Transcript Resonance: 1. When you can draw more than one classic valence bond structure for a compound that differ only in the arrangement of.

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).
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.
[
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
•
H—C
|
H
H
•
|
C
•
C—H
|
H
delocalization of the unpaired electron  resonance
stabilization
proof that the allyl free radical is as proposed:
13CH CH=CH
3
2
+ NBS 
13CH CH=CH
2
2
Br
[
13CH CH=CH
2
2
•

+ 13CH2=CHCH2
Br
13CH =CHCH
2
2
•
]
Dienes:
| |
| |
—C=C—C=C—
conjugated double bonds
|
|
—C=C=C—
cumulated double bonds
| |
|
| |
—C = C — C — C = C —
|
isolated double bonds
nomenclature:
CH2=CHCH=CH2
CH3CH=CHCH2CH=CHCH3
1,3-butadiene
conjugated
2-methyl-1,3-butadiene (isoprene)
conjugated
2,5-heptadiene
isolated
CH3
CH3
CH3
CH2OH
CH3
CH3
Vitamin A
CH3
CH3
H3C
H3C
CH3
CH3
CH3
CH3
beta-carotene
CH3
CH3
(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
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
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!
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
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
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
1,2- plus 1,4-addition of free radicals:
CH2=CHCH=CH2
perox.
+ 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
no resonance is possible with isolated double bonds:
CH2=CHCH2CH=CH2 + HBr  CH2CHCH2CH=CH2
H 
no resonance possible

CH2CHCH2CH=CH2
H Br
 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
polymer 
*
isoprene
all cis- polyisoprene = latex rubber
all trans- polyisoprene = gutta percha
cis-/trans- polyisoprene = chicle
*
polyisoprene
vulcanization of rubber: addition of sulfur and heat to
natural rubber => 1) harder & 2) less soluble in organic
solvents.
synthetic rubber
Cl
CH2 = C—CH = CH2
chloroprene

Cl
-(-CH2—C = C—CH2-)-n
polychoroprene