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Resonance:

Resonance: 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 . If the structures have approximately the same stability, then resonance is important .

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Resonance:

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  1. Resonance: • 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. • If the structures have approximately the same stability, then resonance is important. • 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. • The resonance hybrid is more stable than any of the contributing structures ( resonance stabilization energy).

  2. allylic halogenation of alkenes. • CH2=CHCH3 + X2, heat  CH2=CHCH2 + HX • X • X2 2 X• • CH2=CHCH3 + •X  HX + CH2=CHCH2• • allyl free radical • 3) CH2=CHCH2• + X2  CH2=CHCH2-X + X• • etc.

  3. [ CH2=CHCH2•  •CH2CH=CH2 ] Resonance is important here! H H H | | | H—C- - C- -C—H • Stability of free radicals: allyl > 3o > 2o > 1o > CH3

  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: 13CH3CH=CH2 + NBS  13CH2CH=CH2 + 13CH2=CHCH2 Br Br [13CH2CH=CH2 13CH2=CHCH2 ] • •

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

  7. nomenclature: CH2=CHCH=CH2 CH3CH=CHCH2CH=CHCH3 1,3-butadiene 2,5-heptadiene conjugated isolated 2-methyl-1,3-butadiene (isoprene) conjugated

  8. (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: • are more stable than predicted • are the preferred products of eliminations • give 1,2- plus 1,4-addition products

  9. Heats of hydrogenation (Kcal/mole) for dienes: 1,4-pentadiene 60.8 isolated 1,5-hexadiene 60.5 isolated 1,3-butadiene 57.1 conjugated 1,3-pentadiene 54.1 conjugated 2-methyl-1,3-pentadiene 53.4 conjugated 2,3-dimethyl-1,3-butadiene 53.9 conjugated 1,2-propadiene (allene) 71.3cumulated

  10. Conjugated dienes are more stable (~3/4 Kcal/mole) than predicted. (Isolated dienes are as expected.) Conjugated dienes are the preferred products of eliminations: CH3CH2CHCH2CH=CH2 + KOH(alc)  Br CH3CH2CH=CHCH=CH2 ONLY! CH3CH=CHCH2CH=CH2 none!

  11. isolated dienes: (as expected) 1,5-hexadiene CH2=CHCH2CH2CH=CH2 + H2, Ni  CH3CH2CH2CH2CH=CH2 CH2=CHCH2CH2CH=CH2 + 2 H2, Ni  CH3CH2CH2CH2CH2CH3 CH2=CHCH2CH2CH=CH2 + Br2  CH2CHCH2CH2CH=CH2 Br Br CH2=CHCH2CH2CH=CH2 + HBr  CH3CHCH2CH2CH=CH2 Br CH2=CHCH2CH2CH=CH2 + 2 HBr  CH3CHCH2CH2CHCH3 Br Br

  12. conjugated dienes yield 1,2- plus 1,4-addition: CH2=CHCH=CH2 + H2, Ni  CH3CH2CH=CH2 + CH3CH=CHCH3 CH2=CHCH=CH2 + 2 H2, Ni  CH3CH2CH2CH3 CH2=CHCH=CH2 + Br2 CH2CHCH=CH2 + CH2CH=CHCH2 Br Br Br Br CH2=CHCH=CH2 + HBr  CH3CHCH=CH2 + CH3CH=CHCH2 Br Br peroxides CH2=CHCH=CH2 + HBr  CH2CH=CHCH3 + CH2CH2CH=CH2 Br Br

  13. 1,2- plus 1,4-addition? CH2=CHCH=CH2 + HBr CH2CHCH=CH2 CH2CH=CHCH2 H H  resonance! allyl carbocation: CH3CH--C--CH2   CH2CHCH=CH2 + CH2CH=CHCH2 H Br H Br 1,2-addition 1,4-addition

  14. 1,2- plus 1,4-addition of free radicals: perox. CH2=CHCH=CH2 + HBr CH2CHCH=CH2 CH2CH=CHCH2 Br• Br • resonance! allyl free radical: CH3CH--C--CH2 •  CH2CHCH=CH2 + CH2CH=CHCH2 Br H Br H 1,2-addition 1,4-addition

  15. no resonance is possible with isolated double bonds: CH2=CHCH2CH=CH2 + HBr  CH2CHCH2CH=CH2 H  no resonance possible  CH2CHCH2CH=CH2 H Br

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

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

  18. vulcanization of rubber: addition of sulfur and heat to natural rubber => 1) harder & 2) less soluble in organic solvents. synthetic rubber Cl Cl CH2 = C—CH = CH2 -(-CH2—C = C—CH2-)-n chloroprene polychoroprene

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