Rank the following carbocations from most stable to least stab...

Question

Rank the following carbocations from most stable to least stable:

Three carbocation structures are shown, illustrating varying stability levels for comparison.

Accepted Answer

Hello everyone. Let's do this problem. It says sort the following species in decreasing order of stability, we're given four carbos structure. One is one methyl cyclo hex one e with a positive charge on carbon three structure two is two methyl cyclo hea 13 dying with a positive charge on carbon six structure three is methylene cyclohexane with a positive charge on carbon two and structure four is one methyl cyclo hex one in but the positive charge is on carbon five. So since we are comparing the stability of various carbo Canion, let's discuss a few characteristics that influence Carbocaine stability. The first one is hyper conjugation and this is a phenomenon similar to resonance, but it's a little bit different in that the electrons from a sigma bond are interacting with an empty or partially filled neighboring P orbital. So we can quantify this by counting the number of neighboring sigma carbon hydrogen bonds or you can look at the substitution. So a tertiary Carbocaine is more stable than a secondary, more stable than a primary Carbocaine. All right. So those are different ways to measure hyper conjugation. When looking at the structure, we can also look at the inductive effect, which takes into account the types of groups that are attached to that carbo CAD ion. So if we have electron donating groups, those are going to increase the stability of that carbo CAD ion by making it less intensely positive, right, if you make a charge stronger, that's going to decrease the stability. Which is why electron withdrawing groups are going to decrease the stability of a carbo cannon, right electron withdrawing groups, which are also positive are going to draw away that electron density from the positively charged carbon, making it more positive, which is less stable. OK. Lastly, we can look at resonance and this, this takes precedence over hyper conjugation and inductive effect. So this is the de localization of the positive charge on neighboring double bonds. And that's in this case because we have Carbocaine right, it's not always a positive charge. So let's look at these Carbocaine see if they have any resonance. And if not, we can look at the other factors. OK. So starting with structure one, we have one methyl cyclo he one E so we have our six membered ring with a methyl group, a double bond in the ring on the same carbon in our positive charge on carbon number three. So we have a positive charge neighboring that double bond. So the pi electrons from that double bond can move towards that positively charged carbon to give us one resonance structure where the double bond has now moved between carbons two and three. And that positive charge has moved over to carbon one, making it a tertiary Carbocaine. All right, let's look at structure two, two methyl cyclo hea 13 dyne. We have our six membered ring, a methyl group and two double bonds positive charge on carbon six. So again, that positive charge on carbon six is neighboring the double bond between carbons one and two. So those pi electrons from that double bond can shift over towards that positive charge giving us a second resonance structure, moving that positive charge to carbon two. But now this positive charge is neighboring the other double bond. So those pi electrons can move in between carbons two and three to give us another resonance structure. And the positive charge will end up on carbon number four in our third resonance structure. So notice how this structure has two more resonance forms than the first structure, right. So having more resonance structures is going to make structure two more stable than structure one. OK. Let's keep looking at the other Carbocaine given to us structure three, we have methylene cyclohexane. So we have cyclohexane with that endo cyclic double bond or sorry exo cyclic double bond and a positive charge on carbon number two, which that positive charge is neighboring that double bond. So those pi electrons can move down towards the positive charge giving us now an endo cyclic double bond. But that positive charge will move out towards the end of that methyl group leaving us with a primary Carbocaine. So that is going to be less stable than both of the previous compounds we already looked at right, because like we said in the hyper conjugation section, a primary Carbocaine is less stable than a tertiary. And lastly, we have one methyl cyclo hex one E again, the same structure almost as part one. But our positive charge is in a different location relative to our double bond. It is on carbon number five. So in this case, our pie bond is not neighboring that positive charge. So there is actually going to be no resonance here. And since all of the other compounds had resonance structure four is going to be our least stable compound. So going from most stable to least stable Carbocaine, we have structure two had the most resonance forms. So that will be the most stable then compounds one and three both had one extra resonance form but compound one is tertiary. So that's going to be more stable whereas compound three is primary. So that will be less stable and structure four is the least stable because it has no resonance forms. So that is it for this problem? I hope this video was helpful and thank you for watching.

Rank the following carbocations from most stable to least stable:

Key Concepts

Carbocation Stability

Resonance

Inductive Effect

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