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Unformatted text preview: 14. Conjugated Compounds and Ultraviolet Spectroscopy Based on McMurry's Organic Chemistry, 7th edition Kick it up a Notch! 2 Use Your Time Wisely 3 Remember ACS Multiple Choice Exam at End of Semester 4 Housekeeping Items No Labs this Week Labs Start Next Week I will go at a bit faster pace through the next two chapters. I will be selective as to the material that I will require you to know. Chapter 14: Emphasis given to diene properties and chemistry (addition and Diels-Alder reactions). Diene chemistry is very similar to monoene (alkene) chemistry. Will not discuss UV-visible spectroscopy. Ignore any problems that I have assigned with UV-Vis in it. 5 Chapter 14 Focus on Addition and DielsAlder Reactions 6 Why this Chapter? Conjugated compounds are common in nature Extended conjugation leads to absorption of visible light, producing color (Review this on your own.) Conjugated hydrocarbon with many double bonds are polyenes (lycopene is responsible for red color in tomatoes) Examine properties of conjugated molecules and reasons for the properties
7 Conjugated and Nonconjugated Dienes Compounds can have more than one double or triple bond If they are separated by only one single bond they are conjugated and their orbitals interact The conjugated diene 1,3-butadiene has properties that are very different from those of the nonconjugated diene, 1,5-pentadiene 8 Examples of Conjugated Compounds in Nature
Lycopene is a bright red carotenoid pigment, a phytochemical found in tomatoes and other red fruits. Lycopene is the most common carotenoid in the human body and is one of the most potent carotenoid antioxidants. p. 483 9 14.1 Stability of Conjugated Dienes: Molecular Orbital Theory Typically by elimination in allylic halide Specific industrial processes for large scale production of commodities by catalytic dehydrogenation and dehydration 10 Heat of Hydrogenation of a single C=C Bond H2, Pd H2, Pd H2, Pd H2, Pd Non-conjugated Conjugated H2, Pd Table 14-1, p. 484 11 Measuring Stability Conjugated dienes are more stable than nonconjugated based on heats of hydrogenation Hydrogenating 1,3-butadiene produces 16 kJ/mol less heat than 1,4-pentadiene STABILIZATION DUE TO CONJUGATION 12 Molecular Orbital Description of 1,3Butadiene The single bond between the conjugated double bonds is shorter and stronger than sp3 The bonding -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 (See Figures 14-1 and 14-2) 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 bond system Delocalization leads to stabilization 13 Conjugation of -Bonds Lowers Energy Might expect this to behave like single bond. Instead it has some double bond characteristics, because of p-orbital overlap.
14 atomic orbitals molecular orbitals, but only considering -bonds in this example
Fig. 14-1, p. 485
15 NO -BONDING NO NET BONDING 4 atomic orbitals NET -BONDING generate 4 molecular orbitals NET -BONDING Fig. 14-2, p. 486 16 Fig. 14-3, p. 486 17 14.2 Electrophilic Additions to Alkenes Review: Allylic Carbocations Review: addition of electrophile to C=C Markovnikov regiochemistry via more stable carbocation XS 18 14.2 Electrophilic Additions to Dienes: Allylic Carbocations 1 mol Might expect only one addition product, but two are formed instead. Why? p. 487 19 Carbocations from Conjugated Dienes Addition of H+ leads to delocalized secondary allylic carbocation H+ resonance stabilized or delocalized not resonance stabilized 20 Products of Addition to Delocalized Carbocation Nucleophile can add to either cationic site The transition states for the two possible products are not equal in energy
Br - Br - 21 14.3 Kinetic vs. Thermodynamic Control of Reactions 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) 22 Kinetic and Thermodynamic Control Example Addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct At higher temperature, the product ratio changes and 1,4 adduct predominates 23 A Mechanism which Explains the Products
Composite Struction of Intermediate Carbocation C2 C C C3 H H H H B r C2 C C C2 H H H H
+ H Secondary carbocation Primary carbocation H [ C2 C C C3 H H H B r B r
Lowest Ea C2 C C C3 ] H H H H B r B r C2 C C C3 H H H H
29% The most stable carbocation determines the most kinetically favored product, i.e. lower activation energy. The most interior double bond determines the thermodynamic stability. Kinetically Favored: (0 C) Thermodynamically Favored: (40 C) C2 C C C 3 H H H H
71% 15% Interior double bond 85% 24 Kinetically favored Thermodynamically favored Fig. 14-6, p. 492 25 14.4 The Diels-Alder Cycloaddition Reaction 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) Discovered by Otto Paul Hermann Diels and Kurt Alder in Germany in the 1930's Important method for making ring compounds 26 FRONTIER
NO -BONDING LUMO HOMO NO NET BONDING 4 atomic orbitals NET -BONDING generate 4 molecular orbitals NET -BONDING Fig. 14-2, p. 486 27 View of the Diels-Alder Reaction Woodward (Nobel Prize 1965) and Hoffman showed this shown to be an example of the general class of pericyclic reactions Involves orbital overlap, change of hybridization and electron delocalization in transition state The reaction is called a cycloaddition LUMO HOMO 28 Using Arrow Formalism to Show Mechanism
Y X Y X X H X Draw Molecular Orbital View on Board
29 14.5 Characteristics of the Diels-Alder Reaction The alkene component is called a dienophile C=C is conjugated to an electron withdrawing group, such as C=O or C N Alkynes can also be dienophiles 30 14.5 Dienophile must contain Electron Withdrawing Group to be Active. Notice reduced electron density of -bond. Fig. 14-8, p. 494 31 Stereospecificity of the Diels-Alder Reaction 32 Regiochemistry of the Diels-Alder Reaction 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 33 Conformations of Dienes in the DielsAlder Reaction The relative positions of the two double bonds in the diene are "cis" or "trans" to 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 34 Chapter 14 Focus on Addition and DielsAlder Reactions Make that Monday 35 Housekeeping Items Organic I Lab meets Thursday at 8:00 am Check in and Safety Quiz Organic II Lab meets Tuesday at 8:00, 11:00, and 2:00 Check in, then go to Spartan Lab QUIZ ON MONDAY Chapter 14 BRING YOUR CLICKERS www.iclicker.com/registration/ Return Final Exams from last Semester Review Course Syllabus Work Homework Problems Today!
