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Resonance Tutorial – Chemistry 233, Schemes and Mind Maps of Chemistry

Chemistry

Formal Charges in Resonance Structures​​ Resonance structures will often contain formal charges. It is very important to assign them properly. In the example ...

Typology: Schemes and Mind Maps

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Resonance(Tutorial(–(Chemistry(233(
!
Lone(Pair(Madness:!How!do!I!know!when!and!where!to!put!lone!pair!electrons?!
!
In!skeletal!structures,!it!is!common!for!lone!pair!electrons!to!be!left!off.!You!will!often!need!to!use!your!
knowledge!of!formal!charges!to!determine!how!many,!if!any,!lone!pairs!are!present.!Neutral!
compounds!are!easy!–!Just!assume!enough!lone!pair!electrons!to!give!you!an!octet.!
!
E.g.!If!you!have!a!compound!that!has!oxygen!with!3!bonds!and!a!positive!charge:!!
! Formal!Charge!=!Valence!#!-!#!bonds!-!#!non-bonding!electrons!!
!Oxygen!has!a!valence!of!6!
!Set!up!an!equation:!+1!=!6!-!3!-!?!!!!Solve!for!“?”!à!?!=!2!!!!!
!!!!!!Oxygen!has!2!non-bonding!e-!(1(lone(pair)!!
The(table(below(will(help(guide(you:(
(
Carbon!
4(bonds,(no(lone(pairs(
=Neutral(
!
3(bonds,(no(lone(pairs(
=(+1(charge(
!
3(bonds,(1(lone(pair(
=(-1(charge(
!
Nitrogen!
3(bonds,(1(lone(pair(
=(Neutral(
!
4(bonds,(no(lone(pair(
=(+1(charge(
!
!
2(bonds,(2(lone(pairs(
=(-1(charge(
!
Oxygen!
2(bonds,(2(lone(pairs(
=(neutral(
!
3(bonds,(1(lone(pair(
=(+1(charge(
!
1(bond,(3(lone(pairs(
=(-1(charge(
!
!
Practice:!Fill!in!all!lone!pair!electrons!on!each!of!the!molecules!below.!
!
!
C C
C C
C
CC
Commonly drawn
without the lone
pair. You just need
to know it's there.
=
N
NN
=
Commonly drawn with
just the negative charge.
You need to be able to
figure out how many
lone paris are present.
OCH2
O
You don't have to draw the
lone pairs, just be aware
they are there.
O=O
O=O
Again, its
common to
not draw in
the lone
pairs.
O=O
O
O
NO
NH N
O
Cl
NH
OH
OH
pf3
pf4
pf5

Partial preview of the text

Download Resonance Tutorial – Chemistry 233 and more Schemes and Mind Maps Chemistry in PDF only on Docsity!

Resonance Tutorial – Chemistry 233

Lone Pair Madness : How do I know when and where to put lone pair electrons?

In skeletal structures, it is common for lone pair electrons to be left off. You will often need to use your

knowledge of formal charges to determine how many, if any, lone pairs are present. Neutral

compounds are easy – Just assume enough lone pair electrons to give you an octet.

E.g. If you have a compound that has oxygen with 3 bonds and a positive charge:

Formal Charge = Valence # - # bonds - # non-bonding electrons

Oxygen has a valence of 6

Set up an equation: +1 = 6 - 3 -? Solve for “?” à? = 2

Oxygen has 2 non-bonding e- ( 1 lone pair )

The table below will help guide you:

Carbon 4 bonds, no lone pairs

=Neutral

3 bonds, no lone pairs

= +1 charge

3 bonds, 1 lone pair

= - 1 charge

Nitrogen 3 bonds, 1 lone pair

= Neutral

4 bonds, no lone pair

= +1 charge

2 bonds, 2 lone pairs

= - 1 charge

Oxygen 2 bonds, 2 lone pairs

= neutral

3 bonds, 1 lone pair

= +1 charge

1 bond, 3 lone pairs

= - 1 charge

Practice: Fill in all lone pair electrons on each of the molecules below.

