10-25 - 10/25/2010 Which of the following alkenes is more...

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Unformatted text preview: 10/25/2010 Which of the following alkenes is more stable? Why is cis-2-butene more stable than 1-butene? More resonance Less steric hinderance A B H C H H 1 H C C H H H H H3C H C H C H C H H C resonance stabilization > steric repulsion If there is no bond to the hydrogen atoms in these alkenes why don’t they just drift away? boyfriend cis 2-butene girl friends complete story H C H H C H H C H C H H H3C H C H C H C H H H H H C C H H C H C H H mother Hyperconjugation is an electronic effect where electron delocalization occurs between alkyl groups attached to sp2 atoms. Hyperconjugation is an electronic effect where electron delocalization occurs between alkyl groups attached to sp2 atoms. molecular orbital resonance model model H C H H C H H C H C H H H3C H C H C H C H H 1 10/25/2010 Hyperconjugation is an electronic effect where electron delocalization occurs between alkyl groups attached to sp2 atoms. molecular orbital resonance model model Trans- cyclohexene is more stable than cis- cyclohexene. A. true B. false energy LUMO H HOMO H3C H C C H C H H What is wrong about the following structures of cis and trans cyclohexene? H Trans- cyclohexene is less stable than cis- cyclohexene. H H H H H H H H H H H Trans cyclohexene is more stable than cis cyclohexene. The major product of the following reaction is 1-butene. H H H H Are there any rings that contain trans double bonds? A. true B. false 12 2 10/25/2010 The major product of the following reaction is 2-butene because of electronic not steric effects. The major product of the following reaction is 2-butene because of electronic effects. least hindered H atom A. true B. false H 13 14 What is the geometry of the transition state in a E2 reaction? What are good models for the transition state of an E2 reaction? O H H H Br CH2CH3 H R O H R CH2CH3 H H H Br O H H H 3C Br R O H R O H R CH3 H H H3C Br CH3 H CH3 H H H3C Br a partially formed π-bond in the transition state 15 a partially formed π-bond in the transition state 16 Why does this reaction proceed through a anti periplanar transition state? Why does this reaction proceed through a anti periplanar transition state? O H H H3C Br R O H R O H H H3C Br R O H R CH3 H CH3 H H H3C Br CH3 H CH3 H H H3C Br C-H and C-Br bonds are anti periplanar a partially formed π-bond in the transition state 17 a partially formed π-bond in the transition state 18 3 10/25/2010 Which of the following is a major reason why 2-butene is the major product of this reaction? A. It is produced at a faster rate. (kinetics) B. It is more stable. (thermodynamics) What is the approximate Keq for the following reaction? 1-butene trans-2-butene Keq = trans-2-butene 1-butene A. A. Keq = 1 H OEt H OEt B. Keq = 4 C. Keq = 40 D. Keq = 100 E. Keq = 1000 19% energy Br anti periplanar transition state energy Br Br Br anti periplanar transition state Keq ~ 1012/5.7 = 127 reaction reaction 12 kJ/mol 81% Why is there 19% of 1-butene in this elimination reaction when there is less than 1% at equilibrium? 127 = 1-butene trans-2-butene Changing the base from ethoxide to t-butoxide would increase the proportion of 1-butene. OEt HOEt 19% + 81% A. true H OEt Br B. false energy Br Br anti periplanar transition state 19% Keq ~ 1012/5.7 = 127 reaction 12 kJ/mol 81% Changing the base from ethoxide to t-butoxide increases the proportion of 1-butene. trans-2-butene 127 = 1-butene CH3 O C CH3 H CH3 Predict the major product of the following reaction. A B energy Br Br anti periplanar transition state 53% 12 kJ/mol 46% reaction 4 10/25/2010 The % of alkene will increase when the ethoxide base is replaced by t-butoxide. Both the SN2 and E2 reactions proceed slower when the ethoxide base is replaced by t-butoxide. A. true B. false A. true B. false H H 5 ...
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This note was uploaded on 12/05/2010 for the course CHE 321 taught by Professor Fowler/sampson during the Fall '08 term at SUNY Stony Brook.

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