Chapter 14: Dienes and Conjugation. Topics Dienes: Naming and Properties. Conjugation. 1,2 vs 1,4 addition and the stability of the allyl cation

Transcription

1 rganic hemistry otes by Jim Maxka hapter 14: Dienes and onjugation Topics Dienes: aming and Properties onjugation 1,2 vs 1,4 addition and the stability of the allyl cation Diels Alder eaction Simple rbital Picture of dienes olor and light absorption onnections: X 3 3 X X 3 X l l l l 3 l l l l color with organic chemistry ' 5' 4' 3' ' 14' 13' 12' 11' 10' 9' 8' 7' 6' 2' 1' β,β-carotene 3 20' 3 19' ' 17' 14-1

2 rganic hemistry otes by Jim Maxka Dienes aming: Draw the following: 1,3-Butadiene 1,3,5-exatriene Isoprene (2-Methyl-1,3-Butadiene) Properties Dienes have a new type of conformation. s-cis and s-trans. Draw 1,3-butadiene in the s-is (syn) and in the s-trans (anti) forms: Label the hybridization of each above Make sure you understand the difference between conformation and configuration!!!! The simplest possible diene is not butadiene. Butadiene has s. The simplest diene would have Draw and name the simplest diene. What is the hybridization of the s in allene (common name). onjugation: Are the above molecules (butadiene and allene) conjugated? Why or why not? Draw 1,4-pentadiene Draw 1,3,5-hexatriene Are these molecules conjugated? Why or why not? Below are a ketone and the allyl cation: 2 And 3 and Are these molecules conjugated? Why or why not? What is a good definition of a conjugated molecule? 2 2? ne way to show the stability in a conjugated molecule is to draw resonance structures that go over a long path of the molecule: Worked Example

3 rganic hemistry otes by Jim Maxka Thermochemistry: Stability of alkenes Zaitsev and Markovnikov s rules are based on the most substituted with p-orbitals is the most stable. onsider alkenes first. Which is more stable 1-butene or -2-butene (cis/trans)? Why? Zaitsev s rule states that the most substituted alkene is the the most stable. Why? Zaitsev s rule is related to Markovnikov s rule. For Zaitsev s rule, we speak about substitution of p orbitals in bonds called bonds. For Markovnikov s rule, we speak about substitution of p orbitals: < < < Substitution 0 (methyl) Stability Least stable Most stable For a simple graphic of hyperconjugation check out Thermochemistry- The standard method is to compare a standard reaction for the same chemical structure, but varying the position of the double bond. n 2n (alkene) 2 (Pt) n 2n2 (alkane) Substitution on alkene: The more s attached to the alkene kcal/mole 3 2 E least subst-alkene most subst-alkene kcal/mole Pt 3 3 alkane 26.9 kcal/mole 3 3 To a rough approximation, we see that each alkyl substituent on a double bond stabilizes the alkene about kcal/mole. 14-3

4 rganic hemistry otes by Jim Maxka ations Simple Markovnikov theory will predict one product of the reaction of propene Br 2 2 Br 3 or Br ne of the propryl cations above is more stable than the other. Which one? But conjugation is a more powerful stabilizing influence. ne of the cations below is exceptionally stable. Which one? Why? Draw the resonance structure in the box. 2 or What substitution is the carbocation? Why is it still so stable? Identify the allyl s in the molecule below: Below are allyl cations Practice: Which systems below are allyl? Electrophilic Addition of onjugated Dienes eview In the electrophilic addition of an alkene with Br, the reaction is a two step reaction. Br 3 or 3 Br Step 1 is the formation of the intermediate the slow step. Step 2 is the combination of Br - with the intermediate the fast step. Why? 14-4

5 rganic hemistry otes by Jim Maxka Label the reaction diagram below: B D G G A A= B= = D= 1,2 vs 1,4 addition: In the reaction of propene with Br as shown above, the stability of 2 0 > 1 0 is the key to the outcome of the reaction. For the reaction of conjugated dienes, the _ is the key. Let s follow the course of the reaction of Br with 1,3-butadiene. Step 1: Br reacts with 1,3-butadiene. Which intermediate is stabilized? 2 2 Br What type of cation can be formed? 2 or 2 2 Br - Draw resonance. Step 2: What happens next? earrangement of the allyl cation or reaction with Br: -? Follow both pathways: eaction with Br -- : 2 Br - What is the type of this product? 2 2 Br 1 earrangement followed by reaction with Br --. Br Br This is called the addition. 14-5