36 Quiz 1 Chapter 14 Question #? Which of the following is the most reactive dienophile? (A )
H H C C H H (B )
H H C C C N H (C )
H H C C C3 H H (D )
H H C C C2C3 HH H (E N n o th A o ) o e f e b ve 37 Predict the Product for the following Reaction 1,3-cyclohexadiene + DCl ? 38 Predict the Product for the following Reaction 2 p. 496 39 Predict the Product for the following Reaction Answer O O p. 496 40 Predict the Product for the following Reaction 2 p. 496 41 Predict the Product for the following Reaction p. 496 42 Predict the Product for the following Reaction Answer Enantiomers are formed. Can you explain why? C2C3 OH H C3 H C3C2 HO H HC 3 H H p. 496 43 Predict the Product for the following Reactions p. 511 44 Predict the Product for the following Reactions Answer H H O O Are enantiomers formed here? O
45 Predict the Product for the following Reactions Answer H H O O Are enantiomers formed here? H H O O Endo product dominates O
46 Predict the Product for the following Reaction p. 511 47 Predict the Product for the following Reaction Answer Are enantiomers possible here? Are other products possible? C2C 3 OH H C 2C 3 OH H
48 Asymmetric reactants! Predict the Product for the following Reaction p. 511 49 Predict the Product for the following Reaction H C2C2C3 OHH H H C3C2C 2 HHO H H Are other products possible? H p. 511 50 Predict the Product for the following Reaction Answer H C 2C2C3 OHH H H C3C2C2 HHO H H H H H C 2C2C3 OHH H H C3C2C2 HHO H Which of these pairs is most favored? H
p. 511 51 What Diene and Dienophile would you start with to form these products? p. 512 52 What Diene and Dienophile would you start with to form these products? Answers (a ) O O O O (b ) (c) (d ) C 2C3 OH O
53 Chapter 14 Test question to see if you really grasp the concept. Predict the product of the following reaction. HA ET O ? 54 Chapter 14 - HINT Predict the product of the following reaction. HA ET O ? 55 Chapter 14 - HINT Predict the product of the following reaction. HA ET O O 56 14.6 Diene Polymers: Natural and Synthetic Rubbers Review this material on your own. Conjugated dienes can be polymerized The initiator for the reaction can be a radical, or an acid Polymerization: 1,4 addition of growing chain to conjugated diene monomer 57 Natural Rubber A material from latex, in plant sap In rubber repeating unit has 5 carbons and Z stereochemistry of all C=C Gutta-Percha is natural material with E in all C=C Looks as if it is the head-to-tail polymer of isoprene (2-methyl-1,3-butadiene) 58 Synthetic Rubber Chemical polymerization of isoprene does not produce rubber (stereochemistry is not controlled) Synthetic alternatives include neoprene, polymer of 2-chloro-1,3-butadiene This resists weathering better than rubber 59 Vulcanization Natural and synthetic rubbers are too soft to be used in products Charles Goodyear discovered heating with small amount of sulfur produces strong material Sulfur forms bridges between hydrocarbon chains (cross-links) 60 14.7 Structure Determination in Conjugated Systems: UV Spectroscopy Conjugated compounds can absorb light in the ultraviolet region of the spectrum The electrons in the highest occupied molecular orbital (HOMO) undergo a transition to the lowest unoccupied molecular orbital (LUMO) The region from 2 x 10-7m to 4 x 10-7m (200 to 400 nm) is most useful in organic chemistry A plot of absorbance (log of the ratio of the intensity of light in over light transmitted) against wavelength in this region is an ultraviolet spectrum see Figure 14-11 61 Ultraviolet Spectrum of 1,3-Butadiene Example: 1,4-butadiene has four molecular orbitals with the lowest two occupied Electronic transition is from HOMO to LUMO at 217 nm (peak is broad because of combination with stretching, bending) 62 Quantitative Use of UV Spectra Absorbance for a particular compound in a specific solvent at a specified wavelength is directly proportional to its concentration You can follow changes in concentration with time by recording absorbance at the wavelength Beers' law: absorbance = cl " " is molar absorptivity (extinction coefficient) "c" is concentration in mol/L "l" is path of light through sample in cm 63 14.8 Interpreting UV Spectra: The Effect of Conjugation max: wavelength where UV absorbance for a compound is greatest Energy difference between HOMO and LUMO decreases as the extent of conjugation increases max increases as conjugation increases (lower energy) 1,3-butadiene: 217 nm, 1,3,5-hexatriene: 258 nm Substituents on system increase max See Table 14-2 64 14.9 Conjugation, Color and the Chemistry of Vision Visible region is about 400 to 800 nm Extended systems of conjugation absorb in visible region -Carotene, 11 double bonds in conjugation, max = 455 nm Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain 65 ...
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This note was uploaded on 04/07/2008 for the course CHGN 222 taught by Professor Cowley during the Spring '08 term at Mines.
- Spring '08