2760(Ch14)(Conjugated Systems) PDF

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Summary

Organic Chemistry...

Description

Conjugated Systems Molecular Orbital View of Allyl Cation Conjugated System: When the p orbitals associated with one pi bond overlap with at least one other p orbital (usually another pi bond).

=

# of nodal planes

2

= non-conjugated

conjugated

1

Other conjugated systems: O

0

Significance: In general, conjugated systems are more stable than non-conjugated systems

Cations and anions can also form conjugated systems (are “stabilized by resonance”)

+

+

=

+

allyl cation

+

What about cyclic collections of p orbitals? (Same concepts, slightly more complicated MOs) Interesting observation:

cyclobutadiene

benzene

_

_

=

_

much more stable than expected

allyl anion

much less stable than expected

How to measure energy? Measure energy released during a reaction. e.g.: H2 Pt

+

28.6 kcal/mol

“heat of hydrogenation”

Predict . . .

E

85.8 kcal/mol

57.2 kcal/mol

28.6 kcal/mol

Find . . .

36 kcal/mol

1.8 kcal/mol

E

55.4 kcal/mol

49.8 kcal/mol

28.6 kcal/mol

classified as “aromatic”

resonance stabilization energy...