I am kind of confused on the correlation between reactivity and stability. Why is a primary carbocation more reactive than tertiary? I always assumed more stability results in greater reactivity, such as in addition reactions and stuff.
I am kind of confused on the correlation between reactivity and stability. Why is a primary carbocation more reactive than tertiary? I always assumed more stability results in greater reactivity, such as in addition reactions and stuff.
It depends on the type of reaction we are talking about here. Firstly, a primary carbocation is too unstable to even form for any period of time, so we cant really talk about it being reactive. However, if we were to isolate two carbocations, say a secondary and a tertiary, the more unstable secondary carbocation IS more reactive than the tertiary carbocation in that it is less 'comfortable' existing on its own and will be in more of a hurry to bond with something and relieve the instability.
However, what you may be thinking of is that a tertiary alky halide, or whatever starting compound, is more reactive than a secondary or primary in a reaction involving a leaving group, such as sn1 or e1. We say that the starting compound is more reactive in such a reaction BECAUSE it involves a tertuary carbocation forming. NOT that the tertiary carbocation is more reactive.
I hope that helps, let me know what kind of reaction you have in mind here and i can be more specific.
What @amy_k is referring to is the notion that if we have two reagents, one which will form a primary carbocation during a reaction and one that would form into a tertiary carbocation, the reagent with the tertiary cation will be more reactive. IE: that its conjugate is more stable, thus making the full molecule more reactive. The primary carbocation is an unfavored configuration, whereas tertiary is not. Its just like with acids/bases, if you have a strong acid, generally speaking the conjugate base is weak, meaning it is very un-reactive and stable.So, what I am understanding is that the primary carbocation has to form for it to be more reactive than a tertiary carbocation. But, since it does not form so easily due to its instability, then we don't get much of it for as an intermediate, so most of the product will be the tertiary carbon with some nucleophile.
Can you please clarify what you mean by that? I am a bit confused by it.
Lower stability pretty much always results in increased reactivity (I can't think of an example where it doesn't...)