Why are more stable carbocations faster formed?

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Discussion Overview

The discussion revolves around the relationship between the stability of carbocations and the activation energy of chemical reactions. Participants explore how the thermodynamic stability of carbocations might influence reaction rates, considering various factors such as the structure of reagents and the nature of the transition state.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions how the activation energy could depend on the stability of the carbocation, suggesting it should only depend on the structure of the reagents.
  • Another participant proposes that the structure of the carbon substituents (1°, 2°, 3°) may influence the elimination of the carbocation differently.
  • A participant emphasizes that activation energy is not the only factor, noting that the stability of reactants and products affects the extent of the reaction, including the reversibility of reactions involving carbocations.
  • One participant explains that the activation energy is the energy difference between reactants and the transition state, and that stabilization factors for carbocations also apply to the transition state, which resembles the carbocation more than the neutral reactants.
  • A later reply agrees with the explanation regarding the relationship between activation energy and the stabilization of the transition state.

Areas of Agreement / Disagreement

Participants express differing views on the factors influencing activation energy and reaction rates, with no consensus reached on the primary influences or mechanisms involved.

Contextual Notes

Some assumptions about the nature of activation energy and the role of carbocation stability remain unresolved, and the discussion does not clarify the specific conditions under which these factors apply.

sludger13
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I have no idea how activation energy of a chemical reaction (in either direction) could depend on the thermodynamic stability of formed carbocation, as a product of that reaction. Activation energy of a chemical reaction should depend only on the structure of reagents, right?

The only thing I'm considering is that the more stable is a carbocation, the greater is the reaction enthalpy, thus it could (maybe) better heat the remaining reagents. And with greater Ek of that particles, then the reaction really run faster.
 
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Or, the structure of (1°, 2°, 3°) carbon substitueted REAGENTS really prevent
from elimination of carbocation differently...
 
Activation energy isn't the only factor. The extent to which a reaction occurs is the sum total of the forward and reverse reactions, which happen continuously. Most everything can and does reverse spontaneously to a certain extent, which is dependent on the stability of the reactants and the products.

In the case of carbocations, the molecule separates into its constituent ions. With enough activation energy, they can reverse and reform the full compound. Repeat ad nauseum. Steric hindrance makes the reversal of 3° more difficult than the reversal of 2° and 1°, but it still happens to a certain extent. These relative amounts of hindrance, and therefore the rate of the reverse reaction, are what determine the overall forward rate.

That's how I came to understand it in my organic classes, which were close to 4 years ago now, so somebody else may be able to explain it better or point out if I happened to get something wrong.
 
Activation energy is the difference in energy between the reactands and the transition state. The factors stabilizing the Carbocation are also important in the stabilization of the transition state which resembles more the carbocation than the neutral reactands.
 
DrDu said:
Activation energy is the difference in energy between the reactands and the transition state. The factors stabilizing the Carbocation are also important in the stabilization of the transition state which resembles more the carbocation than the neutral reactands.
This explanation seems fair to me, thanks.
 

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