Kinetic vs thermodynamic control

[Hemichordate]

Advertisement - Members don't see this ad

Just curious about one thing on kinetic/thermodynamic controls:

If the activation energy of a reaction is really low, then at low temperature, the thermodynamic product could form faster than the kinetic product, right?

 

[vsl5]

hard to say, low temp favours thermodynamic product while low activation energy favours kinetic product. Had you said low temp only, I would have told you thermodynamic product is favoured. Thermodynamics has nothing to do with activation energy, (only energy differences between product and reactant),

 

[Rabolisk]

At low temperature, the kinetic product is typically favored. However, if the thermodynamic pathway also has a pretty low activation energy, then you don't want or need a very high temperature. For kinetic control, you want a temperature that is high enough that the reaction will proceed under a reasonable amount of time. For thermodynamic control, you want a temperature that is high enough that the reaction is reversible and approaches equilibrium in a reasonable amount of time. The second temperature is higher than the first temperature if the second reaction also has a higher activation energy.

To answer your question, though, the thermodynamic product wouldn't ever form "faster" than the kinetic product (assuming that we are talking about two competing reactions). The thermodynamic product would predominate at equilibrium only because it is more stable.

 

[t1me]

Just curious about one thing on kinetic/thermodynamic controls:

If the activation energy of a reaction is really low, then at low temperature, the thermodynamic product could form faster than the kinetic product, right?

You're automatically assuming low activation energy = thermodynamic product.

The rule is low temp = more low activation energy reactions, high temp = more high activation energy reactions.

It just happens that for some reactions (1,2 vs 1,4 addition for ex), kinetic products have lower activation energy and thermo products have a higher activation energy. This is not always the case (halogenation of alkanes for ex).