5.5 Physical Properties of Dipole moments

What is direction of dipole moment? Does a methyl group H H donate electrons to the double bond, or does it C C withdraw them? H H

m = 0 D H3C H C C H H m = 0.3 D Dipole moments m = 1.4 D H H C C H H H Cl C C Chlorine is electronegative H H and attracts electrons. m = 0 D H3C H C C H H m = 0.3 D Dipole moments m = 1.4 D Dipole moment H H of 1- C C chloropropene is equal to the H Cl H C H sum of the 3 dipole C C moments of H Cl vinyl chloride H C H and propene. 3 m = 1.7 D C C H H m = 0.3 D Dipole moments m = 1.4 D H H C C Therefore, a H Cl methyl group H3C H donates C C electrons to the double H Cl H C bond. H3C H m = 1.7 D C C H H m = 0.3 D Alkyl groups stabilize sp2 hybridized carbon by releasing electrons

R—C+ is more stable than H—C+

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

R—C+ is more stable than H—C+

. . R—C is more stable than H—C

R—C is more stable than H—C 5.6 Relative Stabilities of Alkenes Double bonds are classified according to the number of carbons attached to them.

R H C C H H monosubstituted R H R R' R H C C C C C C R' H H H H R'

disubstituted disubstituted disubstituted Double bonds are classified according to the number of carbons attached to them.

R R" R R" C C C C R' H R' R"'

trisubstituted tetrasubstituted Substituent Effects on Stability

Electronic

disubstituted alkenes are more stable than monosubstituted alkenes

Steric

trans alkenes are more stable than cis alkenes

Figure 5.4 Heats of

combustion of C4H8 isomers.

2717 kJ/mol + 6O2

2710 kJ/mol

2707 kJ/mol

2700 kJ/mol

4CO2 + 8H2O Substituent Effects on Alkene Stability

Electronic

alkyl groups stabilize double bonds more than H

more highly substituted double bonds are more stable than less highly substituted ones. Problem 5.8

Give the structure or make a molecular model of the most stable C6H12 alkene.

C C Problem 5.8

Give the structure or make a molecular model of the most stable C6H12 alkene.

H3C CH3 C C

H3C CH3 Substituent Effects on Alkene Stability

Steric effects

trans alkenes are more stable than cis alkenes

cis alkenes are destabilized by van der Waals strain Figure 5.5 van der Waals strain cis and trans-2-Butene due to crowding of cis-methyl groups

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

cis-2-butene trans-2-butene van der Waals Strain

Steric effect causes a large difference in stability between cis and trans-(CH3)3CCH=CHC(CH3)3 cis is 44 kJ/mol less stable than trans

CH H C H3C CH3 H3C CH3

H3C C C CH3 C C H H 5.7 Cycloalkenes

Cyclopropene and cyclobutene have angle strain.

Larger cycloalkenes, such as and , can incorporate a double bond into the ring with little or no angle strain.

Stereoisomeric cycloalkenes cis-cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene is 39 kJ/ molmore stable than trans-cyclooctene

H H

H H

cis-Cyclooctene trans-Cyclooctene Stereoisomeric cycloalkenes cis-cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene is 39 kJ/ molmore stable than trans-cyclooctene

cis-Cyclooctene trans-Cyclooctene Stereoisomeric cycloalkenes

trans-cyclooctene is smallest trans- 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

H

H

trans-Cyclooctene 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 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 the ring, trans-cycloalkenes are more stable than cis. The ring is large enough so the cycloalkene behaves much like a noncyclic one.