C C

C C

C C C

Commonly drawn without the lone pair. You just need to know it's there.

N N

Commonly drawn without showing the lone pair.

N

N = N

Commonly drawn with just the negative charge. You need to be able to figure out how many lone paris are present. O CH 2

O

You don't have to draw the lone pairs, just be aware they are there.

O = O

O =^ O

Again, its common to not draw in the lone pairs.

O =^ O

O

O

N O

NH N

O

Cl

N

H

O

H

O

H

Curved Arrows and Resonance

Curved arrows show electron flow; they are used in reaction mechanisms and to show electron

movement in resonance structures.

Every curved arrow has a head and a tail.

The direction a curved arrow is drawn is very important. The tail of the arrow should be at a site of

electron density (lone pair, bond). The head of the arrow shows where those electrons are going.

Rules for Resonance Structures :

• Do not break single bonds.

• Atoms connectivity must not be changed.

• Never exceed an octet for 2nd^ row elements.

• It’s okay to have less than an octet of electrons on second row elements.

• Only electrons from π-bonds and non-bonding electrons (lone pair, radical) can be moved.

Examples of Resonance Violations:

In skeletal structures, it is common for lone pair electrons to be left off. You will often need to use your

knowledge of formal charges to determine how many, if any, lone pairs are present. Neutral

compounds are easy – Just assume enough lone pair electrons to give you an octet.

Practice : For each of the structures below indicate whether or not the curved arrow violates any rules

of resonance.

Not resonance structures! Can't break the single bond, because that changes the atom connecticity. NH H H

NH

H H

Not resonance structures! This violates the octet rule. Here you are exceeding the octet of the carbon (it has 5 bonds). N H H

O OH O

H

O

H

O

C

N

H 3 C N N O^

CH 3

N

tail head

Major Patterns for Resonance Structures

1. Lone Pair Next to a π-bond (allylic lone pair)

• This system requires two curved arrows to show the resonance structure.

• If the atom with the lone pair bears a negative charge, it transfers the negative charge to the

atom that ultimately receives the lone pair.

• If the atom with the lone pair does not have a negative charge, then it will obtain a positive

charge in its resonance structure, while the atom that receives the lone pair will obtain a

negative charge

2. Positive Charge Next to a π-bond (allylic cation)

• This system requires only one curved arrow.

• The atom with the positive charge gets a new π-bond, while the atom that previously had the π-

bond obtains the positive charge.

3. A Lone Pair Next to a Positive Charge

• This system requires only one curved arrow.

• The atom next to the positive charge may have a negative charge. In this case the resonance

structure ends up neutral.

• The atom next to the positive charge may be neutral. In this case the atom with the lone pair

ends up with a positive charge in the resonance structure.

4. A π-Bond Between Two Atoms With Different Electronegativities

• This is basically the reverse of pattern 3.

• This system requires only one curved arrow.

O

O

O

O

O O

O O

O O

O O

Practice: Draw resonance structures for each of the following. In some cases you will be able to draw

multiple resonance structures.

Are all Resonance Structures Equivalent?

NO. Remember that individual resonance structures are not true structures. The true structure of a

molecule is a resonance hybrid. The individual resonance structures that make up a hybrid are not

always equivalent. In many cases, the resonance hybrid looks much more like one resonance structure

than another.

General Rules:

1. Resonance structures that minimize charges are more major contributiors to the hybrid.

2. Resonance structures where all atoms have an octet are more major contributors.

3. Resonance structures that put the negative charge on a more electronegative atom are more major

contributors to the resonance hybrid.

Practice: Draw all of the resonance structures for the compound shown below. Circle the resonance

structure that is the major contributor to the resonance hybrid.

O NH

N

NH 2

O

O

O O

Major Contributor N H

N

H

Major Contributor C does not have an octet all^ atoms^ have^ an^ octet O

H

O

H

Major Contributor O