Ligação e Estrutura Molecular em Compostos Orgânicos

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Ligação e Estrutura Molecular em Compostos Orgânicos 
Paulo Henrique Barcellos França
Maceió, 2014
UNIVERSIADE FEDERAL DE ALAGOAS
ESCOLA DE ENFERMAGEM E FARMÁCIA
Introdução
Para entender química orgânica é necessário conhecer o conceito de ligação química
A ligação química é o cimento molecular!
Para entender química orgânica é necessário conhecer o conceito de ligação química. Pensem em uma casa, cada tijolo é unido a outro pelo cimento. Assim é nas moléculas, onde os átomos são mantidos juntos nas moléculas por meio da ligação química, o cimento molecular. Primeiramente iremos abordar as ideias clássicas sobre a ligação química. Em seguida, iremos considerar modos mais modernos de descrever essa ligação.
2
Introdução
A química ocorre por causa do comportamento dos elétrons nos átomos e moléculas
Esses elétrons estão arranjados nos átomos de tal forma, que podemos sugerir seu arranjo
A química ocorre por causa do comportamento dos elétrons nos átomos e moléculas. Esses elétrons estão arranjados nos átomos de tal forma, que podemos sugerir seu arranjo, sua organização por meio da tabela periódica.
3
Introdução
O aspecto periódico da tabela
O aspecto periódico da tabela - a sua organização em grupos de elementos com propriedades químicas semelhantes - levou à idéia de que os elétrons residem em camadas, ou conchas, sobre o núcleo. A camada de elétrons mais externa de um átomo é chamada de camada de valência, e os elétrons nesta camada são chamados de elétrons de valência. O número de elétrons de valência para qualquer átomo neutro num grupo A da Tabela Periódica (excepthelium) iguala o seu número do grupo. Thus, lithium, sodium, and potassium (Group 1A) have one valence electron, whereas carbon (Group 4A) has four, the halogens (Group 7A) have seven, and the noble gases (except helium) have eight. Helium has two valence electrons.
4
Introdução
Walter Kossel (1916) 
Quando átomos formam íons, eles tendem a ganhar ou a perder elétrons de valência, de modo a ter o mesmo número de elétrons que o gás nobre de número atómico mais próximo (Regra do Octeto).
K – e-  K+
 19 e- 18 e-
Walter Kossel (1888-1956) noted in 1916 that when atoms form ions they tend to gain or lose valence electrons so as to have the same number of electrons as the noble gas of closest atomic number. Thus, potassium, with one valence electron (and 19 total electrons), tends to lose an electron to become K*, the potassium ion, which has the same number of electrons (18) as the nearest noble gas (argon). Chlorine, with seven valence electrons (and l7 total electrons) tends to accept an electron to become the 18-electron chloride ion, Cl-, which also has the same number of electrons as argon. Because the noble gases have an octet of electrons (that is, eight electrons) in their valence shells, the tendency of atoms to gain or lose valence electrons to form ions with the noble-gas configuration has been called the octet rule.
5
Ligação Química
As ligações entre os átomos mantêm as moléculas
Mas o que promove a ligação?
1- Cargas opostas se atraem
2- Cargas iguais se repelem
The bonds between atoms hold a molecule together. But what causes bonding? Two atoms 
form a bond only if their interaction is energetically favorable, that is, if energy — heat, for 
example — is released when the bond is formed. Conversely, breaking that bond requires 
the input of the same amount of energy.
The two main causes of the energy release associated with bonding are based on Coulomb’s 
law of electric charge:
1. Opposite charges attract each other (electrons are attracted to protons).
2. Like charges repel each other (electrons spread out in space).
Each atom consists of a nucleus, containing electrically neutral particles, or neutrons, and 
positively charged protons. Surrounding the nucleus are negatively charged electrons, equal 
in number to the protons so that the net charge is zero.
6
Ligação Química
Ligações envolvem simultaneamente atração coulômbica e troca de elétrons
Força de ligação: energia liberada quando átomos se ligam 
As two atoms approach each other, 
the positively charged nucleus of the first atom attracts the electrons of the second atom; 
similarly, the nucleus of the second atom attracts the electrons of the first atom. As a result, 
the nuclei are held together by the electrons located between them. This sort of bonding is 
described by Coulomb’s* law:Opposite charges attract each other with a force inversely 
proportional to the square of the distance between the centers of the charges.
