Formal Charge
In Class Examples
Assign the charge in the cyanide anion CN¯
Assign the charge in (CH3)3O
Assign the charge in
O
C
CH3
and
CH3NO2
CH2
Resonance
For certain structures we can write more than one line bond or Lewis structure.
Resonance and Benzene Structure
Two resonance structures for benzene
H
C
H
H
H
C
C
C
C
H
H
H
C
H
H
C
H
C
C
C
C
H
C
H
One example is nitromethane. Which of the two is correct? Do we actually observe
different N-O bond lengths?
The answer to the later question is no both N-O bonds have identical length. So in fact
the molecule exists as hybrid of both structures.
O
O
H3C
H3C
N
O
N
O
See the same for the nitrate anion
-1
O
N
O
O
N
N
N
O
O
O
O
O
O
O
O
O
Resonance and Estimation of the Relative Importance of Resonance Forms
Resonance structures exist only on paper. Resonance structures are useful
because they allow us to describe molecules, radicals, and ions for which a single
Lewis structure is inadequate. We write two or more Lewis structures, calling them
resonance structures or resonance contributors. The structures are connected by
double headed arrows, and we say that the real molecule, radical or ion is like a hybrid
of all of them.
In writing resonance structures we are only allowed to move electrons. The
positions of the nuclei must remain the same in all of the structures.
All of the structures must be proper Lewis structures. We should not write
structures in which C is pentavalent for example.
All resonance structures must have the same number of unpaired electrons.
All atoms that are part of a delocalized system must lie in a plane or be nearly
planar.
The energy of the actual molecule is lower than the energy that might be
estimated for any contributing structure.
Equivalent resonance structures make equal contributions to the hybrid and a
system described by them has a large resonance stabilization.
Non equivalent structures do not make equal contributions to the hybrid. The
more stable a structure, the greater is its contribution to the hybrid.
Guidelines to decide how to assign relative importance to different resonance
structures are as follows:-
Two Non Equivalent Resonance Structure of an Enolate Ion
H
H
C
H
H
C
H
C
O
H
C
O
Guideline 1. Full Valence Shell Increases Stability Structures with a maximum of
octets are preferred. In the enolate ion, all component atoms in either structure are
surrounded by octets. Consider, however, the nitrosyl cation, NO+ : The better
resonance form has a positive charge on oxygen with electron octets around both
atoms; the other places the positive charge on nitrogen, thereby resulting in an
electron sextet on this atom. Because of the octet rule, the second structure
contributes less to the hybrid. Thus, the N–O linkage is closer to being a triple than a
double bond and more of the positive charges is on oxygen than on nitrogen.
Nitrosyl cation
N
O
N
Major
resonances
contributor
O
Minor
resonances
contributor
Guideline 2. Charges should be preferentially located on atoms with compatible
electronegativity. Consider again the enolate ion. Which is the preferred resonance
structure? Guideline 2 requires that it is the first, in which the negative charge resides
on the more electronegative oxygen atom.
Looking again at NO+, you might find guideline 2 confusing. The major
resonance contribution to NO+ has the positive charge on the more electronegative
oxygen. In cases such as this, the octet rule overrides the electronegativity criterion;
that is, guideline 1 takes precedence over guideline 2.
Guideline 3. Charge Separation Decreases Stability. Structures with a minimum
of charge separation are preferred. This rule is simple consequence of Coulomb's
law: Separating charges requires energy; hence neutral structures are better than
dipolar ones.
Formic acid
O
H
O
C
O
H
H
C
Major
O
H
Minor
In some cases, to ensure octet Lewis structures, charge separation is acceptable;
that is, guideline 1 takes precedence over guideline 3. An example is carbon
monoxide.
Carbon monoxide
C
O
Major
C
O
Minor
When there are several charge-separated resonance structures that comply with the
octet rule, the most favourable is the one in which the charge distribution best
accommodates the relative electronegativities of the component atoms (guideline 2).
In diazomethane, for example, nitrogen is more electronegative than carbon, thus
allowing a clear choice between the two resonance contributors.
