Rate Constant and Polar solvents

[Dr. To Be]

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Hello!

In Kaplan Ch. 5, it says that polar solvents are favored because it increases the reaction rate. However, I am struggling to grasp the concept behind it. Is it because in polarized bonds the electrons are pulled to one atom causing it to lengthen which then causes it to weaken( unequal electron density?) why the reaction rates increase? Just trying to make sure my thinking it correct here.



Thanks for your help!

 

[aldol16]

Which reaction is favored? Reaction rates always refer to particular reactions. Several effects can be culprit here. Imagine a reaction coordinate diagram of some exergonic reaction. This reaction faces some activation barrier. You have reactants, transition state, and product. So let's consider this simple case here. In order for rate to increase, two things can happen: 1) elevation of the free energy of the reactants and 2) free energy stabilization of the transition state. So let's focus on (1) first. If a polar solvent destabilizes the reactants, this would raise their energy and thereby increase the rate of reaction by simple application of the Arrhenius equation. This can occur in a multitude of ways, although polarity of solvents usually affects not this but rather (2).

So now (2). If you lower the free energy of the transition state, you also decrease the activation energy. How does stabilization of the transition state occur? Well, if your transition state is highly polarized, then polar solvents could stabilize that state and thus bring it down in energy. That would have the net effect of lowering activation energy and again speeding up the reaction.

 

[Dr. To Be]

Which reaction is favored? Reaction rates always refer to particular reactions. Several effects can be culprit here. Imagine a reaction coordinate diagram of some exergonic reaction. This reaction faces some activation barrier. You have reactants, transition state, and product. So let's consider this simple case here. In order for rate to increase, two things can happen: 1) elevation of the free energy of the reactants and 2) free energy stabilization of the transition state. So let's focus on (1) first. If a polar solvent destabilizes the reactants, this would raise their energy and thereby increase the rate of reaction by simple application of the Arrhenius equation. This can occur in a multitude of ways, although polarity of solvents usually affects not this but rather (2).

So now (2). If you lower the free energy of the transition state, you also decrease the activation energy. How does stabilization of the transition state occur? Well, if your transition state is highly polarized, then polar solvents could stabilize that state and thus bring it down in energy. That would have the net effect of lowering activation energy and again speeding up the reaction.

Thanks! I'm still a little confused with how the polar solvents destabilizes the reactants in relation to the Arrhenius equation.

 

[aldol16]

Thanks! I'm still a little confused with how the polar solvents destabilizes the reactants in relation to the Arrhenius equation.

Yes, this is more difficult effect to parse out and in most cases, it won't be very relevant. When you have a metal in solution, for instance, the solvent can coordinate, which would change the reacting species altogether and so that would have a different energy. It's not important because most of the time, you can reason with the transition state energetics - at this level, that will be what most people care about anyway.

 

[Dr. To Be]

Yes, this is more difficult effect to parse out and in most cases, it won't be very relevant. When you have a metal in solution, for instance, the solvent can coordinate, which would change the reacting species altogether and so that would have a different energy. It's not important because most of the time, you can reason with the transition state energetics - at this level, that will be what most people care about anyway.

Thank you!