15.79 Kinetic Control Vs Kinetic Favorability

Arrhenius Equation: $\ln k = - \frac{E_{a}}{RT} + \ln A$

Michelle Dong 1F
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Joined: Fri Sep 29, 2017 7:04 am

15.79 Kinetic Control Vs Kinetic Favorability

In some reactions, two or more different products can be formed by different pathways. If the product formed by the faster reaction predominates, the reaction is considered to be under kinetic control. If the predominant product is the more thermodynamically stable, the reaction is considered to be under thermodynamic control. In the reaction of HBr with the reactive intermediate CH3CH=CHCH2 , at low temperatures the predominant product is CH3CHBrCH=CH2, but at high temperatures, the predominant product is CH3CH=CHCH2Br. (a) Which product is formed by the pathway with the larger activation energy? (b) Does kinetic control predominate at low or high temperatures? Explain your answers.

Why is CH3CHBrCH=CH2 the product formed by the pathway with the larger activation energy? How can you tell that at higher temperatures the reaction has a larger activation energy? Also, is kinetic control the same thing as kinetic favorability?

Leah Thomas 2E
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Joined: Fri Sep 29, 2017 7:06 am

Re: 15.79 Kinetic Control Vs Kinetic Favorability

I'm not 100% sure if this logic is correct and I may be thinking backward here but...if a reaction needs a higher temperature to react it must need to overcome a large activation energy because increasing temperature provides energy to overcome that barrier.

Joyce Lee 1C
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Re: 15.79 Kinetic Control Vs Kinetic Favorability

For part (a), the second product is formed at high temperatures, so a large amount of energy is needed to overcome the energy barrier for the product to form.
For (b), kinetic control predominates at lower temperatures, because the reaction pathways with lower energy barriers have larger rate constants. The larger the rate constant, the faster the reaction. This is proven by the Arrhenius equation.