5.5 Physical Properties of Alkenes

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1 5.5 Physical Properties of Alkenes

2 Dipole moments What is direction of dipole moment? Does a methyl group donate electrons to the double bond, or does it withdraw them? µ = 0 D 3 µ = 0.3 D

3 µ = 1.4 D Dipole moments µ = 0 D 3 l hlorine is electronegative and attracts electrons. µ = 0.3 D

4 Dipole moment of 1-1 chloropropene is equal to the sum of the dipole moments of vinyl chloride 3 and propene. µ = 1.4 D l Dipole moments 3 µ = 1.7 D l µ = 0.3 D

5 µ = 1.4 D Dipole moments Therefore, a methyl group donates electrons to the double bond. 3 l 3 µ = 1.7 D l µ = 0.3 D

6 Alkyl groups stabilize sp 2 hybridized carbon by releasing electrons R + is more stable than + R. is more stable than.

7 Alkyl groups stabilize sp 2 hybridized carbon by releasing electrons R + is more stable than + R. is more stable than. R is more stable than

8 5.6 Relative Stabilities of Alkenes

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

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

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 eats of combustion of 4 8 isomers kj/mol + 6O kj/mol 2707 kj/mol 2700 kj/mol 4O O

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

14 Problem 5.8 Give the structure or make a molecular model of the most stable 6 12 alkene.

15 Problem 5.8 Give the structure or make a molecular model of the most stable 6 12 alkene

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

17 van der Waals strain due to crowding of cis-methyl groups Figure 5.5 cis and trans-2-butene cis-2-butene trans-2-butene

18 van der Waals strain due to crowding of cis-methyl groups Figure 5.5 cis and trans-2-butene cis-2-butene trans-2-butene

19 van der Waals Strain Steric effect causes a large difference in stability between cis and trans-( 3 ) 3 =( 3 ) 3 cis is 44 kj/mol less stable than trans

20 5.7 ycloalkenes

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

22 Stereoisomeric cycloalkenes cis-cyclooctene cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene cyclooctene is 39 kj/ molmore stable than trans-cyclooctene cis-yclooctene trans-yclooctene

23 Stereoisomeric cycloalkenes cis-cyclooctene cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene cyclooctene is 39 kj/ molmore stable than trans-cyclooctene cis-yclooctene trans-yclooctene

24 Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability trans-yclooctene

25 Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability cis-yclododecene trans-yclododecene

26 Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability When there are more than 12 carbons in the ring, trans-cycloalkenes are more stable than cis. The ring is large enough so the cycloalkene behaves much like a noncyclic one.

Structure and Preparation of Alkenes: Elimination Reactions

Structure and Preparation of Alkenes: Elimination Reactions Alkene Nomenclature First identify the longest continuous chain that includes the double bond. Replace the -ane ending of the corresponding unbranched