Bicycloalkane Nomenclature

7/27/2019 Bicycloalkane Nomenclature

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Organic Chemistry

Chapter 7 - Resonance


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Electron Delocalization and Resonance

Localizedelectrons =restricted to a particular region

Delocalizedelectrons do not belong

to a single atom or exclusively to a

bond between 2 atoms


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Benzene C6H6 Rapid Equilibrium between 2 structures

Proposed by Fredrich Kekule(1865 German chemist)

Kekule Structure

Rapid Equilibrium


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*

Kekules Dream


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Kekule Structures of Benzene were

accepted in the 1930s when X-ray

studies showed ALL SIX C-H bondsequal and ALL SIX C-C bonds equal!

Benzene Structure

H

H

H

H

H

H


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Each C is sp2 hybridized

Each C has an unhybridized p orbital

perpendicular to the plane of the ring The 6 p orbitals overlap to form a

cloud

Bonding in Benzene


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A compound with delocalized e- is said

to have resonance

resonance contributor resonance structure

contributing resonance structure

Resonance Hybrid


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Benzene

contributing resonance structures

Resonance Hybrid


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Draw ing resonance hyb r ids

1) Only e- move(not atoms)

2) Only and non-bonding e- move 3) Total # e- stays same(as does unpaired e-)

Resonance Hybrids


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e-can be movedonlyby

e

-

move toward + or toward

bond

Resonance Hybrids


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Nonbonding pair e-

toward a

bond

Resonance Hybrids


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Nonbonding single e-

toward a

bond

Resonance Hybrids


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Drawing resonance hybrids

Resonance Hybrids


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Resonance Hybrids


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Practice Drawing Resonance

Practice

Practice

Practice


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What makes a Resonance Structu re

Have Decreased Stab il i ty?

1) an atom with an incomplete octet

2) a negative charge that is not on the most

electronegative atom

3) a positive charge not on the most

electropositive atom

4) charge separation


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Examples To Examine

B is less stable than AEqual Stability


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1) The greater the predicted stability of aresonance contributor, the more itcontributes to the resonance hybrid.

2) The greater the number of relatively

stable resonance contributors, the greaterthe resonance energy.

3) The more nearly equivalent the resonancecontributors, the greater the resonanceenergy.

Resonance Energy


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Resonance Energy

The more nearly equivalent the resonance contributors,

the greater the resonance energy

St bilit f Di


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Stability of Dienes


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Stability of butadiene


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Butadiene Example

Br

OCH3

[

]

Br

O

CH3

Br

O

CH3

Br

O

CH3


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Allylic cations have delocalized e-

They are more stable due to resonancecontributors:

Allylic Cations


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Benzylic cations have delocalized e-

They are more stable due to resonancecontributors:

Benzylic Cations


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Stability of Cations


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Because of the allyl and benzyl cationsand radicals are more stable, some

products can be formed easier.

Consequences

Mechanisms Mechanisms Mechanisms

Look at how conjugation affects reactions


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Carboxylic acids are stronger acids thanalcohols due to resonance effects

Effects on pKa


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Why is this alcohol as acidic as it is? -OH is on an sp2 hybridized C In phenol, some resonance contributors have a +

charge on O 3 of phenols structures have separated charges

Phenol


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Molecular Orbital Theory can also help

explain increased stability READ THROUGH THIS SECTION OF THE BOOK

Molecular Orbital Theory


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Thermodynamic vs. Kinetic Control

If the kineticproduct and thermodynamicproduct differ, the major product will depend onreaction conditions.

If carried out at mild (low-temp) conditions, thereaction will be irreversible and the kineticproduct will be favored.

If carried out at sufficiently vigorous (hi-temp)conditions, the reaction will be reversible andthe thermodynamic product will be favored.


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Thermodynamic vs. Kinetic

Control

Kineticproduct and thermodynamicproduct:

Formed fastest

Most stable


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Thermodynamic vs. Kinetic

Control

Reaction conditions control products!

Kineticproduct

low temp

Thermodynamic product high temp


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Diels-Alder Reaction

This reaction makes new carboncarbon bonds

Uses a conjugated diene and a

dienophile

Otto DielsKiel University Kiel

Kurt AlderCologne University Cologne


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Electron withdrawing groups (O or N)increases the reactivity of the

dienophile and increase the reactivity

acts as an electrophile




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1,4 addition to a conjugated diene




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Diels-AlderStereochemistry

If a chiral center is created, equalamounts of R, S produced (racemic)




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Diels-AlderStereochemistry

Stereospecific: cis dienophileremains cis

trans dienophile remains trans




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Diels-Alder Examples

Dienophiles can also be C triplebond C




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Bicycloalkanes

Nomenclature parent is the alkane of the same number of carbons as are in two rings

number from a bridgehead, along longestbridge back to thebridgehead, then along the next longest bridge, etc.

show the lengths of bridges in brackets, from longest to shortest

bicyclo[2.2.1]heptane

1

2

3

4

5

6

7


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Bicyclics - Nomenclature

Examples


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Bicyclics - Nomenclature

Examples

Bicycloalkane Nomenclature

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