Conjugated Dienes and Ultraviolet Spectroscopy

Key Words Conjugated Diene Resonance Structures Dienophiles Concerted Reaction Pericyclic Reaction Cycloaddition Reaction Bridged Bicyclic Compound Cyclic Compounds Endo Exo 2

What are Conjugated Dienes? Conjugated Dienes are carbon structures which maintain 2 double bond separated by a single bond. Conjugated Dienes can be found in many different molecules as shown. Examples of Conjugated Dienes 3

Conjugated and Nonconjugated Dienes If Di = two and ene = double bond then Diene = two double bonds. If double bonds are separated by only NE single bond, they are conjugated and their orbitals interact. The conjugated diene 2,4-heptadiene has properties that are very different from those of the nonconjugated diene, 1,5- heptadiene Conjugated Diene Non-Conjugated Diene 4

Polyenes Compounds with many alternating single and double bonds. Extended conjugation leads to absorption of visible light, producing color. Conjugated hydrocarbons with many double bonds are polyenes (lycopene is responsible for red color in tomatoes) Extended conjugation in ketones (enones) found in hormones such as progesterone. 5

Examples of Conjugated Dienes Lycopene H H H Progesterone Benzene 6

Preparation and Stability of Conjugated Dienes Typically by elimination in allylic halide Specific industrial processes for large scale production of commodities by catalytic dehydrogenation and dehydration. NBS = N-Bromosuccimide (You add a bromine (halogen)) KC(CH 3 ) 3 is a strong base (dehydrohalogenation) 7

Preparation Conjugated Dienes Dehydration of Alcohols Removal of hydrogens 8

Stability of Dienes Conjugated dienes are more stable than nonconjugated dienes based on heats of hydrogenation. Hydrogenating 1,3-butadiene releases 15 kj/mol less heat than 1,4-pentadiene. 9

Molecular rbital Description of 1,3- Butadiene The single bond between the conjugated double bonds is shorter and stronger than sp 3 10

Molecular rbital Description of 1,3-Butadiene 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) 11

Molecular rbital Description of 1,3-Butadiene 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 14

Electrophilic Additions to Conjugated Dienes: Allylic Carbocations Review: addition of electrophile to C=C Markovnikov regiochemistry via more stable carbocation 15

Carbocations from Conjugated Dienes Addition of H + leads to delocalized secondary allylic carbocation 16

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 18

Practice Problem 14.1: Products? 19

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) 20

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, product ratio changes and 1,4 adduct predominates (See Figures 14-4 and 14-5) 21

What is the Diels-Alder Reaction? + Conjugated Diene Dienophile Product The Diels-Alder reaction uses a conjugated diene and a dienophile to produce cyclic and bicyclic carbon structures. This is also called the [4 + 2] cycloaddition reaction for the reaction of 4 pi electrons (diene) and 2 pi electron (dienophile). 24

Properties of Conjugated Dienes Conjugated Dienes can undergo resonance which is the movement of a double bond from Conjugated Dienes can often rotate to either form the s-cis or s-trans (s = single) s-trans Rotation s-cis 25

What are Dienophiles? Dienophiles are molecules which maintains a double bond or triple bond. C They are normally bound to electron withdrawing groups or neutral groups. C 26

Diels-Alder Reaction The Diels Alder reaction uses the resonance movement of electrons of the conjugated diene in the s-cis configuration with a dienophile to create a cyclicaddition or bridge bicyclic structure. + New Bond New Bond This reaction works as a concerted reaction or all in one step similar to an SN 2 reaction. 27

Limitations of Diels-Alder Reaction Does not react with s-trans configuration Does not react well with dienophiles with electron donating groups. 29

Products of Diels-Alder Reactions The products of Diels-Alder reaction are cyclic or ring compounds. It is also possible to form Bridged Bicyclic Compound by starting with diene found inside ring structures. + + 30

Cyclic Product The reaction produces only one product. If the reaction occurs with a cis dienophile then the product will be a cis product. + H H H 3 C CH 3 CH 3 If the reaction occurs with a trans dienophile then the product will be a trans product. + H H CH 3 31

