Chapter 13 Conjugated Unsaturated Systems

Chapter 13 Conjugated Unsaturated Systems

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Chapter 13

13.1 Introduction

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Chapter 13

CH2 CH CH2 C C CAllyl radical

CH2 CH CH2 C C CAllyl cation

CH2 CH CH CH2 C C C C1,3-Butadiene

Molecules with delocalized π bonds are called conjugated unsaturated systems.

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Chapter 13

Conjugated unsaturated systems have a p orbital on a carbon adjacent to a double bond

The p orbital can come from another double or triple bondThe p orbital may be the empty p orbital of a carbocation or a p orbital with a single electron in it (a radical)Conjugation affords special stability to the moleculeConjugated molecules can be detected using UV spectroscopy

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Chapter 13

13.2 Allylic Substitution and the Allyl Radical

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Chapter 13

Reaction of propene with bromine varies depending on reaction conditions

At low temperature the halogen adds across the double bondAt high temperature or at very low concentration of halogen an allylic substitution occurs

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Chapter 13

CH2 CH CH3 + X2hv

or CH2 CH CH2X + HX

Allylic substitution

C C

C

HH

H H

H

CH2 CH CH2

Allylic radical

Allylic hydrogen atoms

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Chapter 13

13.2A Allylic Chlorination (High Temperature)

Allylic chlorination can be performed at high temperature in the gas phase

CH2 CH CH3 + Cl2400°C

CH2 CH CH2Cl + HCl

Allylic or allyl > 3° > 2° >1°> vinylic or vinylRelative stability of free radicals:

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13.2B Allylic Bromination with N-Bromo-succinimide (Low Concentration of Br2)

CH2 CH CH3 N Br

O

ON-Bromosuccinimide

(NBS)

+light or ROOR

CCl4CH2 CH CH2Br

N H

O

O

+

NBS provides a continuous low concentration of bromine for the radical reactionA low bromine concentration favors allylic substitution over alkene addition

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The radical reaction is initiated by a small amount of bromine radical formed by exposure of NBS to light or peroxides

N H

O

O

+ Br2N Br

O

O

+ HBr

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Chapter 13

13.3 The Stability of the Allyl Radical

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Chapter 13

13.3A Molecular Orbital Description of the Allyl Radical

C C

C

HH

H H

H sp2 hybridized

Conjugated unsaturated system

Bondingorbital

nonbondingorbital

antibondingorbital

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Chapter 13

The three p orbitals of the allylic system combine to form three molecular orbitals

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Chapter 13

13.3B Resonance Description of the Allyl Radical

CH2 CH CH2 CH2 CH CH2

CH2 CH CH212

12

Resonance structuresA B

CA and B are equivalent resonance structures.The allyl radical is even more stable than a tertiary radical.

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Chapter 13

13.4 The Allyl Cation

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Chapter 13

The allyl cation is intermediate in stability between a tertiary and secondary carbocation

CH2 CH CH2

C C

C

HH

H H

Hsp2 hybridized

Conjugated unsaturated system Bondingorbital

nonbondingorbital

antibondingorbital

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Chapter 13

Stability arises from the delocalization of the positive charge over C1 and C3

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Chapter 13

Resonance theory predicts that the allyl cation is a hybrid of equivalent structures D and E

CH2 CH CH2 CH2 CH CH2

CH2 CH CH212

12

Resonance structuresED

F

Both molecular orbital theory and resonance theory suggest that structure F is the best representation for theallyl cation

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Chapter 13

13.5 Summary of Rules for Resonance

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13.5A Rules for Writing Resonance Structures

1. Resonance structures exist only on paper.2. In writing resonance structures we are only

allowed to move electrons.3. All of the structures must be proper Lewis

structures.4. All resonance structures must have the same

number of unpaired electrons.5. All atoms that are a part of the delocalized

system must lie in a plane or be nearly planar

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Chapter 13

6. The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure.

7. Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization.

8. The more stable a structure is (when taken by itself), the greater is its contribution to the hybrid.

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13.5B Estimating the Relative Stability of Resonance Structures

a) The more covalent bonds a structure has, the more stable it is.

b) Structures in which all of the atoms have a complete valence shell of electrons(i.e., the noble gas structure) are especially stable and make large contributions to the hybrid.

c) Charge separation decreases stability.

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Chapter 13

CH2 CH CH CH2 CH2 CH CH CH2

CH2 CH CH CH2CH2 CH CH CH2

CH2 CH CH CH2 CH2 CH CH CH2

CH2 CH CH CH2

1 2

34

5 6

7

Relative stability: 1 > 4, 5, 6, 7 > 2, 3