Draw all resonance structures of the following carbanion and c...

Question

Draw all resonance structures of the following carbanion and carbocation.

(a)

(b) Chemical structure of a carbocation with a positive charge, illustrating resonance structures for educational purposes.

Accepted Answer

Alright. Hi, everyone. So first question, let's consider the following. And here we're given one carbo cat ion and one carbon ion. The question is asking us to provide all the resonance structures of the given carbo cion and carbon ion which I've gone ahead and redrawn on the bottom here. So before we begin talking about the answer to this question, let's just go ahead and recall a few things about resonance and resonance structures in general, right? So recall that resonance structures are representations of the same molecule that differ only in the movement of electrons, right. So the structure, the base structure of a molecule stays the exact same. But the only thing that moves is electrons, right? So either lone pairs or pie bonds. Now here, when we're talking about illustrating a resonance structure, we use curved arrows to indicate the movement of electrons, right? And specifically this double headed arrow indicates that there are two electrons moving around at once. So here it's important to acknowledge that when we're talking about the movement of electrons, electrons are inclined to move from an area of higher density tune area of lower density, right. So for both of these scenarios we have to consider which region has the higher electron density and which region has the lower electron density and then act accordingly. Right? So let's start with our carbo cion. And what you'll notice about a carbo cion is that there is of course a region that has a positive charge, right, that indicates electron deficiency, right? This particular carbon is electron deficient and therefore it is the region of lower electron density. So our carbo cion here is the area of the lower electron density. And on the other hand, right, we do have a P bo close to that carbo c. So because the pi bond has more electrons than a regular signal bond, our pi bond here are all keen is the area of the higher electron density. So what that means for the sake of a resonance structure is that the pi electrons in our structure here in between carbons three and four to be exact, they are going to move towards the electron deficient carbo CIA. So let's go ahead and illustrate that. So here our pi electrons are going to shift one sigma bond over and that's going to form another carbo cion just in a different position, right? Because notice how in moving the position of the pi bond we've generated another carbo cion where the pi bond used to be. So let me illustrate what I mean by that. Yeah. Right. So here what you'll notice is that there is now a double bond where the carbo cion used to be. But during that process, we've left another carbon electron deficient, right. We've left this tertiary carbon here that used to be part of the alkene electron deficient instead, but that's slightly more favorable, right? Because recall that tertiary carbo C ions are more stable than secondary ones. So that's OK. And now let's talk about the car ban aisle because in the carbon ion, right, we have a pi bond but we also have a carbon that has a loan pair of electrons, right? Because what you'll notice is that we have an all keen in between carbons two and three here, but the lone pair of electrons sits at carbon four now because there's a lone pair, carbon four has a formal charge of negative one, right. So that indicates that the lone pair actually is the region of the higher electron density. And by contrast, the pi bond therefore, is the region of lower electron density. So because of that for the resonance structure of a carbon ion, like the one that we have here, it's going to be the lone pair of electrons that moves around, therefore, displacing the pie bo, right. So what you'll see here is that the loan pair of electrons on carbon four is going to jump in between carbons three and four. And during that process, the pi bond between carbons two and three is instead going to jump towards carbon two because notice what would happen if we end up with two pythons on carbon three, right? That would violate the octet because then carbon three would have too many bonds. So it's best to avoid that. And now let's go ahead and you are the product of this process. We still have a carbon ion, right? But notice how the positioning has shifted a little bit because now the carbon ion is on position two and the pi bond is in between carbon three and four. So these two structures right here are going to be your final answers, right. So the important takeaway here is that electrons in a resonance structure move from an area of higher electron density to an area of lower electron density. So because both carbo C ions and carbon ions have different distributions of electron density, then the resonance structures look a little bit different. But the principle is the same. So with that being said, if you stuck around to the end, thank you so much for watching. And I hope you found this helpful.

Draw all resonance structures of the following carbanion and carbocation.
(a)
(b)

Key Concepts

Resonance Structures

Carbanions

Carbocations

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