14. Conjugated Compounds and Ultraviolet Spectroscopy Based on McMurry’s Organic Chemistry, 7 th edition

2 Why this Chapter? Conjugated compounds are common in nature Extended conjugation leads to absorption of visible light, producing color 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

3 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

4 Conjugated Systems Conjugation occurs whenever p-orbitals can overlap on three or more adjacent atoms. Two common conjugates systems are: i) 1,3-dienes ii) Allylic Carbocation

5 Conjugated Systems

6 1,3-dienes Contain two Carbon – Carbon double bonds joined in a single sigma bond e.g. 1,3 –butadiene Each C atom of the 1,3-diene is bonded to three other atoms and has no non-bonded electron pairs Thus, each C atom is sp 2 hybridized and has one p orbital containing an electron The four p orbitals on adjacent atoms make a 1,3-diene a conjugated system

7 1,3-dienes What is special about conjugation?  Having three or more p-orbitals on adjacent atoms allows p orbitals to overlap and electrons to delocalize  When p orbitals overlap, the electron density in each of the pi bonds is spread out over a large volume, thus lowering the energy of the molecule and making it more stable

8 Isolated dienes Compounds having two double bonds separated by more than one sigma bond E.g. 1,4-pentadiene  The electron density in each pi bond of an isolated diene is localized between two carbon atoms

9 Exercise 1

10 Allylic Carbocations Allylic carbocation is another example of a conjugated system The three C atoms are sp 2 hybridized with a p orbital The p orbitals for the double bond C atoms each contain an electron, whereas the p orbital for the carbocation is empty.  Three p orbitals on the three adjacent atoms, even if one of the p orbitals is empty, make the allyl carbocation conjugated

11 Allylic Carbocations Conjugation stabilizes the allyl carbocation because overlap of three p orbitals delocalizes the electron density of the pi-bond over the three atoms

12 Exercise 2

13 Resonance and Allylic Carbocations Resonance – two or more different Lewis structures for the same arrangement of atoms Structures differ in the placement of pi-bond and non- bonding electrons Conjugated allylic carbocation is an example of species for which two resonance structures can be drawn. The true structure of the allyl carbocation is a hybrid of the two resonance structure.

14 Resonance and Allylic Carbocations Delocalizing electron density lowers the energy of the hybrid, thus stabilizing the allyl carbocation and making it more stable than a normal 1 o carbocation and comparable to in stability to a more highly substituted 2 o Carbocation

15 Recap on Resonance Four common bonding patterns for which more than one Lewis structure can be drawn.

16 Recap on Resonance Four common bonding patterns for which more than one Lewis structure can be drawn.

17 Recap on Resonance Four common bonding patterns for which more than one Lewis structure can be drawn.

18 Recap on Resonance Four common bonding patterns for which more than one Lewis structure can be drawn.

19 Resonance Hybrid Although the resonance hybrid is some combination of all of its valid resonance structures, the hybrid more closely resembles the most stable resonance structure. Use three rules to evaluate the relative stabilities of two or more valid resonance structures

20 Resonance Hybrid Although the resonance hybrid is some combination of all of its valid resonance structures, the hybrid more closely resembles the most stable resonance structure. Use three rules to evaluate the relative stabilities of two or more valid resonance structures

21 Resonance Hybrid Although the resonance hybrid is some combination of all of its valid resonance structures, the hybrid more closely resembles the most stable resonance structure. Use three rules to evaluate the relative stabilities of two or more valid resonance structures

22 Exercise 3

23 Exercise 3_Solution

24 Exercise 4

25 Exercise 5 Use Resonance theory and the Hammond postulate to explain why 3-chloro-1-propene is more reactive than 1-chloropropane in S N 1 reactions?

26 Measuring Stability of Conjugated Dienes Conjugated dienes are more stable than non-conjugated based on heats of hydrogenation E.g Hydrogenating of 1,3-butadiene produces 16 kJ/mol less heat than 1,4-pentadiene. Conjugated dienes release less energy on hydrogenation since they contain less energy to start of with. 1,3-butadiene is approx. 16kJ/mol more stable than expected

27 Measuring Stability of Conjugated Dienes

28 Exercise 6

29 Valence Bond Theory Description of 1,3-Butadiene Electrons in sp 2 orbitals are closer to the nucleus Thus, The single bond between the conjugated double bonds is shorter and stronger than sp 3 Greater s orbital character in sp2 forming the single bond

30 Molecular Orbital 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 In addition, the single bond between the two double bonds is strengthened by overlap of p orbitals (more double-bond character) In summary, we say electrons in 1,3-butadiene are delocalized over the  bond system Delocalization leads to stabilization

31 Isomers of Conjugated Dienes Three isomers are possible for 1,3-dienes with alkyl groups bonded to each end carbon of the diene

32 Isomers of Conjugated Dienes Two possible conformations result from rotation around the C-C bond that joins the two double bonds

33 Isomers of Conjugated Dienes Note: stereoisomers are discrete molecules, whereas conformations interconvert. E.g 2,4-hexadiene