H
Diazomethane
H
C
N
H
N
C
N
H
Major
Minor
N
Representation of Structural Formulas
Different types of formula are dot formula, dash formula condensed formula and Bondline formula.
Often lone pairs are omitted but it is a good idea to include them on all structures where
they are present.
Condensed Formulas
In these widely used structural formulas, the bonding is implied by the
order in which the atoms appear. The sequence of central atoms is
written from left to right. The hydrogens and other atoms that are
bonded to a central atom appear immediately after that central atom.
A Constitutional Isomer of C2H6O
H
H
H C O C H
H
H
Lewis Dot Structure
H
H
H C O C H
H
H
Dash Formula
CH3OCH3
Condensed Formula
Representations of Structural Formulas
Dot Structures (Lewis Structures)
There are a number of ways to write structural formulas. Each
conveys some idea or information about the structure.
These structures emphasize the number of valence electrons. They help
in recognizing a correct valence bond structure in conjunction with the
octet rule. But using dots for covalent bonds is tedious, and they are
rarely used .
Dash or Line Formulas
These structural formulas where each pair of electrons in a covalent
bond is represented as a line are widely used. When the nonbonding
electron pairs are shown as dots, these formulas provide an
accounting of all the valence electrons. Sometimes the nonbonding
electrons are omitted.
Equivalency of Dash Structural Formulas
Dash structural formulas may, to the inexperienced eye, appear to be
nonequivalent, when, in fact, they represent the same compound.
Because of rotation around single bonds, it is possible to have several
perspectives of a structure that are drawn in different dash formulas .
They all represent the same structure.
H H
H C C OH
H H
H
H
H
OH
H
H
ethyl alcohol
H
may be shown as H C
H
H
H C
H
H
C H
OH
OH
C H
H
equivalent
Condensed Structural Formulas
These shorthand formulas with implied bonding are widely used.
Sometimes some partial line formulas are included to emphasize a
structural detail.
Dash or Line Formulas
Condensed Structural Formulas
(All covalent bonds are shown.)
H H
H C C OH
H H
H H H H
H C C C C H
H Cl Cl H
(Note use of parentheses to enclose groups.)
H H H H
H C C C C H
H
H H
H C H
H
(Bonds are implied.)
CH3CH2OH
CH3CHClCHClCH3
or
CH3CHCHCH3
Cl Cl
CH3CH(CH3)CH2CH3
or
CH3CHCH2CH3
CH3
or
(CH3)2CHCH2CH3
Bond-Line Formulas and Cyclic Molecules
Bond-line formulas are short-hand representations where atoms are
not explicitly shown. The carbon skeleton is drawn by lines. Only
atoms and groups other than hydrogen are shown. Each intersection of
two or more lines is a carbon center. Multiple bonds are shown..
Cl
CH3CH2CH3
CH3CHCH2CH3
CH3
CH3CHClCH3
CH3CHCHBrCH3
CH3
Br
Cyclic structures are shown as regular polygons.
H2
C
H2
C
H2C
H2C
CH2
H 2C
Multiple bonds are shown.
CH3
CH3C=CHCH3
CH2
CH2
Three-Dimensional Representations
Three-dimensional representations of structures are often important.
By convention, a solid wedge is a bond projected out of the plane of
paper or screen towards the viewer. A dashed wedge is a bond
projected out of the plane away from the viewer. A solid line is a
bond in the plane.
towards viewer
in plane
away from viewer
Examples
H
Br
H
C
H
CH3Br
H
Cl
C H
H
OH
CH3OH
H3C
H
C
H
CH3CH2Cl
Quiz Chapter 1 Section 17
Are the following pairs of structural formulas the same or different
compounds?
(CH3)2CHCH2CH3
CH3CH2CHCH3
CH3
CH3CH2CHClCHCH3
CH3CH2CHClCHCH2CH3
CH2CH3
CH3
CH3CHCH2CHCH2CH3
CH3
Cl
(CH3)2CHCHClCH2CH2CH3