7
Ligação Química
Distância interatômica x energia
When the atoms reach a certain closeness, no more energy is released. The distance 
between the two nuclei at this point is called the bond length(Figure 1-1). Bringing the 
atoms closer together than this distance results in a sharp increasein energy. Why? As 
stated above, just as opposite charges attract, like charges repel. If the atoms are too close, 
the electron – electron and nuclear – nuclear repulsions become stronger than the attractive 
forces. When the nuclei are the appropriate bond length apart, the electrons are spread 
out around both nuclei, and attractive and repulsive forces balance for maximum bonding.
8
Ligação Química
Ligação iônica
Envolve a transferência de elétrons, gerando espécies eletricamente carregadas (íons) 
An alternative to this type of bonding results from the complete transfer of an electron 
from one atom to the other. The result is two charged ions:one positively charged, a cation,
and one negatively charged, an anion(Figure 1-3). Again, the bonding is based on coulombic 
attraction, this time between two ions.
9
Ligação Química
Dois átomos de podem ser ligados por mais do que uma ligação covalente.
Two atoms in covalent compounds may be connected by more than one covalent bond. The
following compounds are cornmon examples:
Ethylene and formaldehyde each contain a double bond-a bond consisting of two electron
pairs. Acetylene contains a triple bond-a bond involving three electron pairs.
Covalent bonds are especially important in organic chemistry because all organic molecules contain covalent bonds.
10
Ligação Química
Na maioria das ligações covalentes, os elétrons não são compartilhados igualmente. Ex.: HCl
Como determinar se uma ligação é polar?
In many covalent bonds the electrons are not shared equally between two bonded atoms. Consider, for example, the covalent compound hydrogen chloride, HCl. (Although HCI dissolves
in water to form H,O+ and Cl- ions, in the gaseous state pure HCl is a covalent compound.)
The electrons in the H-Cl covalent bond are unevenly distributed between the two atoms;
they are polarized, or "pulled," toward the chlorine and away from the hydrogen. A bond in
which electrons are shared unevenly is called a polar bond. The H-Cl bond is an example
of a polar bond.
How can we determine whether a bond is polar? Think of the two atoms at each end of the
bond as if they were engaging in a tug-of-war for the bonding electrons. The tendency of an
atom to attract electrons to itselfin a covalent bond is indicated by irs electronegativity. The
electronegativities of a few elements that are important in organic chemistry are shown in
Table I . 1. Notice the trends in this table. Electronegativity increases to the top and to the right
of the table. The more an atom attracts electrons, the more electronegative it is. Fluorine is
the most electronegative element. Electronegativity decreases to the bottom and to the left of
the periodic table. The less an atom attracts electrons, the more electropositive it is. Of the
common stable elements, cesium is the most electropositive.
If two bonded atoms have equal electronegativities, then the bonding electrons are shared
equally. But if two bonded atoms have considerably different electronegativities,
then the electrons are unequally shared, and the bond is polar. (We might think of a polar covalent bond as
a covalent bond that is hying to become ionic!) Thus, a polar bond is a bond between atoms
with significantly dffirent electronegativities.
11
Ligação Química
Cargas parciais sobre os átomos
Mapas de potencial eletrostático
In this notation, the delta (6) is read as "partially" or "somewhat," so that the hydrogen atom
of HCI is "partially positive," and the chlorine atom is "partially negative."
Another more graphical way that we'll use to show polarities is the electrostatic potential
map. An electrostatic potential map (EPM) of a molecule starts with a map of the total electron density. This is a picture of the spatial distribution of the electrons in the molecule that
comes from molecular orbital theory, which we'll learn about in Sec. 1.8. Think of this as a
picture of "where the electrons are." The EPM is a map of total electron density that has been
color-coded for regions oflocal positive and negative charge. Areas ofgreater negative charge
are colored red, and areas of greater positive charge are colored blue. Areas of neutrality are
colored green.
The EPM of H-{l shows the red region over the Cl and the blue region over the H, as we
expect from the greater electronegativity of Cl versus H. In contrast, the EPM of dihydrogen
shows the same color on both hydrogens because the two atoms share the electrons equally.
The green color indicates that neither hydrogen atom bears a net charge.
12
Ligação Química
Como medir a polaridade de uma ligação?