6 rganic hemistry otes by Jim Maxka If there were no barrier to the formation of either the 1,2 or the 1,4 addition, there would be mostly 1,4 product. Why? more heat hotter-more kinetic energy Br eaction ecap The intermediate in the 1,2 addition is what type of cation? _ 0 c cation Draw it: The intermediate in the 1,4 addition is what type of cation? _ 0 c cation Draw it: What is the relationship between these two structures? Which one is more stable? Why? What are thermodynamic conditions? What are kinetic conditions? eaction with dihalogens The reaction is similar to Br but now we make 1,2 and 1,4 dibromides: Br - Br 2 2 Br Br 2 Practice for 1,2 vs 1,4 electrophilic addition 1. What would be the Br addition products of 1,3-hexadiene? 2. Why is there no cis/trans for 1,3-butadiene? What about 1,3-hexadiene? 3. Is 2,3-hexadiene conjugated? Explain. is/trans here? 4. 2,3-hexadiene has enantiomers. an you draw them? Why is this unusual. 5. Which is favored by high T, 1,2 or 1,4? Which is favored by low T, 1,2 or 1,4? 14-6

7 rganic hemistry otes by Jim Maxka Diels Alder eaction rbital Model of onjugation Pi Bonds are areas for electron density outside of the normal sigma bond space. Bonding model of pi bond in ethylene: First consider each p orbital independent. Individual p orbitals in-phase combination out-of-phase combination ote this the in-phase combination of the two p orbitals is bonding (pi bond), the out-of-phase combination is antibonding (pi*) and higher in energy. If we build a bonding correlation diagram, we can see that since each atomic p orbital brings 1 e - to the molecule, we have e - total for the molecule. Fill in all the electrons for the ethylene system below. Antibonding (1 node) LUM 2 p orbitals Bonding (0 nodes) M What is the significance of a node? The M (ighest ccupied) Is this molecular orbital electron rich or poor?. The LUM (Lowest Unoccupied) Is this molecular orbital electron rich or poor?. ow onsider the p orbital arrangement for butadiene. ow many nodes* and bonding molecular orbitals can be formed? olor the orbitals. ount the number of electrons. Which is M, which is LUM? odes* Bonding? odes* Bonding? odes* Bonding? odes* Bonding? *ode= odal plane. What is a good definition of node? Diels Alder eaction is a completely electronic reaction between orbitals. Formally, 14-7

8 rganic hemistry otes by Jim Maxka EWG EWG ew sigma bonds Diene 1,3-Butadiene EWG Dienophile ote cis arrangement of EWG groups ew pi bond EWG Still cis arrangement of EWG. otes: (1) There must be a diene and a dienophile. (2) The reactivity of the diene is increased by EWG (electron withdrawing groups. (3) EWG are highly E groups (without reactive LP) or groups with oxygens and nitrogens (highly E) positioned by one atom away. The electrons are pulled away creating a positive charge next to the reactive group. Y Y Pick the following systems that have electron withdrawing groups: The molecules that do not have EWG have EDG (electron donating groups). The eaction Identify the diene and the dienophile and predict the product of the following Diels-Alder eactions. ote the EWG and EDG. Diene reactivity is increased by EDG; Dienophile reactivity is increased by EWG. l 3 l Some reactions can make bicyclic products: 14-8

9 rganic hemistry otes by Jim Maxka Sorting out egiochemistry of Diels Alder When bicyclic molecules are formed there may be the possibility of up or down attachment. The reaction below can be endo or exo LY the endo is formed: 2 up exo 2 down endo There are a couple of ways of looking at the exclusion of the exo product: (1) Sterics (2) Electronics ot all reactions have endo/exo: onformational equirements for Diels Alder The diene must by in the s-cis conformation for the reaction to occur. The following molecule does not react by Diels Alder. Why? Anything that stops the formation of the s-cis conformation will inhibit or slow down the reaction. This is not to say that the a diene written in the s-trans form will not react. 14-9

