Wednesday, May 18, 2022

# How To Draw A Resonance Structure

## Lewis Structure For Resonance Form Of Clo2

Resonance Structures, Basic Introduction – How To Draw The Resonance Hybrid, Chemistry

Need to draw a lewis structure for resonance form of ClO2- showing the lowest possible formal charges / give oxidation numbers of atoms

so for oxidation numbers I got :

O = -2

also, I THINK this is the structure not 100% sure

O=Cl–O^-

with 4 dots on first O, 4 dots on Cl, and 5 dots on the last O ?

## Using Formal Charges To Identify Viable Resonance Structures

While each resonance structure contributes to the total electronic structure of the molecule, they may not contribute equally. Assigning Formal charges to atoms in the molecules is one mechanism to identify the viability of a resonance structure and determine its relative magnitude among other structures. The formal charge on an atom in a covalent species is the net charge the atom would bear if the electrons in all the bonds to the atom were equally shared. Alternatively the formal charge on an atom in a covalent species is the net charge the atom would bear if all bonds to the atom were nonpolar covalent bonds. To determine the formal charge on a given atom in a covalent species, use the following formula:

Rules for estimating stability of resonance structures

• The greater the number of covalent bonds, the greater the stability since more atoms will have complete octets
• The structure with the leastnumber of formal charges is more stable
• The structure with the least is more stable
• A structure with a negative charge on the more electronegative atom will be more stable
• Positive charges on the least electronegative atom is more stable
• Resonance forms that are equivalent have no difference in stability and contribute equally
• Example \: Thiocyanate Ion

Consider the thiocyanate ) ion.

Solution

1. Find the Lewis Structure of the molecule.

2. Resonance: All elements want an octet, and we can do that in multiple ways by moving the terminal atom’s electrons around .

Strategy:

## Curved Arrows In Organic Reaction Mechanisms

Curved arrows are a formal notation to help us understand the electron flow in organic reactions. This makes it easier to keep track of the bonds forming and breaking during the reaction as well as visualizing and explain more advanced features such as the region and stereochemistry of certain reactions.

The electrons always flow from a high electron density region to a low electron density region. The molecules with a high electron density are nucleophiles i.e. love nucleus. A molecule with a low electron density is classified as an electrophile i.e. loves electrons.

The following is a nucleophilic addition reaction which is a very important class of organic reactions:

The arrow starting from the lone pair on the sulfur and pointing to the positively charged carbon makes a new covalent bond between them by a nucleophilic attack.

Another common important class of reactions that we can consider for learning the curved arrows is the acid-base reactions:

For example:

Here, the hydroxide ion is the base and it attacks the proton connected to the carbon. So, this curved arrow shows a bond-forming between the oxygen and the hydrogen. The second arrow indicates breaking the bond between the hydrogen and the nitrogen as otherwise, the hydrogen would have left with two bonds which is not possible.

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## Rule #: Place Negative Charges On The Atom Best Able To Stabilize It

Given that neutral resonance structures are preferred overall, when a resonance structure absolutely must bear a negative charge somewhere, place it on the atom best able to stabilize that charge. Since, in essence, basicity is the opposite of stability, this is the same as saying, put the negative charge on the least basic atom.

The good news here is that if you understand the factors that affect acidity, you also understand the factors which stabilize negative charge.

There are four main trends to consider here:

• Electronegativity: across a row of the periodic table, negative charge becomes more stable as electronegativity is increased.
• Polarizability: down a column of the periodic table, negative charge becomes more stable as polarizability increases
• Electron withdrawing groups stabilize negative charge through inductive effects.
• Hybridization: negative charge becomes more stable as the s-character of the atom is increased. sp > sp2> sp3 (least stable

Note again that stability is the opposite of basicity.

## Resonance Structures Of Carbonate Ion

How to draw resonance structures

Carbonate ion is a common polyatomic ion found in limestone, baking powder and baking soda. Addition of acid to the carbonate ion causes the formation of carbonic acid, which decomposes rapidly into water and carbon dioxide. In baking the carbon dioxide that is released causes bread to rise and makes its texture lighter.

Our first attempt at drawing the lewis dot structure of the carbonate ion results in the structure shown below. Carbonate has 24 electrons, 2 of them responsible for the -2 charge, probably electrons from calcium, sodium or whatever salt resulted in a cation that donated electrons to the carbonate anion. The carbon atom in pure structure still needs to share electrons to satisfy the octet rule. Which atom is most likely involved in sharing electrons.

Using the formal charges on the atoms, we could reconfigure our electrons to participate in a double bond with the carbon. At this point the positive charge on the carbon atom is gone and all the valence is filled the octet rule is satisfied. The sum of the formal charges is equivalent to the charge on the carbonate ion. This is a good Lewis dot structure for carbonate.

