Chemistry 18B Ch 14 Delocalized Pi system PDF

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Summary

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Description

Summary of CH14 Delocalized

Systems

I. Definition Delocalized systems: extended overlap of three or more contiguous p orbitals bond bond

VI. Allylic Grignard and allylithium reagents • Preparations X R2 R1 C

C H

CH2

Mg/Et2O

R2 R1 C

C H

CH2MgX

allylic Grignard

allylic halide p orbital R2R1C CH

CH3

R2 R1 C

n-BuLi/TMEDA

C H

CH2Li

allyllithium

Allylic systems (C+, C•, C –)

• Allylic Grignard and allyllithium reagents react with electrophiles like other organometallic compounds.

Conjugated dienes

II. Stability of delocalized -systems VII. Conjugated Dienes Synthesis The Heck Rection Pd, or Ni CH2 CHCl + CH2 CH2

• -electron delocalization stabilizes the entire -system C+: substituted allylic > 3° > allyl > 2° > 1° C•: substituted allylic or allyl > 3° > 2° > 1° C–: CH 2=CH-CH2– > CH3-CH2-CH2– • Review resonance theory -know how to draw resonance structures and determine their relative stabilities. -explain the formal charge (C+, C•, C–) distribution in a delocalized -system -know how resonance structures contribute to the stability of the actual molecule • MO theory -numbers of AOs is equal to numbers of MOs

CH2 H

Br2 (low conc. high temp.)

H2C

C H

R1 C R2

CH

X

allylic halide Br

C R2

poor Nu– SN1

R1

inversion

Nu H2C

C H

or X–X

H2C

C H

C CH2 X– H H (or X)

C

H2C R2

R1

C H

C

racemic mixture

1° or 2° allylic halides undergo SN1 reactions with poor nucleophiles and SN2 reactions with good nucleophiles and 3° allylic halides always undergo SN1 reactions.

IX. Diels-Alder cycloadditions • Diels-Alder reaction is a concerted reaction with a high stereospecificity

• Kinetically controlled addition favors endo products while thermodynamically controlled addition favors exo products. • Most Diels-Alder reactions are reversible.

CH2

Nu

SN2

CH2

thermodynamic product

V. Nucleophilic substitutions C H

C R

• Ionic mechanism via a delocalized C+ intermediate • 1,2-addition is the kinetic product while 1,4-addition is the

• Radical substitution (favored by the formation of an allylic radical) vs. ionic addition • NBS in the presence of ROOR or hv (radical mechanism) gives exclusive allylic bromination

H2C

C R

X

H2C

or NBS/hv or ROOR

good Nu–

H2C

R1 R2 Nu

X. Intra molecular Electrocyclic reactions or hu

C H

H C

CH2

H (or 1,2-addition X)

H–X

H2C

III. Kinetic vs. Thermodynamic control • Kinetic control: the ratio of products of a reaction is determined by the relative rate. • Conditions favor kinetic control: short reaction time, lower reaction temp, irreversible reactions • Thermodynamic control: the ratio of products of a reaction that reaches equilibrium is determined by the relative stabilities of the products • Conditions favor thermodynamic control: longer reaction time, higher reaction temperature, reversible reactions IV. Allylic halogenation CH

H2C

(X: Cl, Br)

-node and its relationship to stability of MO -explain the formal charge (C+, C•, C–) distribution according to individual MO -Fill up the bonding MOs with electrons lowers the energy of the system (stabilization)

H2C

VIII. Electrophilic 1,2 vs 1,4-additions of H-X or X2 to conjugated dienes

C H

X

C

CH2

H H (or 1,4-addition X)...