1. Resonance: 1.When you can draw more than one classic valence bond structure for a compound that differ only in the arrangement of the electrons, there is resonance. 2.If the structures have approximately the same stability, then resonance is important. 3.If resonance is important, none of the classic structures adequately represent the compound. It is better represented as a hybrid of the classic valence bond structures. 4.The resonance hybrid is more stable than any of the contributing structures ( resonance stabilization energy).

2. allylic halogenation of alkenes. CH 2 =CHCH 3 + X 2, heat  CH 2 =CHCH 2 + HX X 1)X 2  2 X 2)CH 2 =CHCH 3 + X  HX + CH 2 =CHCH 2 allyl free radical 3) CH 2 =CHCH 2 + X 2  CH 2 =CHCH 2 -X + X etc.

3. [ CH 2 =CHCH 2  CH 2 CH=CH 2 ] Resonance is important here! H H H | | | H—C- - C- -C—H Stability of free radicals: allyl > 3 o > 2 o > 1 o > CH 3

4. H | C H — C C — H | | H H delocalization of the unpaired electron  resonance stabilization

5. proof that the allyl free radical is as proposed: 13 CH 3 CH=CH 2 + NBS  13 CH 2 CH=CH 2 + 13 CH 2 =CHCH 2 Br Br [ 13 CH 2 CH=CH 2  13 CH 2 =CHCH 2 ]

6. Dienes: | | | | | | | | | — C = C — C = C — —C = C — C — C = C — | conjugated double bondsisolated double bonds | | — C = C = C — cumulated double bonds

7. nomenclature: CH 2 =CHCH=CH 2 CH 3 CH=CHCH 2 CH=CHCH 3 1,3-butadiene 2,5-heptadiene conjugated isolated 2-methyl-1,3-butadiene (isoprene) conjugated

9. (cumulated dienes are not very stable and are rare) isolated dienes are as you would predict, undergo addition reactions with one or two moles…  conjugated dienes are unusual in that they: 1)are more stable than predicted 2)are the preferred products of eliminations 3)give 1,2- plus 1,4-addition products

10. Heats of hydrogenation (Kcal/mole) for dienes: 1,4-pentadiene60.8isolated 1,5-hexadiene60.5isolated 1,3-butadiene57.1conjugated 1,3-pentadiene54.1conjugated 2-methyl-1,3-pentadiene53.4conjugated 2,3-dimethyl-1,3-butadiene53.9conjugated 1,2-propadiene (allene)71.3cumulated

11. Conjugated dienes are more stable (~3/4 Kcal/mole) than predicted. (Isolated dienes are as expected.) Conjugated dienes are the preferred products of eliminations: CH 3 CH 2 CHCH 2 CH=CH 2 + KOH(alc)  Br CH 3 CH 2 CH=CHCH=CH 2 only! CH 3 CH=CHCH 2 CH=CH 2 none!

12. isolated dienes: (as expected) 1,5-hexadiene CH 2 =CHCH 2 CH 2 CH=CH 2 + H 2, Ni  CH 3 CH 2 CH 2 CH 2 CH=CH 2 CH 2 =CHCH 2 CH 2 CH=CH 2 + 2 H 2, Ni  CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 2 =CHCH 2 CH 2 CH=CH 2 + Br 2  CH 2 CHCH 2 CH 2 CH=CH 2 Br Br CH 2 =CHCH 2 CH 2 CH=CH 2 + HBr  CH 3 CHCH 2 CH 2 CH=CH 2 Br CH 2 =CHCH 2 CH 2 CH=CH 2 + 2 HBr  CH 3 CHCH 2 CH 2 CHCH 3 Br Br

13. conjugated dienes yield 1,2- plus 1,4-addition: CH 2 =CHCH=CH 2 + H 2, Ni  CH 3 CH 2 CH=CH 2 + CH 3 CH=CHCH 3 CH 2 =CHCH=CH 2 + 2 H 2, Ni  CH 3 CH 2 CH 2 CH 3 CH 2 =CHCH=CH 2 + Br 2  CH 2 CHCH=CH 2 + CH 2 CH=CHCH 2 Br Br Br Br CH 2 =CHCH=CH 2 + HBr  CH 3 CHCH=CH 2 + CH 3 CH=CHCH 2 Br Br peroxides CH 2 =CHCH=CH 2 + HBr  CH 2 CH=CHCH 3 + CH 2 CH 2 CH=CH 2 Br Br

14. 1,2- plus 1,4-addition? CH 2 =CHCH=CH 2 + HBr  CH 2 CHCH=CH 2  CH 2 CH=CHCH 2 H  H  resonance! allyl carbocation: CH 3 CH--C--CH 2   CH 2 CHCH=CH 2 + CH 2 CH=CHCH 2 H Br H Br 1,2-addition 1,4-addition

15. 1,2- plus 1,4-addition of free radicals: perox. CH 2 =CHCH=CH 2 + HBr  CH 2 CHCH=CH 2  CH 2 CH=CHCH 2 Br Br resonance! allyl free radical: CH 3 CH--C--CH 2  CH 2 CHCH=CH 2 + CH 2 CH=CHCH 2 Br H Br H 1,2-addition 1,4-addition

16. no resonance is possible with isolated double bonds: CH 2 =CHCH 2 CH=CH 2 + HBr  CH 2 CHCH 2 CH=CH 2 H  no resonance possible  CH 2 CHCH 2 CH=CH 2 H Br

17.  conjugated dienes are unusual in that they: 1)are more stable than predicted 2)are the preferred products of eliminations 3)give 1,2- plus 1,4-addition products

18. isoprene polyisoprene all cis- polyisoprene = latex rubber all trans- polyisoprene = gutta percha cis-/trans- polyisoprene = chicle polymer 

19. vulcanization of rubber: addition of sulfur and heat to natural rubber => 1) harder & 2) less soluble in organic solvents. synthetic rubber Cl Cl CH 2 = C—CH = CH 2  -(-CH 2 —C = C—CH 2 -)- n chloroprenepolychoroprene