Review Of Organic Chemistry Page 17
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34Organic Chemistry I Review: Highlights of Key Reactions, Mechanisms, and Principles
17
Impact of Conjugation
4. Stability: Conjugation is stabilizing because of p-orbital overlap (Sections 15.2, 4, 7)
• Note: In the allyl family, resonance = conjugation
One p
Two p’s
Three p’s
Four p’s
Six p’s in circuit
Unstabilized
Allyl type
Butadiene type
Aromatic
π-bond
Isolated
C=C
C=O
O
C=N
O
O
O
NH
O
2
OH
O
OR
O
5. Reactivity: Conjugation-induced stability impacts reactivity (Sections 15.4-7)
• If the product of a rate-determining step is stabilized, the reaction rate will go faster
(product stability-reactivity principle)
o Common when allylic cations, radicals, or carbanions are involved
• If the reactant in the rate-determining step is stabilized, the reaction rate will go slower
(reactant stability-reactivity principle)
o Why aromatics are so much less reactive
o Why ester, amide, and acid carbonyls are less electrophilic than aldehydes or ketones
6. Molecular shape (Sections 15.3, 8, 9)
• The p-orbitals must be aligned in parallel for max overlap and max stability
2
• The sp
centers must be coplanar
2
All four sp
carbons must be flat for the p's to align
2
7. Bond Length: Bonds that look like singles but are actually between conjugated sp
centers are
shorter than ordinary single bonds
1.54 A
1.48 A = Shortened
1.33 A
Shortened
Shortened
Shortened
normal
normal
and Strengthened
and Strengthened
and Strengthened
and Strengthened
single
double
conjugated single
NH
O
OR
O
OH
2
O
• In amides, esters, and acids, the bond between the carbonyl and the heteroatom is shortened
•
2
8. Bond Strength: Bonds that look like singles but are actually between conjugated sp
centers
are stronger than ordinary single bonds
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