Download - Resonance:
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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).
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allylic halogenation of alkenes.
CH2=CHCH3 + X2, heat CH2=CHCH2 + HX
X
1) X2 2 X•
2) CH2=CHCH3 + •X HX + CH2=CHCH2• allyl free radical
3) CH2=CHCH2• + X2 CH2=CHCH2-X + X•
etc.
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[ 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
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H | C
H — C C — H | | H H
delocalization of the unpaired electron resonance
stabilization
••
•
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proof that the allyl free radical is as proposed:
13CH3CH=CH2 + NBS 13CH2CH=CH2 + 13CH2=CHCH2
Br Br
[ 13CH2CH=CH2 13CH2=CHCH2 ] • •
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Dienes:
| | | | | | | | |— C = C — C = C — —C = C — C — C = C — |
conjugated double bonds isolated double bonds
| | — C = C = C —
cumulated double bonds
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nomenclature:
CH2=CHCH=CH2 CH3CH=CHCH2CH=CHCH3
1,3-butadiene 2,5-heptadiene
conjugated isolated
2-methyl-1,3-butadiene (isoprene)
conjugated
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CH3
CH3
CH3
CH2OH
CH3CH3
Vitamin A
CH3
CH3
CH3
CH3CH3
CH3 CH3
H3CH3C
CH3
beta-carotene
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(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
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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) 71.3 cumulated
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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!
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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
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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
peroxidesCH2=CHCH=CH2 + HBr CH2CH=CHCH3 + CH2CH2CH=CH2
Br Br
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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
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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
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no resonance is possible with isolated double bonds:
CH2=CHCH2CH=CH2 + HBr CH2CHCH2CH=CH2
H
no resonance possible
CH2CHCH2CH=CH2
H Br
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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
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isoprene polyisoprene
all cis- polyisoprene = latex rubber
all trans- polyisoprene = gutta percha
cis-/trans- polyisoprene = chicle
polymer **
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vulcanization of rubber: addition of sulfur and heat to natural rubber => 1) harder & 2) less soluble in organic solvents.
synthetic rubber
Cl ClCH2 = C—CH = CH2 -(-CH2—C = C—CH2-)-n
chloroprene polychoroprene