Chem 141 – Organic Chemistry I
Wednesday, 2-18-09
1) Formal charge
2) Lewis structures & exceptions to the octet
rule
3) Resonance contributors
•
•
Rules for writing acceptable resonance
contributors
Relative importance of contributing structures
4) Use everything you know to draw complete
and correct Lewis or skeletal structures
Rules for Writing Lewis Structures (again)
1)
2)
3)
4)
5)
Write molecular skeleton
Assume all bonds are covalent
Count # of available valence electrons
Put single bonds between bonded atoms
(represents 2 e-), then give each atom a total of 8 e(2 for H).
Count electrons in structure & compare with #
derived in step 3.
If same, structure is correct
If # in trial structure > # available valence
electrons, add one or more multiple bonds.
Formal Charge
‹Formal
charge: the charge on an atom in a
molecule or a polyatomic ion
‹To derive formal charge
1. write a correct Lewis structure for the molecule or ion
2. assign each atom all its unshared (nonbonding)
electrons and one-half its shared (bonding) electrons
3. compare this number with the number of valence
electrons in the neutral, unbonded atom
Formal
charge
N umber of
= valence electrons
in th e neutral,
un bonded atom
One h alf of
All
un shared + all sh ared
electrons
electrons
Formal Charge
Formal charge
• The difference in the number of electrons owned by an atom in a
molecule and by the same atom in its elemental state
• Assigned to specific atoms within a molecule
• Dimethyl sulfoxide CH3SOCH3
•
•
Sulfur atom has three bonds rather than the usual two and
has a formal positive charge
Oxygen atom has one bond rather than the usual two and
has a formal negative charge
Formal Charges
Show the formal charges, if any, for atoms in the molecules shown
H
O
H
O
H
H
H
B
H
B
H
H O
H
H
H
H B H
H
H
none
H
H O
H
H
H B H
H
H
O
H
O
F
H C N
H C N
none
none
F
Show the formal charges, if any, for atoms in the molecules shown
H
H N
H
H
H N H
H
H
H N
H
H
H N H
H
H N H
H
C
H
C C
H
H
H
H C
H
H
H C
H
none
H
H
H
C
H
H
H N H
C N
H
H
H
C N none
H
H
H C C
H
C C
H
H
H C C
H
Exceptions to the Octet Rule
‹ Atoms
of third-period elements have 3d orbitals
and may expand their valence shells to contain
more than 8 electrons
• phosphorus may have up to 10
:
H- O-P- O-H
:
Cl :
:
Cl :
:O:
: :
: Cl
P
: : : :
O-H
Phosphoric
acid
:
CH3
Trimethylphosphine
: : : :
:
CH3 -P- CH 3
: Cl
: Cl :
Phosphorus
pentachloride
Exceptions to the Octet Rule
• sulfur, another third-period element, forms compounds
in which its valence shell contains 8, 10, or 12
electrons
H-O- S-O-H
:
CH 3 -S-CH 3
:
:
:
:
H-S- H
:
: O:
:
: O:
:O :
Hydrogen
sulfide
Dimethyl
sulfoxide
Sulfuric
acid
Resonance
• Many molecules and ions are best described by writing two or more
Lewis structures.
• Individual Lewis structures are called contributing structures. They
are imaginary, not real.
• The actual structure of the molecule or ion is a composite, or hybrid of
the various contributing structures.
• We use double-headed (resonance) arrows to connect contributing
structures.
• We use single-headed arrows to show electron movement.
O
O
H
H
H
H
H
H
O
O
lower energy
contributor
• Lower energy resonance structures are more important contributors.
• Resonance leads to stability
Resonance
‹
Examples: equivalent contributing structures
CH3
O:
C
: O ::
N itrite ion
(equivalent con trib uting
s tru ctures)
O:
CH3
C
:
:O ::
:
O:
: O :-
O:
:N
:
:N
:
:
:
:O: -
A cetate ion
(equ ivalen t contributin g
s tru ctures)
Resonance
‹
Allowed types of electron redistribution:
1) Move non-bonded electrons from an atom into a new
connecting π bond. In this case, the electrons must move
toward a + charge, or toward another π bond.
2) Move electrons from a π bond
a) Onto a connected atom, or
b) Into a new adjacent π bond. In this case, the π electrons
must move toward a + charge, or toward another π bond.
‹
Electron pushing is a survival skill in organic
chemistry. Learn it well!
Resonance
‹
All contributing structures must:
1. have the same number of valence electrons
2. obey the rules of covalent bonding (follow octet rule)
• no more than 2 electrons in the valence shell of H
• no more than 8 electrons in the valence shell of a 2nd period
element
• 3rd period elements, such as P and S, may have up to 12
electrons in their valence shells
3. differ only in distribution of valence electrons (π bond
electrons & non-bonded electrons); the position of all nuclei
must be the same.
Evaluating Importance of Resonance
Contributors
1) Structures with more filled octets are preferred
2) Structures with > # of covalent bonds contribute more
than structures with fewer covalent bonds
3) Structures with a minimum of separation of unlike
charges are preferred.
4) If separation of charge does occur, electronegative
elements should gain the negative charge, and less
electronegative elements should bear the positive
charge.
5) Rules are listed in order of importance
Resonance
‹ Preference
1: filled valence shells
• structures in which all atoms have filled valence shells
contribute more than those with one or more unfilled
valence shells
+
CH3 O
••
C
H
H
Greater contribution;
both carbon and oxygen have
complete valence shells
••
CH3 O
••
+
C
H
H
Lesser contribution;
carbon has only 6 electrons
in its valence shell
Resonance
‹ Preference
2: maximum number of covalent
bonds
• structures with a greater number of covalent bonds
contribute more than those with fewer covalent bonds
CH3
+
O
••
••
C
H
H
Greater contribution
(8 covalent bonds)
CH 3 O
••
+
C
H
H
Lesser contribution
(7 covalent bonds)
Resonance
‹ Preference
3: least separation of unlike charge
• structures with separation of unlike charges contribute
less than those with no charge separation
Greater contribution
(no separation of
unlike charges)
:
:
O:
CH3 -C- CH3
:O: -
CH3 -C- CH3
Lesser contribution
(separation of unlike
charges)
Resonance
‹ Preference
4: negative charge on the more
electronegative atom
• structures that carry a negative charge on the more
electronegative atom contribute more than those with
the negative charge on the less electronegative atom
O
(1)
C
H3 C
O
O
CH3
(a)
Less er
con trib ution
(2)
C
H3 C
CH3
(b)
Greater
contribu tion
C
H3 C
CH3
(c)
S hould n ot
be d raw n
Draw Lewis Structures and Resonance Structures
POCl3
HONO2
(HNO3)