10 rganic hemistry otes by Jim Maxka onsider the reaction below. It does occur. Draw the product and explain: s-trans More on the Electronic equirements of Diels Alder ow let s go through an explanation of electronic requirements of the Diels Alder: For the Diels Alder reaction to work, the diene needs groups and the dienophile needs groups. The reason for this has to do with the orbitals that interact. For the M of the diene to become more reactive, that is more electron donating, it needs EDG. What is the EDG group here? For practice color in the p-lobes to give the phases of the the M of the diene and the LUM of the dienophile. 3 2 EWG 2 EWG Diene Dienophile The dienophile reacts from the LUM. EWG promote reaction in the LUM because they pull electron density out of the molecule bringing the molecule closer in energy to the diene s M. Similarly, the EDG pump energy into the diene. This brings the orbitals closer together and increases reactivity. pump electrons in empty LUM electron poor M electron rich pull electrons out Diels Alder Practice: What dienes and dienophiles would react to give the following D A products. 2 3 l l l 14-10

11 rganic hemistry otes by Jim Maxka Molecular color The EM Spectrum: X ray Far UV ear UV Visible I Microwave E wavelength The colorwheel concept. Light absorbed is not transmitted. blue absorbed = what color? red absorbed = what color? The phenomena of absorption and emission in Absorbtion out in Emission out pi* hv relax hv pi Ground State Excited State Ground State The longer the conjugation length, the longer the wavelength and the lower the energy of absorption of light. Beta-arotene from carrots What color? What color would ethylene be? butadiene? redder color benzene? Ultraviolet

12 rganic hemistry otes by Jim Maxka Why should nylon materials not be exposed to direct sunlight? ( 2 )n ( 2 )n ( 2 )n ( 2 )n 14-12

13 rganic hemistry otes by Jim Maxka Do you know the use of the compound called PABA? 2 Draw a resonance structure starting from the LP on. What is this phenomenon called? S 3 - a S 3 - a FWA-1 (Disodium 4,4'-bis[(4-anilino-6-morpholino-1,3,5-triazin-2-yl)amino]stilbene-2,2'- disulphonate (AS-o )) is a Fluorescent Whitening Agent (FWA) mainly used (more than 90%) in household detergents in concentrations ranging from 0.05 to 0.15%. It is also used to a far lesser extent (less than 10% in total) in textiles and paper. FWA-1 behaves like colorless direct cotton dyes. Its specific molecular structure is responsible for the high affinity to cotton. What is the role of sulfate groups? Is this a UV absorber? Summaries 1. ame dienes and draw dienes from names. 2. Understand the difference between cis/trans and s-cis/s-trans of conjugated dienes. 3. Find conjugated regions in molecules. Determine when dienes are conjugated or not. 4. Determine when a cation or alkene is stabilized by conjugation or hyperconjugation. 5. Which is stronger stabilization? onjugation or hyperconjugation. 6. ecognize allylic cations. 7. Predict the products of 1,2 and 1,4 addition of a halo acid. 8. Determine when a reaction will lead to kinetic or thermodynamic products based on heat or cold. 9. Be able to write molecular orbital diagrams for simple p-orbital systems. 10. Differentiate between pi and pi* orbitals. 11. Determine which orbitals will be filled and which ones will be unfilled. 12. Be able to identify EWG and EDG. 13. Determine the reactivity of the Diels Alder reaction in terms of diene and dienophile. 14. Determine the reactivity of the Diels Alder reaction in terms of EWG and EDG. 15. Determine the reactivity of the Diels Alder reaction in terms of stability of the s-cis conformation of the diene. 16. Predict whether the Diels Alder reaction will produce endo or exo products. Be able to draw these products. Be able to identify these products. 17. Understand the difference between color absorption and emission. 18. ecognize how conjugation affects color of organic molecules. 19. What is the difference between UV and VIS absorption? 14-13