	Momento de dipolo (m) 
	m = qr
O momento de dipolo é uma quantidade vetorial 
How can bond polarity be measured experimentally? The uneven electron distribution in a
compound containing covalent bonds is measured by a quantity called the dipole moment,
which is abbreviated with the Greek letter p (mu).The dipole moment is commonly given in
derived units called debyes, abbreviated D, and named for the physical chemist Peter Debye
(1884-1966), who won the 1936 Nobel Prize in Chemistry. For example, the HCI molecule
has a dipole moment of 1.08 D, whereas dihydrogen (Hr), which has a uniform electron distribution, has a dipole moment of zero.
The dipole moment is defined by the following equation:
p: qr (1.4)
In this equation, q is the magnitude of the separated charge and r is a vector from the site of
the positive charge to the site of the negative charge. For a simple molecule like HCl, the magnitude of the vector r is merely the length of the HCI bond, and it is oriented from the H (the
positive end of the dipole) to the Cl (the negative end). The dipole moment is a vector quantity, and p and r have the sarne directiotx-Iiom the positive to the negative end of the dipole.
As a result, the dipole moment vector for the HCI molecule is oriented along the H-Cl bond
from the H to the Cl:
13
Ligação Química
Moléculas polares são aquelas que apresentam um momento de dipolo permanente
Moléculas com ligações polares podem ser apolares
Os vetores se anulam!
= 1,08 D
Molécula polar
= 0 
Molécula apolar
Molecules that have permanent dipole moments are called polar molecules. HCI is a polar
molecule, whereas H, is a nonpolar molecule. Some molecules contain several polar bonds.
Each polar bond has associated with it a dipole moment contribution, called a bond dipole.
The net dipole moment of such a polar molecule is the vector sum of its bond dipoles.
Because the CO, molecule is lineat the C-O bond dipoles are oriented in opposite directions. Because they have equal magnitudes, they exactly cancel. (T[vo vectors of equal magnitude oriented in opposite directions always cancel.) Consequently, CQ, is a nonpolar molecule,
even though it has polar bonds .
14
Ligação Química
Se uma molécula contém vários dipolos de ligação que não se cancelam, estes dipolos são dicionados vetorialmente para dar o momento de dipolo resultante. 
In contrast, if a molecule contains several bond dipoles that do
not cancel, the various bond dipoles add vectorially to give the overall resultantdipole moment.
For example, in the water molecule, which has a bond angle of 104.5o, the O-H bond dipoles
add vectorially to give a resultant dipole moment of 134 D, which bisects the bond angle. The
EPM of water shows the charge distribution suggested by the dipole vectors-a concenfation
of negative charge on oxygen and positive charge on hydrogen.
15
Estrutura Molecular
Estruturas de Lewis
Modo de ilustrar pares de elétrons de valência compartilhados ou não ligações 
Predição da geometria e polaridade de compostos orgânicos
Os símbolos “:” ou “–” são usados para ilustrar um par de elétrons
Lewis structures are important for predicting geometry and polarity (hence reactivity) of 
organic compounds, and we shall use them for that purpose throughout this book. In this 
section, we provide rules for writing such structures correctly and for keeping track of 
valence electrons. Lewis structures are important for predicting geometry and polarity (hence reactivity) of 
organic compounds, and we shall use them for that purpose throughout this book. In this 
section, we provide rules for writing such structures correctly and for keeping track of 
valence electrons.
16
Estrutura Molecular
Regras para desenhar estruturas de Lewis
1- Desenhe o esqueleto molecular desejado.
2- Conte o número de elétrons de valência.
3- (A regra do octeto) Descreva todas as ligações covalentes por dois elétrons compartilhados, dando à maior quantidade possível de átomos um octeto de elétrons ao redor, exceto para H, que requer um dueto.
Estrutura Molecular
Regras para Desenhar estruturas de Lewis
4- Atribua cargas formais aos átomos da molécula
Estrutura Molecular
Carga Formal x Polaridade de Ligação
O flúor é mais eletronegativo que o boro!