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## Rules To Draw Resonance Structures

• Locations of nuclei of atoms should not be changed with structures. Structures are changed only with their electrons configuration.
• Number of total electron pairs should be same in every structure. Total electron pairs can be simplified as bonds and lone pairs. Otherwise we can say, number of electron pairs = bonds + bonds + lone pairs.If unpaired electrons exist, they also should be same number in every molecules. .
• Energy of different structures should be same or close.
• Higher electronegative atoms have more chance to hold negative charges to obtain more stability to the molecule or ion.As an example, when sulfur and oxygen is in the molecule, oxygen has the higher ability to hold the negative charge.
• ## Resonance Structures Of O3

The ozone molecule consists of a central oxygen atom which is singly bonded to one oxygen atom and doubly bonded to another. There is no net charge on this molecule, but the Lewis structures of this molecule show a +1 charge on the central oxygen and a -1 charge on the singly bonded oxygen. The two resonance structures of the ozone molecule are illustrated below.

The resonance hybrid of ozone has a +1 charge associated with the oxygen at the centre and a partial charge of – associated with the other oxygen atoms.

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## Drawing Resonance Structures: 3 Common Mistakes To Avoid

How To Draw Resonance Structures: Avoiding Common Mistakes

No discussion of resonance structures would be complete without mention of how to royally screw them up. This isnt something to feel bad about, by the way: there isnt a chemist alive who hasnt made one of these mistakes at some point. Think of it as a rite of passage. The trick is to make the mistakes while doing problems, not while doing an exam.

There are at least three common categories of mistakes regarding resonance structures:

• Unbalanced equations
• Incorrect drawing of resonance arrows

Lets deal them each in turn.

• How To Avoid Making Mistakes Drawing Resonance Structures
• ## Drawing A Resonance Structure:

How to draw resonance structures (1)
• Is there a ring with alternating double bonds? If so, move the double bonds around in a circle so that they are in the alternate positions.
• N CH 3

• Look for Y=Z, YZ, or =Y=Z. In these cases, push the lone pair on X to make an additional bond between X and Y, and break one of the bonds between Y and Z and make it a new lone pair on Z. The X atom increases its formal charge by 1 and the Z atom decreases its formal charge by 1.
• H O O

• Look for Y=Z, YZ, or =Y=Z. In these cases, use the single-headed or fishhook electron-flow arrow to show the movement of single electrons. Push the unshared electron on X to start an additional bond between X and Y, take one electron from a bond between Y and Z and use it to complete the additional bond between X and Y, and take the other electron from the bond between Y and Z and make it a new unshared electron on Z. There are no changes in formal charges.

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## Common Resonance Structure Mistake #: Drawing Unbalanced Resonance Equations

Lets first talk about unbalanced resonance equations, where something has been added or subtracted. Remember that in drawing resonance forms were only allowed to move electrons, and nothing more. That means that the two resonance forms can neither differ in the number of their electrons nor can they differ in the number of atoms.

## How Do You Find A Molecules Resonance Structures

When drawing resonance structures, here are a few things you want to keep in mind:

• Only move the electrons .
• This includes lone pair electrons and pi bonds
• Never break a single bond .
• Work in one region at a time.
• This means focus on the three atoms closest together and make your way down the molecule.
• You will want to begin in the electron-dense regions.
• This means you will be pushing the electrons from the negative charge to the positive.
• Typically, you will be turning lone pairs into bonds and bonds into lone pairs.
• Follow the and try to stay away from large formal charges.
• #### Example: How would you draw the resonance structures of Ozone ?

##### What is Ozone?

First, lets take a look at ozone. You have probably heard of the ozone layer in the earths stratosphere. The reason it is named the ozone layer is because of its high concentration of ozone, which absorbs the majority of the suns ultraviolet radiation. In chemistry, it is one of the most powerful oxidizing agents as it has a large standard reduction potential.

##### Drawing the Lewis Structure of Ozone

To find the resonance structure of ozone, we will draw the lewis structure of ozone. Since the molecular formula is O3, we know there are 18 valence electrons . After placing all the electrons, we will have a double bond and a single bond.

##### Drawing the Resonance Structure

Step 1: Transfer the electrons on the oxygen with the negative charge and turn it into a double bond.

## Rule #: Full Octets Are Preferable To Empty Octets

Resonance forms where all atoms have full octets will be more significant than resonance forms where atom lack a full octet. Importantly, its a good general rulenever to place less than a full octet on nitrogen or oxygen, as in the acetone example . Since these atoms are highly electronegative, these resonance forms are extremely unstable and will be insignificant.

## Not All Resonance Forms Are Of Equal Significance So How Do We Evaluate How Important Each One Is

How To Draw The Lewis Structure of NO3- (Nitrate Ion)

In the case of the acetate ion and the allyl cation, both resonance forms are equal in energy, so the hybrid is a 1:1 mixture of the two. However, this is only rarely the case.

Take the ketone below for which we can draw 3 different resonance forms.

In cases like these, how do we evaluate the relative importance of each resonance form?

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## Resonance Structures Of Nitrobenzene

The electron density in the aromatic ring of nitrobenzene is less than that of benzene owing to the presence of an electron withdrawing group, which has a double bond that is adjacent to the phenyl ring of nitrobenzene as illustrated by the resonance structures of nitrobenzene.

