[Gauss]

Which of the 4 is the most stable carbocation?

1) A carbocation with its carbocation-carbon connected to 3 cyclopropyl groups.

2)A carbocation with its carbocation-carbon connected to 3 phenyl groups.

3)A carbocation with its carbocation-carbon connected to 3 cyclohexyl groups

4)tert-butyl carbocation

Better if you can grade them.

[Dan]

Forum rules. Show an attempt.

[AWK]

There is a plenty good sites on this subject, eg
http://web.chem.ucla.edu/~harding/tutorials/cc.pdf
http://www.chemguide.co.uk/mechanisms/eladd/carbonium.html
http://chemistry.umeche.maine.edu/CHY556/carbocat.html
http://www.personal.psu.edu/the1/carbocat.htm

These come from the first page of Google.

[Gauss]

Sorry, is it compulsary that we ask only homework questions and show an attempt?

This was asked in our test and none of us got it right. I thought that the question was quite interesting and was not so obvious. Sorry if I am wrong.

However I know the answer and it is (1). We were surprised coz most of us chose (2). Now I know the reason too.

[Dan]

Sorry, is it compulsary that we ask only homework questions and show an attempt?

This was asked in our test and none of us got it right. I thought that the question was quite interesting and was not so obvious. Sorry if I am wrong.

Posting homework questions is not compulsary.
Showing an attempt is compulsary.

The reason is that simply giving you an answer is only useful to you if you are given the same question again. If you show an attempt, we can help you work out and understand the answer to this question, because we can see why you got it wrong in your test in the first place, and show you how to approach general problems of this nature. In order to give you the most effective help we have to understand how you are approaching the problem to point you in the right direction. It is for your benefit.
The question is not necessarily obvious, but that shouldn't discourage you from throwing some ideas around.

[Gauss]

Alright then. The reason I chose (2) was bcoz it had 6 stable resonance structures (leaving those in which aromaticity is destroyed) and resonance gives an extra edge as all of them are tertiary C-Cs.

But I later got to know from my teacher that (1), through hydride shifts was equivalent to 4 stable tertiary C-Cs. He also said that (3) had a disadvantage that cyclohexane rings are not planar. So (1) is the most stable one.
But how would you grade the others? I think it should be (2)>(3)>(4).
In such cases where there is severe competition, how can we estimate which has a greater effect?(i.e. in case of 1 & 2 ) 

[english]

Consider the varying differences between 2° and 3°.

Why is a 2° carbocation with 2 methyl groups roughly the same stability as a 2° carbocation with two ethyl groups?  Now, why is a 2° cation with 2 isopropyl groups more stable than one with two t-butyl groups?

Think about the bonds; how many C-H sigma bonds overlap with the cation? 


Now compare 2° to 3°.

[AWK]

You can qualitatively distinguish easily between 3 and 1, 2 and 4 together. More - should be calculated

[Gauss]

As for what g english has said, I know quite well  what you are saying. But my real question is that how do you compare resonance and other factors like hybridisation, Inductive effect etc. in situations like this? The general notion is that resonance has a much greater stabilization than the others. But here that doesn't work well. Bcoz my teacher said that (1) is so stable that it can stay in the carbocation form in a solution for quite sometime.

Now suppose I go on adding a +R-effect exerting group like -NH₂ on the para and ortho posns. of the phenyl grps. in (2) one by one and can anyone tell me when it is going to overtake (1) ?

So, is it not better to have numbers? Like why don't we say by what factor resonance stabilization is better than the others rather than just say resonance stabilization is much better than the others.(At least for an approximation)