Bond polarity is also useful because it gives us some insight that we can apply to the concept of formal charge. It's important to keep in mind that formal charge is only a bookkeeping
device for keeping track of charge. In some cases, formal charge corresponds to the actual
charge. For example, the actual negative charge on the hydroxide ion, -OH, is on the oxygen,
because oxygen is much more electronegative than hydrogen. In this case, the locations ofthe
formal charge and actual charge are the same. But in other cases the formal charge does not
correspond to the actual charge. For example, in the -BF* anion, fluorine is much more electronegative than boron (Table I . 1). So, most of the charge should actually be situated on the
fluorines. In fact, the actual charges on the atoms of the tetrafluoroborate ion are in accord
with this intuition:
19
Estrutura Molecular
A regra do octeto nem sempre prevalece
Exceção 1: Espécies com número ímpar de elétrons
You will have noticed that all our examples of “correct” Lewis structures 
contain an even number of electrons; that is, all are distributed as bonding or lone pairs. 
This distribution is not possible in species having an odd number of electrons, such as 
nitrogen oxide (NO) and neutral methyl (methyl radical, ?CH3
; see Section 3-1)
20
Estrutura Molecular
Exceção 2: Compostos com deficiência de elétrons na camada de valência
Some compounds of the early second-row elements, such as BeH
2and BH3
, 
have a deficiency of valence electrons.
Because compounds falling under exceptions 1 and 2 do not have octet configurations, they 
are unusually reactive and transform readily in reactions that lead to octet structures. For 
example, two molecules of ?CH3react with each other spontaneously to give ethane, CH
3– CH3
, 
and BH3reacts with hydride, H
2
, to give borohydride, BH
4
2
21
Estrutura Molecular
Exceção 3: Átomos de elementos além do segundo período (expansão da camada
de valência)
Beyond the second row, the simple Lewis model is not strictly applied, and 
elements may be surrounded by more than eight valence electrons, a feature referred to as 
valence-shell expansion.For example, phosphorus and sulfur (as relatives of nitrogen and 
oxygen) are trivalent and divalent, respectively, and we can readily formulate Lewis octet 
structures for their derivatives. But they also form stable compounds of higher valency, 
among them the familiar phosphoric and sulfuric acids. Some examples of octet and 
expanded-octet molecules containing these elements are shown below.
22
Geometria Molecular
A estrutura de uma molécula é conhecida por meio de sua conectividade atômica e de sua geometria molecular 
A geometria molecular especifica o quão distantes os átomos estão e como estão situados no espaço
Depende dos comprimentos e ângulos de ligações
We know the structure of a molecule containing covalent bonds when we know its atomic connectivity and its molecular geometry. Atomic connectivity is the specification of how atoms in
a molecule are connected. For example, we specify the atomic connectivity within the water
molecule when we say that two hydrogens are bonded to an oxygen. Molecular geometry is
the specification ofhow far apart the atoms are and how they are situated in space.
Given the connectivify of a covalent molecule, what else do we need to describe its geometry? Let's start with a simple diatomic molecule, such as HCl. The structure of such a molecule is completely defined by the bond length, the distance between the centers of the bonded
nuclei. Bond length is usually given in angstroms; 1 [ 
: 
1g-to m : 
l0-8 cm 
: 
100 pm (picometers). Thus, the structure of HCI is completely specified by the H-Cl bond length,
1.274 A.
When a molecule has more than two atoms, understanding its structure requires knowledge
of not only each bond length, but also each bond angle, the angle between each pair of bonds
to the same atom. The structure of water (HrO) is completely determined, for example, when
we know the O-H bond lensths and the H-O-H bond ansle.
23
Geometria Molecular
Comprimentos de ligação
a) Comprimentos de ligação aumentam significativamente para períodos superiores da tabela periódica.
l. Bond lengths increase significantly toward higher periods (rows) of the periodic table.
This trend is illustrated in Fig. 1.2. For example, the H-S bond in hydrogen sulfide is
longer than the other bonds to hydrogen in Fig. 1.2; sulfur is in the third period of the
periodic table, whereas carbon, nitrogen, and oxygen are in the second period. Similarly,
a C-H bond is shorter than a C-F bond, which is shorter than a C-Cl bond. These
effects all reflect atomic size. Because bond length is the distance between the centers
of bonded atoms, larger atoms form longer bonds.
24
Geometria Molecular
Comprimentos de ligação
b) Os comprimentos de ligação diminuem com o aumento de ligações múltiplas 
Bond lengths decrease with increasing bond order. Bond order describes the number of
covalent bonds shared by two atoms. For example, a C-C bond has a bond order of 1,
a C:C bond has a bond order of 2, and a C:C bond has a bond order of 3. The decrease of bond length with increasing bond order is illustrated in Fig. 1.3. Notice that the
bond lengths for carbon--carbon bonds are in the order C-C > C:C ) C:C.
