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Topic E

Introduction to Resonance

1. Applying Electronegativity Differences To Understand Electron Densities In Molecules With Pi Bonds – Simple Examples

An analysis of electronegativities can give an accurate idea of where the electron densities are in pi bonds
*think the dipole arrows that you draw on molecules
this model can be used to find the reactive sites in molecules
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2.Two Cases Where “Applying Electronegativity Differences” Fails To Accurately Predict Electron Density

with the information above we would assume that the most electroneg. atom would act as the Nu. and the least electroneg. atom would act as the E., but this is not always true due to resonance!
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4.In These Two Cases The “True” Electron Densities Are Reflected By A “Hybrid” of Two Resonance Structures

resonance: when we can draw two or more forms of the same molecule that differ only in the placement of electrons
the true structures of the molecules above is a hybrid of two resonance forms(shown by a double headed arrow)
This means that the formal charge(ex acetate ion) is distributed evenly between the two oxygens
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How to Use Curved Arrows To Interchange Resonance Forms

1. What Bonds Formed, What Bonds Broke?

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While both atoms on the right and left contain all the same atoms, the electrons have been moved around
NOTE: there are two places to find electron pairs, in bonds or as lone pairs

2. Introducing Curved Arrows, A Tool For Showing The Movement Of Electrons Between Resonance Structures

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3. Every Resonance Form For A Molecule Can Be “Found” Through The Application Of Three Electron-Pushing Arrow “Moves”


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4. Some Common “Dumb” Questions About Curved Arrows (That Are Not Dumb)

Q: Are we allowed to give an atom more than 8 electrons?
A: No(not w C,N,O,F)

Evaluating Resonance Forms (1) – The Rule of Least Charges

1. The First Rule Of Evaluating Resonance Structures: Minimize Charges

the most important rule when evaluating resonance forms is: the resonance form with the least number of charges will be the most stable and common “resonance hybrid”

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2.Evaluating The Resonance Structures Of Ethene(“Ethylene”)

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The net charge of both structures is equal(zero), so there is no point in moving the electrons because it will not reveal anything about the molecule’s electronegativity.

3.Evaluating The Resonance Structures Of The Allyl Cation

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How To Find The Best Resonance Structure By Applying Electronegativity

1.Always Break A Pi Bond So As To Put The Electrons On The More Electronegative Atom

what about when you have a pi bond between two dissimilar atoms?

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if we must make a charged resonance form, it makes sense to put the negative charge on the atom that will stabilize it the easiest( aka the most electroneg. atom)
NOTE: if you look at the five key factors that influence acidity, one of the most important is the electronegativity of an atom

2.Evaluating The Resonance Structures Of Acetone (Propanone)

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the best resonance form is the neutral form with full octets
the second-best resonance form is the one with a negative charge on oxygen (and less than a full octet on carbon), and
the resonance form where there is a negative charge on carbon and less than a full octet on oxygen is insignificant.

3.Evaluating The Resonance Structures Of An Imine

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4.Evaluating The Resonance Structures Of The Acetate Ion ​
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Drawing Resonance Structures: 3 Common Mistakes To Avoid

Mistake #1: Unbalanced Resonance Equations

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the two resonance forms can neither differ in number of electrons or atoms

Mistake #2 :Moving Atoms Around

it is not a resonance form if single bonds(opposed to pi bonds) have been broken and thus moved the location of one or several atoms. This would create a constitutional isomer
NOTE: DONT MOVE HYDROGENS

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Mistake #3 : Incorrectly Drawing Curved Arrows

A) Breaking the Octet Rule:
do not give atoms more bonds than they can accept. This would be inconcievable and therefore not a resonance contributor.

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A- Carbon cant have 5 bonds
B- Carbon cant have 5 bonds
C-Carbon cant have 5 Bonds
D- Nitrogen can have 4 bonds but cant have it in this example because of the lone pair
E- You cant move electrons from a cation because cations are electron deficient
F-Again you cant move electrons from a cation because cations are electron deficient
B) Missing a Curved Arrow :
some illegal moves can be amended through adding an arrow. Try these:

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A- [o-c pi bond]→ [O atom]
B- *push all alkenes one bond clockwise
correct answer: [c-c pi bond] → [C atom]
note: I think both would work in this case, but the first would result in another major resonance contributor while the second would give a minor resonance contributor
C-[c-n pi bond]→[N atom]
D- [c-c pi bond] → [C atom]
E-[c-c pi bond] → [C atom]

How to apply electronegativity and resonance to understand reactivity

1.Applying Electronegativities To Understand The Electron Density Of A Molecule

The dipoles of an atom will give you clues about the electron densities of the two atoms.
NOTE: this is different than formal charge

2. Applying Resonance To Understand The Electron Density Of A Molecule

when you know the most stable resonance forms, you will have a good idea of what a resonance hybrid looks like. This resonance hybrid will tell you electron densities which gives clues to the reactivity of a molecule.

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3.Some Examples Of Resonance Hybrids

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4.Applying These Two Factors To Figure Out Plausible Chemical Reactivity: Areas Of High Electron Density Will Seek Out Areas Of Low Electron Density

Now that we know the partial charges on a molecule, we can use this information to figure out plausible chemical reactivity
“Opposite charges attract, like charges repel”
so any region of negative charge on a molecule will have some attraction to a region of positive charge on another molecule
electrons flow from areas of high electron density to low electron density
the partial negative charge will go to a region of partial positive charge
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