. PRACTICE: Provide the mechanism and final product for the following reaction. Page 38

39 CONCEPT: CLAISEN REARRANGEMENT A -activated [3,3]-sigmatropic shift that involves an ether Can be differentiated from other pericyclic reactions due to lack of conjugation Molecule may require rotation to visualize the 3,3-location A final tautomerization step is required for molecules in which the enol-form is favored. EXAMPLE: Circle the more favored tautomer of the following Claisen Rearrangement products EXAMPLE: Provide the mechanism and final product for the following reaction. You may skip the tautomerization mechanism if one is required. Page 39