Which carbocation in each pair is more stable?

Name: Problem 13, Chapter 8: Organic Chemistry
Uploaded: 2020-07-24T04:48:18-08:00
Duration: 6 min 30 s
Channel: Jake Rempel
Description: Which carbocation in each pair is more stable?

Key Concepts

Inductive Effects

Inductive effects describe the electron-donating or electron-withdrawing influence exerted by substituents through sigma bonds. Electron-donating groups can help stabilize a carbocation by pushing electron density towards the positively charged center, whereas electron-withdrawing groups can have a destabilizing effect by pulling electron density away.

Hyperconjugation

Hyperconjugation involves the interaction of filled ?-bond orbitals (typically from C–H or C–C bonds adjacent to the positively charged carbon) with the vacant p orbital of the carbocation. This electron donation through overlap helps to disperse the positive charge, thereby enhancing the stability of the carbocation.

Carbocation Stability

Carbocation stability refers to how readily a positively charged carbon species can exist based on its ability to delocalize or mitigate the positive charge. Factors influencing this stability include the number and type of substituents attached to the charged carbon, which can stabilize the cation through electronic effects such as resonance or hyperconjugation.

Resonance Stabilization

Resonance stabilization occurs when the positive charge of a carbocation can be delocalized over multiple atoms through the overlap of p orbitals or adjacent ? bonds. This delocalization distributes the charge over a larger part of the molecule, significantly lowering the energy of the carbocation and increasing its stability.

Transcript

00:01 Let's start by drawing a hexane ring.

00:03 In a, we have a ring with a double bond here, and then a bond over to a carbon, and then another bond over that way.

00:17 This carbon has a positive charge on it, and we are considering this compound or an alternative that looks very similar with one key difference.

00:29 And that difference is that we've moved the double bond down here.

00:34 And we have, again, both those carbons with a positive charge over here.

00:40 So to consider which of these is going to be more stable, let's go ahead and draw any resonant structures that are possible.

00:48 So in the first compound, we have a resonant structure that will form when we bump these electrons over this way and form a double bond over here.

00:59 And that will give us a move of the positive and a new compound that will look like this, where we have a positive down here.

01:13 And in this case, we actually don't have any resonant structures possible because of this sp3 carbon right here.

01:22 So as a result, we can say that the first compound, this guy right here, is going to be, more stable, and that's due to resonance that we can form.

01:38 All right, let's go on and draw our compounds that we're working with in b.

01:43 In b, we have two compounds that are very similar.

01:49 They both have carbon on each side, and then an oxygen hanging out in the middle.

01:56 So the first one, we have a positive charge sitting on this carbon down here.

02:01 And in the second one, we have a positive charge, again sitting on that carbon, but instead of an oxygen, we have a nitrogen with a hydrogen bonded to it.

02:13 And then again, that positive charge down here.

02:17 All right.

02:17 So let's go ahead and add the loan pairs on the oxygen and the nitrogen, just so we can get a better look at what we're working with.

02:24 So in both of these cases, we can bring a lone pair down and form a double bond.

02:31 Like so, and that's going to give us resonant structures that are going to look like this.

02:37 We have our oxygen double bonded off to a carbon and the oxygen holding a positive charge with a single loan pair of electrons on it.

02:47 Or the alternative where we have our nitrogen, still with its hydrogen, no lone pair this time, and a double bond again off to that carbon and a positive charge on that nitrogen.

03:01 So in this case, we need to consider which of these is going to be happier with that positive charge.

03:08 Does an oxygen prefer a positive charge or does nitrogen prefer the positive charge? the answer there is that nitrogen prefers that charge because remembering our order of electronegativity, we have fluorine as being more electronegative than oxygen, which is more electronegative than nitrogen...

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