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5.5 Physical Properties of Alkenes

5.5 Physical Properties of Alkenes. H. H. C. C. H. H. H 3 C. H. C. C. H. H. m = 0.3 D. Dipole moments. What is direction of dipole moment? Does a methyl group donate electrons to the double bond, or does it withdraw them?. m = 0 D. m = 1.4 D. H. H. C. C. H. H. H. Cl.

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5.5 Physical Properties of Alkenes

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  1. 5.5Physical Properties of Alkenes

  2. H H C C H H H3C H C C H H m = 0.3 D Dipole moments What is direction of dipole moment? Does a methyl group donate electrons to the double bond, or does it withdraw them? m = 0 D

  3. m = 1.4 D H H C C H H H Cl C C H H H3C H C C H H m = 0.3 D Dipole moments Chlorine is electronegative and attracts electrons. m = 0 D

  4. m = 1.4 D H H C C H Cl H3C H C C Cl H H3C H m = 1.7 D C C H H m = 0.3 D Dipole moments Dipole moment of 1-chloropropene is equal to the sum of the dipole moments of vinyl chloride and propene.

  5. m = 1.4 D H H C C H Cl H3C H C C Cl H H3C H C C H H m = 0.3 D Dipole moments Therefore, a methyl group donates electrons to the double bond. m = 1.7 D

  6. R—C+ H—C+ R—C H—C Alkyl groups stabilize sp2 hybridizedcarbon by releasing electrons is more stable than . . is more stable than

  7. R—C+ H—C+ R—C H—C R—C H—C Alkyl groups stabilize sp2 hybridized carbon by releasing electrons is more stable than . . is more stable than is more stable than

  8. 5.6Relative Stabilities of Alkenes

  9. H R C C H H H R C C C C C C R' H disubstituted Double bonds are classified according tothe number of carbons attached to them. monosubstituted R' H R R R' H H H disubstituted disubstituted

  10. R" R" R R C C C C R' H R' R"' trisubstituted tetrasubstituted Double bonds are classified according tothe number of carbons attached to them.

  11. Substituent Effects on Alkene Stability Electronic disubstituted alkenes are more stable than monosubstituted alkenes Steric trans alkenes are more stable than cis alkenes

  12. Figure 5.4 Heats of combustion of C4H8isomers. 2717 kJ/mol + 6O2 2710 kJ/mol 2707 kJ/mol 2700 kJ/mol 4CO2 + 8H2O

  13. Substituent Effects on Alkene Stability Electronic alkyl groups stabilize double bonds more than H more highly substituted double bonds are morestable than less highly substituted ones.

  14. C C Problem 5.8 Give the structure or make a molecular model of the most stable C6H12 alkene.

  15. C C Problem 5.8 Give the structure or make a molecular model of the most stable C6H12 alkene. H3C CH3 H3C CH3

  16. Substituent Effects on Alkene Stability Steric effects trans alkenes are more stable than cis alkenes cis alkenes are destabilized by van der Waalsstrain

  17. Figure 5.5 cis and trans-2-Butene van der Waals straindue to crowding ofcis-methyl groups cis-2-butene trans-2-butene

  18. Figure 5.5 cis and trans-2-Butene van der Waals straindue to crowding ofcis-methyl groups cis-2-butene trans-2-butene

  19. H3C CH3 CH3 H3C CH3 H3C C C C C H H van der Waals Strain Steric effect causes a large difference in stabilitybetween cis and trans-(CH3)3CCH=CHC(CH3)3 cis is 44 kJ/mol less stable than trans

  20. 5.7Cycloalkenes

  21. Cycloalkenes Cyclopropene and cyclobutene have angle strain. Larger cycloalkenes, such as cyclopenteneand cyclohexene, can incorporate a double bond into the ring with little or no angle strain.

  22. H H H H Stereoisomeric cycloalkenes cis-cyclooctene and trans-cycloocteneare stereoisomers cis-cyclooctene is 39 kJ/ molmore stablethan trans-cyclooctene cis-Cyclooctene trans-Cyclooctene

  23. cis-Cyclooctene trans-Cyclooctene Stereoisomeric cycloalkenes cis-cyclooctene and trans-cycloocteneare stereoisomers cis-cyclooctene is 39 kJ/ molmore stablethan trans-cyclooctene

  24. H H Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability trans-Cyclooctene

  25. Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability cis-Cyclododecene trans-Cyclododecene

  26. Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability When there are more than 12 carbons in thering, trans-cycloalkenes are more stable than cis.The ring is large enough so the cycloalkene behavesmuch like a noncyclic one.

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