Draw resonance structures for the following species: (FIGURE...

Key Concepts

Resonance Structures

Resonance structures are different Lewis structures for a molecule in which the connectivity of the atoms remains the same, but the distribution of electrons differs. They are used to represent the delocalization of electrons within molecules, especially in systems where multiple valid Lewis structures can be drawn. Each individual resonance form does not represent the actual molecule on its own but contributes to the overall electronic structure.

Resonance Hybrid

The resonance hybrid is the true representation of a molecule's structure and is a weighted average of all the possible resonance forms. It reflects the idea that the electrons are delocalized over the entire system rather than being confined to a single structure, resulting in bonds that have characteristics intermediate between those depicted in the individual resonance forms.

Electron Delocalization

Electron delocalization refers to the phenomenon where electrons, particularly those in ? bonds or lone pairs, are spread out or shared over several atoms instead of being localized between a specific pair of atoms. This delocalization can lead to increased stability of the molecule and influences its reactivity and physical properties.

Formal Charge

Formal charge is a calculation used in drawing Lewis structures that helps to determine the most stable resonance forms. By assigning electrons in bonds and lone pairs to atoms, chemists can determine where extra negative or positive charges are likely to reside in a molecule. Often, the most stable resonance structures are those in which the formal charges are minimized or placed on atoms best able to accommodate them.

Electron Pushing Mechanism (Curved Arrow Notation)

The electron pushing mechanism, commonly represented by curved arrows, is a tool used to indicate the movement of electrons when interconverting between resonance structures. It helps in visualizing the shifts in electron density and understanding how the delocalization occurs across the molecule. This notation is crucial for keeping track of electron pairs and ensuring the conservation of charge and electron count in theoretical transformations.

Transcript

00:01 Okay, this problem is asking us to draw out resonance forms from the compounds that were given.

00:05 So the first one that was given was this one.

00:07 It was an eight carbon compound with three double bonds and a positive charge on the rightmost carbon.

00:14 So what are resonance forms? essentially, they replace positive charges by relocating them across the molecule.

00:21 So this carbon has a positive charge.

00:24 We do not want a positive charge on that carbon.

00:26 We want to move it around so that it can go onto another carbon if possible.

00:30 So if i were to move the electrons from this double bond and put it onto this single bond, i will have created a new double bond.

00:36 By creating a new double bond, i will have relieved this carbon of its positive charge, and i will have relocated that positive charge and put it onto this carbon.

00:45 Okay, so let's clean that up and draw the effect.

00:50 I'll be using a double -headed arrow because double -headed arrows mean that i'm going to end up with the same molecule, but just a different form.

00:56 So i have my new alkyne, i have a new positive charge because i relocated.

01:01 It and i have my unaffected double bonds.

01:05 Okay, so next up, let's see what would happen if i were to relieve this carbon of its positive charge using the electrons from this double bond.

01:13 If i do that, i will end up with an alkyne here and a positive charge here.

01:17 That's back to where i started, so i don't want to use the electrons from that double bond.

01:21 Instead, i want to use the electrons from this double bond.

01:25 By doing that, i will have created an alkyen here, relieve this carbon of its positive charge, and made a new positive charge on that carbon.

01:32 Okay, so let's draw the effect of that.

01:35 Double -headed arrow, signifying a new form, a carbon compound, unaffected alken, unaffected alken, new al -keen, and positive charge on this carbon.

01:48 Okay, in orange, what would happen if i relieve this carbon of its positive charge using the electrons from this double bond? i'll be back to where i started.

01:56 So not those electrons, but the electrons next over.

01:59 So using these electrons, i'm going to place them onto that single bond.

02:03 Creating a double bond relieving this of its positive charge and creating a new positive charge on that carbon okay so the effect of that would be as such new double bond new double bond and unaffected double bond as well so i have my new positive charge on this carbon okay what would happen if i were to relieve that positive charge using the electrons from my carbon hydrogen bonds so this carbon has three hydrogens attached to it each of those has electrons attached to it but i do not have a base.

02:39 If i had a base, then i could deprotate one of those hydrogens and relieve this positive charge, making it an alken.

02:44 But in this problem, i'm not given any bases.

02:46 I'm just asked to draw the residence forms.

02:48 So if i were to be asked a problem like that, then i could relieve this positive charge even further, but i cannot.

02:54 So i must use the electrons from what i'm given.

02:58 What i'm given is a double bond right next to it.

03:01 But if i were to use those double bonds, those electrons from that double bond, i'll be back to where i started, so i know that i'm done.

03:07 I'm done drawing my resonance forms.

03:11 So from my starting material, i have one, two, three new resonance forms.

03:16 Okay, next one.

03:19 Here i have cyclopentane.

03:22 I have lone pairs attached to it on the rightmost carbon, and i have double bonds attached to it as well.

03:28 Okay, so a resonance form is moving positive charges around, and it is also moving negative charges around.

03:35 So let's try moving these.

03:37 Loan pairs around.

03:39 I'm going to move them in a clockwise direction.

03:44 So let's move them onto the single bond.

03:47 If i move them onto that single bond, i will have created a double bond.

03:50 If i created a double bond there, and this carbon has a hydrogen attached to it, and another set of another alken, then i know i have exceeded my octet for this carbon.

03:59 So i have to move electrons around to relieve that carbon of its exceeded octet.

04:05 So i'm going to move the electrons onto this carbon.

04:10 If i do that, then i'll end up with this cyclopentane with a new double bond, new set of lone pairs, and my unaffected double bond.

04:26 Okay, so once again, i need to move those electrons around, so i'm going to move them onto this single bond.

04:33 Moving them onto that single bond, and with the presence of this hydrogen, this carbon has exceeded its octet, so i need to relieve that of its exceeded octet.

04:40 So i'm going to move those onto this carbon...

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