CHEM 109A
CLAS
Drawing Resonance Structures - KEY
Resonance structures (a.k.a. resonance contributors) are structures drawn to show
the possible locations of delocalized electrons. Use a double headed arrow (↔)
between structures to indicate that they are resonance contributors, and curved arrows
to show how e-s “moved”. Resonance structures differ from each other ONLY in the
location of the e-s and lone pairs (they must have same atoms connected in the same
order. If the connectivity or orientation of the atoms changes then the structures are
considered isomers which we will discuss in Ch 5). Resonance contributors are
representations of what the molecule looks like at a given instance in time. The true
molecular structure is a combination or hybrid of the most significant (read stable)
resonance contributors.
Drawing Resonance Strucutures/Contributors:
See also Special Topic V: Drawing Resonance Contributors in Study Guide and
Solutions Manual of text.
1. Draw a Lewis structure (be sure to include all lone pairs)
2. Use curved arrows to show how electrons (e-) move:
Tail of a curved arrow shows where e- came from (multiple bond or lone pair,
never a + charge or empty space).
Head of curved arrow shows where e- go to (point between atoms to form a
multiple bond or point to an atom – usually with a positive charge or fairly
electronegative – to form lone pair, never into a – charge and only very rarely into
empty space (EX. Diels-Alder reactions, radical reactions)).
Two rules/commandments
1. Do NOT break a single bond.
2. Do NOT exceed an octet for second-row elements (C, N, O, F) – can
have less than an octet (use formal charge) but NOT more – or 12
electrons for a third-row element (P, S, etc).
O
R
N
+
N
violates rule 2
OK
violates rule 1
For example…
1. For each set of drawings below, draw the curved arrow(s) that get from one
structure to the next.
a.
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CHEM 109A
CLAS
Drawing Resonance Structures - KEY
same stability
N+
b.
N
A > B b/c A has all octets & “extra” bond
c.
same stability
d.
B,C,D > A,E b/c 2oC+ more stable than 1oC+ (hyperconjugation)
O-
O
e.
-O
O
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CHEM 109A
CLAS
Drawing Resonance Structures - KEY
same stability
2. For each of the compounds below, draw all resonance contributors.
O
a.
O
O-
O-
O
O
+
O
O
A > (B > C)
Factor
Filled octet
FC
Structure A Structure B
All atoms
Not C+
None
+ on C and –
on adjacent
O
# bonds
5
4
Structure C
All atoms
+ on O and
– on O
separated
further
5
N
b.
N+
N
B>A
Factor
Filled octet
FC
# bonds
Structure A
Not C+
+ on C
8
Structure B
All atoms
+ on N
9
O-
O-
O
c.
Cl
O
Cl
Cl
Cl+
A > (B > C)
Factor
Structure A Structure B
Structure C
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CHEM 109A
CLAS
Drawing Resonance Structures - KEY
Filled octet
FC
All atoms
None
Not C+
+ on C and –
on adjacent
O
# bonds
4
3
All atoms
- on O and
+ on Cl
separated
further
4
O
N
d.
O
O-
N
N
B>A
Factor
Filled octet
FC
# bonds
Structure A
All atoms
- on C
5
Structure B
All atoms
- on O
5
OH
e.
OH
OH
OH
OH
OH
A=B>C=E>D
Factor
Filled octet
FC
Structure A Structure B
All atoms
All atoms
None
None
# bonds
11
11
Structure C
All atoms
+ on O and
– on C
11
Structure D
All atoms
+ on O and
– on C
11
Structure E
All atoms
+ on O and
– on C
11
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CHEM 109A
CLAS
Drawing Resonance Structures - KEY
Predicting Relative Stability of Resonance Structures/Contributors:
Factors that INCREASE Stability
Factors that DECREASE Stability
complete octets on all atoms
incomplete octets
no formal charge (FC)
separated charges/large FC/
like charges on adjacent atoms
more covalent bonds
fewer covalent bonds
negative FC on more EN atoms
negative FC on less EN atoms
positive FC on less EN atoms
positive FC on more EN atoms
Some examples comparing stabilities
O-
O
HO
H
+HO
H
less stable
O+
C
O-
O
-
C+
most stable
least stable
N
N-
N
+
+N
H3 C
N-
N
+
H 3C
less stable
H 2C
H 2C
X
N-
N
+
N
H 2C
H 3+ C
N
N
N
N
N-
less stable
less stable
The more stable a resonance structure, the lower its energy and the more it
contributes to the structure of the hybrid/actual molecule. The hybrid can be thought
of as a weighted (based on stability) average of the resonance structures. In general,
the more resonance structures you can draw for a particular compound, the more
stable that compound – this is attributed to Resonance Stabilization Energy (RSE).
RSE is defined at the difference in energy between the energy of the hybrid and the
estimated energy of the lowest energy resonance contributor.
RSE = Ehybrid-Elowest E contributor
EX. Which is more stable, methoxide ion or methanoate ion?
O
O-
H
O-
The methanoate ion is more stable b/c is has resonance (RSE) whereas methoxide
ion does not.
O-
O
H
O-
H
O
For example…
3. Circle the most significant resonance contributor(s) in 1 & 2 above.
See answers above
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CHEM 109A
CLAS
Drawing Resonance Structures - KEY
Some examples of BAD resonance structures: Why is each bad?
X
Atoms (nuclei) are not in the same positions – not the same connectivity between
atoms.
X
The two structures do NOT have the same number of unpaired electrons
is OK, but not common to use these “no-bond”
resonance structures
X
O
O-
These structures can NOT conform to any molecular geometry – too much strain
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