alkene reactivity

[aviary17]

Hi. It says in EK OC (p37) that "the most reactive alkenes in electrophilic addition are the most thermodynamically stable. This is because they also have the lowest activation energy when forming carbocations."

Why is that? I thought that the more unstable alkene would be more reactive because it will start off at a higher energy and thus take less energy to react. Thanks for your help.

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[amm]

Common mistakes organic students make is to misunderstand the relationship between stability and reactivity. Because a molecule is stable (thermodynamic property) doesn't mean it is not reactive (kinetic property). Thermodynamic (stability/total energy of the molecule) and kinetics (speed/activation energy) are separate properties.

To answer your question: A more stable alkene is one where the double is more substituted (meaning there are carbons attached to the sp2 carbons in the double bond). In electrophilic addition, one hydrogen is added to one sp2 carbons of the double bond and the other sp2 carbon becomes a carbocation. If the double bond was more substituted, the more stable the carbocation will become.

The more stable the resulting carbocation, the faster it can be formed and the faster it can react. On the other hand, if the double bond was not substituted, it would be less likely to undergo electrophilic addition because resulting carbocation would be very unstable; therefore, it would not easily form, and the reaction would not easily proceed.

 

[bobsagat]

Hi. It says in EK OC (p37) that "the most reactive alkenes in electrophilic addition are the most thermodynamically stable. This is because they also have the lowest activation energy when forming carbocations."

Why is that? I thought that the more unstable alkene would be more reactive because it will start off at a higher energy and thus take less energy to react. Thanks for your help.

I think you're right to be suspect of the statement, because it doesn't make intuitive sense. Orgo really isn't my strong point, but I think this statement is true because in this case the stability of the carbocation is a function of the stability of the overall structure. So, the activation energy gets lower as the stability of the molecule increases. (which, I believe is the opposite of the general case)

Someone check me on this.

 

[requeim]

yeah, that is the opposite of the general case.

i wouldn't really focus too much or get too worked up over it bceause alkenes are a topic supposedly NOT tested by the aamc. that and benzene rings as well.

 

[bryanjb]

the fact that since carbocation formation is the rate limiting step of the reaction may also have something to do with it. also, more stable intermediates (higher substituted) have lower energy so it requires less energy input (endothermic) to form the carbocation.