01 Guide to drawing resonance structures PDF

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Guide to use curved arrows for drawing resonance structures Introduction: This set of supplementary lecture notes is designed to help you build the basic skill of using electron pushing to draw resonance structures. Please note that these notes will not be covered in class. You are expected to read and do exercise yourselves. Should you need help, please don’t hesitate to ask. Principles of resonance    

Sometimes a single Lewis structure does not describe an organic compound accurately Resonance method attempts to describe the structure by drawing resonance structures Resonance structures are not real. There is only one structure for the actual molecule which is a hybrid of all the resonance structures All resonance structures contribute to the stability of the hybrid structure. If the resonance structures are not identical, they contribute differently to the stability of the hybrid.

Rules for drawing resonance structures 1. The relative position of the atoms in all resonance structures must be the same 2. The formal charge is conserved: The sum of formal charge on each resonance structure must be the same 3. The number of electrons is conserved: Each resonance structure must have the same number of electron pairs 4. Octet rule must be observed: Although it is possible to have less than 8 electrons in an atom, it must not have more than 8. Therefore, no pentavalent carbon is allowed. You have to be very careful especially if you use angle-line formulas. This is because pentavalent carbon can be generated without noticing. For example:

This is because what really happens is: Pentavalent carbon

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How to draw resonance structures Curved arrow or “electron pushing” depicts the movement of an electron-pair (i.e. 2 electrons). = Movement of a pair of electron A curved arrow

When you draw resonance structures you have to decide on two things: 1 2

Which pair of electron to be moved: Identify the pushable electron To where should the electron pair be moved: The receptor for the electron being pushed.

Here are the principles for drawing resonance structures: 1. Pushable electrons can be the unshared electron pairs on a single atom 2. Electron pairs in multiple bonds (pi bond) are also pushable

3. Never push single bonds (sigma bond) 4. Receptor of electron pairs can be: a. Atoms with positive charge b. Atoms that can tolerate negative charge. For example, an atom with octet does not tolerate negative charge c. Atoms possess unshared electron pairs and / or electronegative

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Drawing resonance structures It is very important for you to know which electron pairs are pushed and to where it goes when you draw resonance structures. Here are a few different scenarios. When there is a positive charge in the molecule. Remember: Always push electron pairs towards positive charge. Electron pair from pi bond is pushed towards the positive charge Please also note that electron number and formal charge are conserved

It is crucial that the curved arrow indicates precisely what electrons are being pushed (where it starts) and where it goes. In this case, the curved arrow indicates the pair of pi electrons between the C=C bond is pushed to form another C=C pi bond between carbon 1 and 2. If there is another double bond conjugated to the positive charge, another resonance structure is possible as:

Please be careful to do it step by step (i.e. pushing electron pair one at a time). Very often, we may be tempted to do it in one single step as the following shows:

As can be seen, one resonance structure is missing if you jump step.

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Sometimes, the pushable electron can be lone pair electron as the following shows:

Alternatively, it can be a double bond which is adjacent to the positive formal charge as:

Unshared electron pair from oxygen is pushed to form a double bond with carbon. The positive formal charge is now on oxygen. Formal charge is therefore conserved

Key to this exercise will be provided in due course

Exercise:

Draw resonance structure for the following using curved arrows. Please note that in some cases, no additional resonance structures can be generated. Just write none if that happens. (Be very careful to show where the curved arrow starts and where it ends)

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H H 3C

C

C

O

CH3

CH3 H 4

More than one resonance structure can be drawn

When there is a negative charge in the molecule. Remember: Always push electron pairs away from the atom bearing negative charge. The electron pair should be pushed to form a pi bond with the next atom as the following shows: A pentavalent carbon is generated!

As can be seen, forming a pi bond with the adjacent carbon would generate a pentavalent. To solve the problem, the original pi bond needs to be pushed towards the oxygen as:

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Exercise: Draw resonance structure for the following using curved arrows. Please note that in some cases, no additional resonance structures can be generated. Just write none if that happens. Also, beware that more than one resonance structures can be generated in some cases. (Be very careful to show where the curved arrow starts and where it ends)

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When it is a benzene or an aromatic compound When there are three pi bonds in an aromatic ring, it is possible to draw another resonance structure by pushing all the pi electrons as:

Draw two resonance structures for the following aromatic compound by using curved arrows:

Exercise: Draw resonance structures for the following using curved arrows. Remember to push electron pair step by step. (Be very careful to show where the curved arrow starts and where it ends) 15)

O

You can expect to have three more resonance structures for this anion. Hint: Move negative charge on oxygen. Don’t draw resonance structure within the aromatic ring.

You can expect to have three more resonance structures for this cation. Don’t draw resonance structure within the aromatic ring.

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17) H H2N Br

You can expect to have three more resonance structures for this cation.

You can expect to have three more resonance structures for this cation.

You can expect to have two more resonance structures for this anion. Ignore resonance structure within the aromatic ring.

You can expect to have three more resonance structures for this anion.

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