Bridged Bicyclic Products ften the attachment to the diene moves up creating a bridge while the dienophile binds beneath it. H H No stereoselectivity H H H H The diene can bind three ways 1) without stereoselectivity 2) endo and 3) exo. Endo Exo H H 33

Endo Product This is where the dienophile attaches (down) opposite the bridge or functional groups. H H f the Diels-Alder reactions with stero selectivity the Endo product is preferred due to decreased steric strain. Endo 34

Exo Product This is where the dienophile attaches (up) same the bridge or functional groups. f the Diels-Alder reactions with stero selectivity the Exo product is less favorable due to increased steric strain. Exo H H 35

Diels-Alder Examples H + + C 2 Et C 2 Et H Endo Major Product C 2 Et C 2 Et + X NR Not in the s-cis config + The molecules rotates into the s-config 36

Easy Retrosynthesis Find the double bond Remove the double bond. Add double bonds to the adjacent bonds. Move 2 bond in both directs, remove these new bonds. Add a double bond to the final bond. + + 37

Diels Alder Reaction Can create carbon carbon single bonds by reacting conjugated diene and a dienophile to produce cyclic and bicyclic carbon structures. Reacts with electron withdrawing dienophiles or neutral groups. Works with conjugated dienes in the s-cis configuration. The Diels-Alder reaction is stereoselective giving cis and trans configuration to the product. 38

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 If the two bridges are equal, the product with the substituent endo to the new double bond is formed. 40

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 42

Practice Problem 14.2: 43

Solution: 44

Problem 14.7 (p. 478): 45

Reaction Mechanism: 46

Solution: 47

Unreactive Dienes 48

Reactive Diene: cyclopentadiene 49

Experiment 49: 50

Problem 14.33: Diels-Alder Products? 51

Problem 14.40: Diels-Alder Reactants? 52

Problem 14.45: Structure of Product? 53

First Diels-Alder Reaction: 54

Second Diels-Alder Reaction: 55

Diene Polymers: Natural and Synthetic Rubber 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 56

Natural Rubber A material from latex, in plant sap In rubber, the repeating unit has 5 carbons and Z stereochemistry of all C=C double bonds Gutta-Percha is natural material with E in all C=C They are head-to-tail polymers of isoprene (2- methyl-1,3-butadiene) 58

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

Vulcanization: 61

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 and solvents better than rubber 62

Neoprene: 63

Structure Determination in Conjugated Systems: UV Spectroscopy Conjugated compounds can absorb light in the ultraviolet region of the spectrum The region from 2 x 10-7 m to 4 x 10-7 m (200 to 400 nm) is most useful in organic chemistry 64

Structure Determination in Conjugated Systems: UV Spectroscopy The electrons in the highest occupied molecular orbital (HM) undergo a transition to the lowest unoccupied molecular orbital (LUM) 65

Structure Determination in Conjugated Systems: UV Spectroscopy 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 1,3-butadiene below 66

Ultraviolet Spectrum of 1,3- Butadiene Example: 1,4-butadiene has four molecular orbitals with the lowest two occupied Electronic transition is from HM to LUM at 217 nm (peak is broad because of combination with stretching, bending) 67

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 (kinetic experiment) Beers law: absorbance (A) = ecl e is molar absorptivity (extinction coefficient c is concentration in mol/l l is path of light through sample in cm 69

Interpreting UV Spectra: Effect of Conjugation max : wavelength where UV absorbance for a compound is greatest Energy difference between HM and LUM decreases as the extent of conjugation increases 70

Interpreting UV Spectra: Effect of Conjugation 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 for examples 71

Conjugation, Color and the Chemistry of Vision Visible region is about 400 to 800 nm Extended systems of conjugation absorb in visible region b-carotene, 11 double bonds in conjugation max = 455 nm 73

Conjugation, Color and the Chemistry of Vision b-carotene is converted to Vitamin A, which is converted to 11-cis-retinal: 75

Conjugation, Color and the Chemistry of Vision 11-cis-retinal is converted to rhodopsin in the rod cells of the retina. Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain 76