Thus, the phenyl ring of nitrobenzene is less nucleophilic than benzene. From the resonance structures that the ortho and the para positions are positive. Thus, for an electrophilic aromatic substitution reaction, the electrophile will not react at these positions, but instead at the meta position. Thus, it appears that if a double bond is in conjugation with the phenyl [ring, the electrophilic aromatic substitution product will be the meta substituted product.

## Resonance Hybrid And Movement Of Electrons

I know that I have just told you that curved arrows show the movement of electrons but I also need to tell you something that goes against this.

Even though we use the curved arrows and move the electrons around in resonance structures, you need to know that the electrons do not actually move in the sense of jumping from one atom to another as we show them in resonance structures.

In reality, the electrons are spread among the atoms and none of the resonance forms is the correct representation of the molecule.

The accurate representation of the molecule is given by the resonance hybrid.

Having the resonance forms in brackets is to indicate that they represent one entity, which is the resonance hybrid where the charge are spread over the two atoms.

You may wonder now why do we then draw resonance structures and use the curved arrows? And the answer to this is that some properties and reactions of molecules are better explained by the individual resonance structures and we use them with curved arrows to keep track of electrons and explain these properties.

Think about a hybrid fruit nectarine. It is a mix of a peach and a plum and to explain its color, texture and the taste, we refer to the individual fruits.

This, however, does not mean that the nectarine exists as peach for some time and then turns into a plum. None of them is a correct representation of the nectarine just like none of the resonance structures is the correct representation of the given molecule.

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## Rule #: Place Any Empty Octets On The Atoms Best Able To Stabilize Them

As we said above, full octets are best. However, when dealing with a resonance structure where there absolutely must be an atom with less than a full octet, then follow these principles:

• Place the empty octet on carbon, never oxygen or nitrogen
• Place place the empty octet on the most substituted carbon
• Avoid placing positive charge adjacent to electron withdrawing groups if possible
• Place positive charge preferentially on alkyl carbocations as opposed to alkenyl or alkynyl carbons.
• ## Resonance Structures Of Nitrite Ion

How to Draw the Resonance Structures for Benzene (C6H6)

Nitrogen atom has the greatest possibility to be the middle atom than oxygen atom according to theory of an atom which has a greater valence should be the middle atom. Therefore, three oxygen atoms are located around the nitrogen atom. When we draw the structure of the nitrite ion by following “VSEPR rule”. Then it is as the figure 1.a .

#### Keep octal of oxygen and nitrogen atoms

Remember that, octal of oxygen and nitrogen atoms cannot be exceeded because both oxygen and nitrogen atoms are located in the second period and have only s and p orbitals.

In the nitrite ion, there is a -1 charge. According to the rule number 4, an oxygen atoms should keep negative charges because electronegativity of oxygen is higher than nitrogen. In NO3-, there are two -1 charges on two oxygen atoms and +1 charge on nitrogen atom.

Now we have one structure. Now we try to draw more structures by changing the bonds and lone pairs keeping locations of atoms without changing .

#### Bond to lone pair, lone pair to bond

A lone pair is converted to a bond. With that, a bond should be converted to a lone pair by keeping the octal of nitrogen and oxygen atoms. So it follows rule number 2 which says number of total electron pairs should be same in every structure.

With conversion of a bond to a lone pair and lone pair to a bond, double bond becomes a single bond and single bond become double bond respectively. Study the resonance structures of NO2- ion

#### Three resonance structures

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## S Of Drawing Lewis Structure Of So32

Following steps are required to draw the SO32- lewis structure and they are explained in detail in this tutorial.

• Find total number of electrons of the valance shells of sulfur and oxygen atoms
• Total electrons pairs
• Put lone pairs on atoms
• Check the stability and minimize charges on atoms by converting lone pairs to bonds.
• Drawing correct lewis structure is important to draw resonance structures correctly

## Remember That Resonance Structures Are Not In Equilibrium With Each Other They Represent Contributions To An Overall Resonance Hybrid

Heres an important point about resonance forms. It is tempting to think that these resonance forms are in equilibrium between each other. Avoid this common mistake!

Instead, the true state of the molecule will be a hybrid of these resonance forms.

For example in the acetate and allyl cation examples below, the true structure of the molecule is represented through a 50:50 combination of the two resonance forms.

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## Common Resonance Mistake #: Moving Atoms Around

A second category of common mistake is to move atoms around. Although the two structures shown below have the same number of atoms and electrons, they are not resonance forms because we have broken single bonds and thus moved the location of one or several atoms. The easiest way to screw this up is to move hydrogens. While these molecules are related, they are actually pairs of constitutional isomers, not resonance structures.

One way to avoid making these types of mistakes is to try to interconvert the structures using curved arrows. There are only three legal arrow-pushing moves for drawing resonance structures. Double check to make sure you arent breaking the rules.

## What Is A Major Resonance Structure

How to draw resonance structures (48)

A major resonance contributor is one that has the lowest energy. We can often write more than one Lewis structure for a molecule, differing only in the positions of the electrons. Each individual structure is called a resonance contributor. The most stable structures contribute most to the resonance hybrid.

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