25
Geometria Molecular
Comprimentos de ligação
c) Ligações diminuem de tamanho ao longo de um determinado período da tabela periódica.
Bonds of a given order decrease in length toward higher atomic number (that is, to the
right) along a given row (period) of the periodic table. Compare, for example, the H-C,
H-N, and H-O bond lengths inFig. L.2. Likewise, the C-F bond in H,C-fl at 1.39
A, is shorter than the C-C bond in H3C-CH3, at 1.54 A. Because atoms on the right of
the periodic table in a given row are smaller, this trend, like that in item 1, also results from
differences in atomic size. However, this effect is much less significant than the differences
in bond length observed when atoms of different periods are compared.
26
Geometria Molecular
Ângulos de ligação
Teoria de Repulsão de Pares de Elétrons da Camada de Valência
Elétrons estão organizados em volta de um átomo central de modo que estejam o mais distante possível.
Minimizar a repulsão entre elétrons livres e elétrons em ligações
The bond angles within a molecule determine its shape. For example, in the
case of a triatomic molecule such as HrO or BeHr, the bond angles determine whether it is
bent or linear. To predict approximate bond angles, we rely on valence-shell electron-pair repulsion theory, or VSEPR theory, which you may have encountered in general chemistry.
According to VSEPR theory, both the bonding electron pairs and the unshared valence elecffon pairs have a spatial requirement. The fundamental idea of VSEPR theory is that bonds
and electron are arranged about a central atom so that the bonds are as 
far 
apart as possible.
This arrangement minimizes repulsions between electrons in the bonds. Let's first apply
VSEPR theory to three situations involving bonding electrons: a central atom bound to four,
three, and two groups, respectively.
27
Geometria Molecular
Ângulos de ligação
When four groups are bonded to a central atom, the bonds are farthest apart when the central atom has tetrahedral geometry. This means that the four bound groups lie at the vertices of
a tetrahedron. A tetrahedron is a three-dimensional object with four triangular faces (Fig.
l.4a,p. 16). Methane, CHo;has tetrahedral geometry. The central atom is the carbon and the
four groups are the hydrogens. The C-H bonds of methane are as far apart as possible when
the hydrogens lie at the vertices of a tetrahedron. Because the four.C-H bonds of methane
are identical, the hydrogens lie at the vertices of a regular tetahedron, a tetrahedron in which
all edges are equal (Fig. l.ab). The tetrahedral shape of methane requires a bond angle of
109.5" (Fig. 1.4c).
When three groups surround an atom, the bonds are as far apart as possible when all bonds
lie in the same plane with bond angles of 120'. This is, for example, the geometry of boron trifluoride. In such a situation the surrounded atom (in this case boron) issaid to have trigonat planar
geometry.
When an atom is surrounded by two groups, maximum separation of the bonds demands a
bond angle of 180e. Thisjs the_sinrationwith each carbon in acetylene, H-C7C-H. Each
carbon is surrounded by two groups: a hydrogen and another carbon. Notice that the triple
bond (as well as a double bond in other compounds) is considered as one bond for purposes of
VSEPR theory because all three bonds connect the same two atoms. Atoms with 180' bond
angles are said to have linear geometry. Thus, acetylene is a linear molecule
28
Geometria Molecular
Ângulos de ligação em moléculas com elétrons não compartilhados
Now let's consider how unshared valence electron pairs are treated by VSEPR theory. An
unshared valence electron pair is treated as if it were a bond without a nucleus at one end. For
example, in VSEPR theory, the nitrogen in ammonia, :NHr, is surrounded by four "bonds":
three N-H bonds and the unshared valence electron pair. These "bonds" are directed to the
vertices of a tetrahedron so that the hydrogens occupy three of the four tetrahedral vertices.
This geometry is called trigonal pyramidal because the three N-H bonds also lie along the
edges of a pyramid.
VSEPR theory also postulates that unshared valence electron pairs occupy more space
than an ordinary bond. lt's as if the electron pair "spreads out" because it isn't constrained by
a second nucleus. As a result, the bond angle between the unshared pair and the other bonds
are somewhat larger than tetrahedral, and the N-H bond angles are colrespondingly smaller.
In fact, the H-N-H bond angle in ammonia
is 107.3'.