34 Exercise 7 Draw the structures consistent with each description.

35 Worksheet 1

36 Preparation of Conjugated Dienes Conjugated dienes can be prepared by methods previously discussed for preparing alkenes Typically by base-induced elimination of HX from an allylic halide

Reactions of Conjugated Dienes Conjugated dienes can be used for specific industrial processes for large scale production of commodities Simple conjugated dienes are used in polymer synthesis include 1,3-butadiene, chloroprene (2-chloro-1,3- butadiene) and isoprene ( 2-methyl-1,3-butadiene) Isoprene can be prepared industrially by several methods e.g. acid-catalyzed dehydration of 3-methyl-1,3-butanediol

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

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

40 Products of Addition to Delocalized Carbocation Nucleophile can add to either cationic site, leading to two products i.e. 1,2-addition and 1,4-addition The transition states for the two possible products are not equal in energy

41 Products of Addition to Delocalized Carbocation Nucleophile can add to either cationic site, leading to two products i.e. 1,2-addition and 1,4-addition The transition states for the two possible products are not equal in energy

42 Products of Addition to Delocalized Carbocation Mechanism

43 Exercise 8 1) Give the structures of the products from the reaction of 1 eq. of HCl with 2-methyl-1,3- cyclohexadiene 2) draw the products formed when each diene is treated with 1eq. of HCl

44 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

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 product (Kinetic Control)

Kinetic vs. Thermodynamic Control of Reactions Recap on chapter 6

Kinetic vs. Thermodynamic Control of Reactions Recap on chapter 6

Kinetic vs. Thermodynamic Control of Reactions

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

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 C=C are less electron-rich than in ethene Alkynes can also be dienophiles

51 View of the Diels-Alder Reaction Woodward and Hoffman showed this 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

52 View of the Diels-Alder Reaction Woodward and Hoffman showed this 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

53 View of the Diels-Alder Reaction

54 View of the Diels-Alder Reaction

55 Exercise 9

56 Rules Governing the Diels-Alder Reaction

57 Exercise 10

58 Rules Governing the Diels-Alder Reaction

59 Exercise 11

60 Rules Governing the Diels-Alder Reaction

61 Rules Governing the Diels-Alder Reaction

62 Exercise 12

63 Stereospecificity of the Diels-Alder Reaction Another stereochemical feature of the rxn is that the diene and dienophile partners orient so that the endo product and not the exo product is formed.

64 Stereospecificity of the Diels-Alder Reaction The product of the Diels-Alder reaction of a cyclic 1,3-diene is bycyclic, but the carbon atoms shared by both rings are non-adjacent. Thus, the bycyclic product differs from the fused ring system obtained when the dienophile is cyclic. Bycyclic ring system in which the two rings share non-adjacent carbon atoms is called a bridged ring system. the terms endo and exo are used to indicate the position of Z (electron withdrawing group of the dienophile)

65 Stereospecificity of the Diels-Alder Reaction The product of the Diels-Alder reaction of a cyclic 1,3-diene is bycyclic, but the carbon atoms shared by both rings are non-adjacent. Thus, the bycyclic product differs from the fused ring system obtained when the dienophile is cyclic. Bycyclic ring system in which the two rings share non-adjacent carbon atoms is called a bridged ring system. the terms endo and exo are used to indicate the position of Z (electron withdrawing group of the dienophile)

66 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

67 Regiochemistry of the Diels-Alder Reaction Reactants align to produce endo (rather than exo) product

68 Recap - Conformations of Dienes in the Diels-Alder 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 rxn

69 Fused Ring vs Bridged Ring System

70 Exercise 13 Draw the product of each of the Diels-Alder reaction.

71 Retrosynthesis of Diels-Alder Reaction

72 Retrosynthesis of Diels-Alder Reaction

73 Worksheet 2 What diene and dienophile are needed to prepare each of the following products?

Diene Polymers: Natural and Synthetic Rubbers 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

75 Natural Rubber A material from latex, in plant sap In rubber, repeating unit is isoprene (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)

76 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

77 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 - vulcanization Sulfur forms bridges between hydrocarbon chains (cross- links) Degree of hardness can be varied

78 Synthesis of Steroids Steroids – tetracyclic lipids containing three six membered rings and one five membered ring. The four rings are designated A,B,C and D Steroids have a range of biological functions, depending on the substitution pattern of functional groups on the ring

79 Synthesis of Steroids Steroids have a range of biological functions, depending on the substitution pattern of functional groups on the ring Cholesterol – component of cell membrane, implicated in cardiovascular disease Estrone – hormone which regulates the menstrual cycle Cortisone – hormone controlling inflammation & regulation of carbohydrate metabolism

80 Synthesis of Steroids Diels-Alder rxn used to prepare C ring of estrone and B ring of cortisone

81 Synthesis of Steroids Lovastatin - Cholesterol lowering drug isolated from bacterium Aspergillus terreus key step involves intramolecular Diels-Alder reaction of a triene

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 m to 4 x m (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

83 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)

84 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

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

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