resonance:

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 — C C — H | | H H

delocalization of the unpaired electron resonance

stabilization

••

•

proof that the allyl free radical is as proposed:

13CH3CH=CH2 + NBS 13CH2CH=CH2 + 13CH2=CHCH2

Br Br

[ 13CH2CH=CH2 13CH2=CHCH2 ] • •

Dienes:

| | | | | | | | |— C = C — C = C — —C = C — C — C = C — |

conjugated double bonds isolated double bonds

| | — C = C = C —

cumulated double bonds

nomenclature:

CH2=CHCH=CH2 CH3CH=CHCH2CH=CHCH3

1,3-butadiene 2,5-heptadiene

conjugated isolated

2-methyl-1,3-butadiene (isoprene)

conjugated

CH3

CH3

CH3

CH2OH

CH3CH3

Vitamin A

CH3

CH3

CH3

CH3CH3

CH3 CH3

H3CH3C

CH3

beta-carotene

(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

peroxidesCH2=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:

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

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

isoprene polyisoprene

all cis- polyisoprene = latex rubber

all trans- polyisoprene = gutta percha

cis-/trans- polyisoprene = chicle

polymer **

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