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Next Steps:. Resonance. Formal Charge. When atoms do not exhibit ‘normal’ bonding patterns, they will contain a ‘formal charge’. Formal Charge does not indicate an actual ionic charge – it indicates the distribution of electrons. Dimethyl Sulfoxide (DMSO). - PowerPoint PPT Presentation


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Next Steps:ResonanceFormal ChargeWhen atoms do not exhibit normal bonding patterns, they will contain a formal charge.Formal Charge does not indicate an actual ionic charge it indicates the distribution of electronsDimethyl Sulfoxide (DMSO)

Normally, Sulfur owns 6 valence electrons, but in this structure, it only owns 5Therefore, Sulfur has formally lost 1 electron and has a + chargeLikewise, Oxygen normally owns 6 valence electrons in this structure it owns 7, so it has a formal - chargeCalculating Formal ChargeFC = #valence e- - [(1/2 bonded e-) + nonbonding e-]

Easier Calculation:FC = #valence e- - bonds dots

You Try It: Calculate any fcs for nonhydrogen atoms

H3C-CN-ONote:From now on, lone pairs or formal charges must be shown when needed. You may show both, but it is not necessary.Atoms that exhibit normal bonding patterns may assumed to have a formal charge of zero

Read pages 10-19 & try problems

ResonanceThis is why we study formal charge:Consider Nitromethane:

Nitromethane EPMExperiments show that each N-O bond is equivalent. Examine electron distribution:

Why?The true structure is a resonance hybrid. The electrons are distributed evenly with both oxygen atoms bearing equal negative charge.

Remember:Resonance structures are not real. They only help us to envision electron distribution. Only by knowing the contributing structures can we envision the real structure.


2 Major Rules for ResonanceNever break a single bond

Never exceed an octet for 2nd row elements

For more practice see handout problems 2.2 2.12 pgs 26-27

Drawing Arrows to Show Movement of Electrons: Pushing ElectronsWhere the electrons come fromWhere the electrons are moving toExample:

You try itDraw arrows that show how one structure becomes the other through resonance:

More problems: pg 29; 2.14 2.19Patterns for Drawing Resonance Structures:Lone pair next to pi bondLone pair next to a positive chargePi bond next to a positive chargePi bond between two atom where one of those is electronegativePi bonds going all the way around a ring

Pi bond next to a free radical1. Lone Pair Next to a Pi BondNext to a lone pair is separated from a pi bond by exactly one single bond

2. Lone pair next to + chargeRemember a + charge means that there is less electron density than usual, so there is an empty orbital available


3. Pi Bond next to + charge+4. Pi Bond between two atoms where one is electronegativeAn electronegative atom can support an additional pair of electrons and a formal negative charge

5. Pi bonds going all the way around a ring

PhenanthreneHow many resonance structures for this example?

6. Pi bond next to free radicalWhat is a free radical?radical - (free radical) a neutral substance that contains a single, unpaired electron in one of its orbitals, denoted by a dot () leaving it with an odd number of electrons.Radicals are highly reactive and unstableRadicals can form from stable molecules and can also react with each other.

Showing resonance of free radicalsUse half-arrows to represent the movement of single electrons

You try itShow all of the resonance forms for the following structure:

A look at PyridineThe lone pair on the nitrogen does not participate in resonance due to its position in an sp2 hybrid orbital

Draw All Resonance Structures for Pyridine

Significant Resonance StructuresNot all resonance structures are significant. Three rules help us choose structures that are significant1. Minimize Charges2. Electronegative atoms can bear positive charge only if they have a full octet3. Avoid resonance structures in which two carbon atoms bear opposite charges1. Minimize Charges2. Electronegative Atoms & positive charge3. Avoid Carbons with opposite charges