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Reactivity and stability

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DingDongD

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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.
 

amy_k

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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.
 
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NextStepTutor_1

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Stability refers to the compounds "desire," if you will, to stay as is. More specifically, stability means the compound doesn't need additional compounds, atoms, or electrons, and thus is not going to react readily. A simple example can help to paint the picture - atoms want to fill their outer octet, but only noble gases in their monoatomic form are capable of this on their own. Remember that noble gases are very stable, and also remember from the periodic table trends and tidbits that you know that those noble gases do not react readily - and again this makes sense since they have a filled octet and do not need additional electrons that another atom could share. Noble gases are stable, noble gases are not readily reactive. Now think about fluorine - it does not have a filled octet, and in it's monoatomic form it prefers to bind to another atom to fill it's octet. Monoatomic fluorine is thus reactive.
 
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Doctor Dream

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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.

Lower stability pretty much always results in increased reactivity (I can't think of an example where it doesn't...)

If the examples other people provided don't help, you could also think of stability and reactivity in the context of acids or bases. A strong acid for example is very reactive and donates a proton very easily. It's therefore thermodynamically favorable for the acid to lose its proton and go from a state of higher energy to lower energy state with more stability. This is why strong acids always give rise to more stable conjugate bases. Note, the stronger the acid, the more stable the conjugate base.

I remember the idea that a strong acid gives rise to a stable conjugate base took me a while to understand but when you understand it, it totally makes sense.
 
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DingDongD

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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.

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.

Can you please clarify what you mean by that? I am a bit confused by it.
 

EParker37

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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.
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.
 
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amy_k

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I was trying to understand where the op got the idea that a more stable carbocation was more reactive. For example if we had 3-bromo butane versus 2-bromo butane and were doing sn1, 3-bromo butane might be described as more reactive in sn1 than 2-bromo, because it forms the more stable tertiary carbocation intermediate and is therefore more favorable. This would be describing the starting compound 3-bromo butane as more reactive, not the intermediate tertiary carbocation.
 
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amy_k

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In terms of what i said about a primary carbocation, we cant say that this is more reactive (even though it would be), because it is too unstable to even form in the first place (i.e. because it is unlikely to even exist). Hope that helps to clarify.
 
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whitepatchMD

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Simply look at the strucure. A primary carbocation does not have the luxury of having electron donating groups, like methyl groups, adjacent to it. The tertiary carbocation has electron donating groups adjacent so it is able to stablize the positive charge much more easily. This is in essence why you have hydride shifts with alkenes
 

Teleologist

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Lower stability pretty much always results in increased reactivity (I can't think of an example where it doesn't...)

I can think of many examples.

If orgo 1 and 2 weren't enough, then here it is: things can react in a variety of ways. Oxidations, reductions, 1,5 hydrogen shifts, 1,2 alkyl shifts, conjugate additions, [4+2] cycloadditions, halogenation, substitution, addition, etc. etc.

So something can obviously be stable (impervious) to one reaction pathway ... yet totally unstable/reactive toward another reaction pathway. Remembering something now? Benzene doesn't do 1,2 alkyl shifts too well ... it doesn't any alkyl groups to be pushing around. Yet it can undergo electrophilic aromatic substitution.

Another example:


Think of CO2. Completely inert to further oxidation. So someone might come along and say, hey, CO2 is pretty god damn stable!!!! And he'd be right. In only one way (of a million).

I'll come along and correct his smart-ass and say that hey, CO2 has a relatively low LUMO and is highly electrophilic and prone to being attacked by even water (CO2 contamination of de-ionized water is what makes it acidic)!
 
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