Chapter 10 Conjugation in Alkadienes and Allylic...

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cook.chem.ndsu.nodak.edu/chem341 ©2012 Gregory R Cook Chapter 10 Conjugation in Alkadienes and Allylic Systems part 01 CHEM 341: Spring 2012 Prof. Greg Cook cook.chem.ndsu.nodak.edu/chem341 Wednesday, April 18, 12

Transcript of Chapter 10 Conjugation in Alkadienes and Allylic...

Page 1: Chapter 10 Conjugation in Alkadienes and Allylic Systemscook.chem.ndsu.nodak.edu/.../2012/04/Chem341-Chapter10-pt01.pdf · Chapter 10 Conjugation in Alkadienes and Allylic Systems

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Chapter 10Conjugation in Alkadienes and

Allylic Systemspart 01

CHEM 341: Spring 2012

Prof. Greg Cook

cook.chem.ndsu.nodak.edu/chem341

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Conjugation

• Conjugation - conjugare (Latin) to link together - when pi bonds are extended beyond 2 carbons

2

conjugated not conjugated

CH

H

β-Carotene - orange colored constituent of carrots

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Conjugation and Color

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lycopene (red color of tomatos)

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Conjugation and Color

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O

OHR

R

OROH

HO NaOH O

OR

R

OROH

O

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Allylic Conjugation

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allylic carbocation allylic free radical allylic carbanion

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Allylic and Vinylic Positions

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allylic alcohol

OH

allylic bromide

Br

CC

C

H

H

H

H

HH

allylic hydrogens

vinylic hydrogens

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Allylic Carbocations

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Allylic Carbocations

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Allylic Halide Substitution Reactions

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3° and allylic bromide

relative ratefor SN1 Subst.

3° bromide

123 1

Cl Cl

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Allylic Halide SN1 Substitution

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85%

ClClH2O

Na2CO3

H2O

Na2CO3OH

HO+

15%

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Allylic Halide SN2 Substitution

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Cl

relative ratefor SN1 Subst. 80 1

Cl

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Allylic Free Radicals

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allylic free radical

spin densities

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Allylic Positions are Activated

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H

H

+ H

+ H

ΔH° = +101 kcal/mol

ΔH° = +88 kcal/mol

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Free Radical Halogenation

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R H + X2 R X + HX

HCl2 Cl

NBS

heat or light

heat or light

Br

N

O

O

Br

NBSN-bromosuccinimide

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Allylic Bromination

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Allylic Anions

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allylic carbanion

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Allylic Anions

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H

H

pKa = 62

base

base

pKa = 43

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Isolated and Conjugated Dienes

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1,4-pentadiene 1,3-pentadiene

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Cumulated Dienes

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H2C C CH2

an alleneC C C

H

HH

H

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Cumulated Dienes

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H2C C CH2

an alleneC C C

H

HH

H

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Relative Stability of Dienes

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Relative Stability of Dienes

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ΔH° = -70.5 kcal/molH2C C CH2

H2Pd/C

H2Pd/C

ΔH° = -29.9 kcal/mol

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Conformations of Conjugated Dienes

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s-Cis conformation

s-Trans conformation

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