Stabilitas Struktur: TS/Intermediate/ Produk - Spada UNS

Stabilitas Struktur:

TS/Intermediate/

Produk

• What makes a Resonance Structure Have Decreased Stability?

• 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

Examples To Examine

B is less stable than A Equal Stability

• 1) The greater the predicted stability of a resonance contributor, the more it contributes to the

resonance hybrid.

• 2) The greater the number of relatively stable

resonance contributors, the greater the resonance energy.

• 3) The more nearly equivalent the resonance contributors, the greater the resonance energy.

Resonance Energy

Resonance Energy

The more nearly equivalent the resonance contributors, the greater the resonance energy

Stability of Dienes

Stability of butadiene

Butadiene – Example

• Allylic cations have delocalized e^-

• They are more stable due to resonance contributors:

Allylic Cations

• Benzylic cations have delocalized e^-

• They are more stable due to resonance contributors:

Benzylic Cations

• *

Stability of Cations

• Because of the allyl and benzyl cations and radicals are more stable, some products can be formed

easier.

Consequences

Mechanisms Mechanisms Mechanisms Look at how conjugation affects reactions

• Carboxylic acids are stronger acids than alcohols due to resonance effects

Effects on pK

_a

• 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 O3 of phenol’s structures have separated charges

Phenol

• Molecular Orbital Theory can also help explain increased stability

Molecular Orbital Theory

Thermodynamic vs. Kinetic Control

• If the kinetic product and thermodynamic product differ, the major product will depend on reaction conditions.

• If carried out at mild (low-temp) conditions, the reaction will be irreversible and the kinetic product will be favored.

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

Thermodynamic vs. Kinetic Control

• Kinetic product and thermodynamic product:

Formed fastest

Most stable

Thermodynamic vs. Kinetic Control

• Reaction conditions control products!

Kinetic product – low temp

Thermodynamic product – high temp