29
Estruturas de Ressonância
Alguns compostos não são descritos precisamente por uma única estrutura de Lewis
Estruturas e híbrido de ressonância 
This Lewis structure shows an N-O single bond and an N:O double bond. From the preceding section, we expect double bonds to be shorter than single bonds. However, it is found experimentally that the two nitrogen-oxygen bonds of nitromethane have the same length, and this
length is intermediate between the lengths of single and double nitrogen-oxygen bonds found in
other molecules. We can convey this idea by writing the structure of nitromethane as follows.
When two resonance structures are identical, as they are for nitromethane, they are equally
important in describing the molecule. We can think of nitromethane as a 1:1 average of the
structures in Eq. L6. For example, each oxygen bears half a negative charge, and each nitrogen-oxygen bond is neither a single bond nor a double bond, but a bond halfway in between.
If 
30
Estruturas de Ressonância
Para compostos com estruturas de ressonância diferentes, aquela de maior estabilidade contribui mais para o híbrido de ressonância
Estrutura de maior importância
Todos os átomos apresentam octetos
If two resonance structures are not identical, then the molecule they represen t is a weighted
average of the two. That is, one of the structures is more important than the other in describing the molecule. Such is the case, for example, with the methoxymethyl cation.
It turns out that the structure on the right is a better description of this cation because all atoms
have complete octets. Hence, the C-O bond has significant double-bond character, and most
of the formal positive charge resides on the oxygen.
31
Estruturas de Ressonância
Como saber qual a estrutura mais importante?
Regra 1: Estruturas com maior número de octetos são mais importantes.
Estrutura de maior importância
Guideline 1. Structures with a maximum of octets are most important.In the enolate ion, 
all component atoms in either structure are surrounded by octets. Consider, however, the 
nitrosyl cation, NO
1
: One resonance form has a positive charge on oxygen with electron 
octets around both atoms; the other form 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 charge is on oxygen than on nitrogen. Similarly, the 
dipolar resonance form for formaldehyde (p. 20) generates an electron sextet around carbon, 
rendering it a minor contributor. The possibility of valence shell expansion for third-row elements (Section 1-4) makes the non-charge-separated picture of sulfuric acid with 12 electrons 
around sulfur a feasible resonance form, but the dipolar octet structure is better.
32
Estruturas de Ressonância
Como saber qual a estrutura mais importante?
Regra 2: Cargas devem ser colocadas preferencialmente sobre átomos com eletronegatividades compatíveis.
Estrutura de maior importância
Guideline 2. Charges should be preferentially located on atoms with compatible electronegativity.Consider again the enolate ion. Which is the major contributing resonance form? 
Guideline 2 requires it to be the first, in which the negative charge resides on the more 
electronegative oxygen atom. Indeed, the electrostatic potential map shown in the margin 
on p. 20 confirms this expectation.
Looking again at NO
1
, you might fi nd guideline 2 confusing. The major resonance 
contributor to NO
1
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 2. Charges should be preferentially located on atoms with compatible electronegativity.Consider again the enolate ion. Which is the major contributing resonance form? 
Guideline 2 requires it to be the first, in which the negative charge resides on the more 
electronegative oxygen atom. Indeed, the electrostatic potential map shown in the margin 
on p. 20 confirms this expectation.
Looking again at NO
1
, you might fi nd guideline 2 confusing. The major resonance 
contributor to NO
1
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.
33
Estruturas de Ressonância
Como saber qual a estrutura mais importante?
Regra 3: Estruturas com menor separação de cargas opostas são mais importantes que aquelas cuja separação é maior.
Estrutura de maior importância
Guideline 3. Structures with less separation of opposite charges are more important resonance contributors than those with more charge separation.This rule is a simple consequence 
of Coulomb’s law: Separating opposite charges requires energy; hence neutral structures are 
better than dipolar ones. An example is formic acid, shown below. However, the influence of 
the minor dipolar resonance form is evident in the electrostatic potential map in the margin: 
The electron density at the carbonyl oxygen is greater than that at its hydroxy counterpart.
In some cases, to draw octet Lewis structures, charge separation is necessary; that is, guideline 1 takes precedence over guideline 3. An example is carbon monoxide. Other examples 
are phosphoric and sulfuric acids, although valence-shell expansion allows the formulation 
of expanded octet structures (see also Section 1-4 and guideline 1).
When there are several charge-separated resonance forms that comply with the octet 
rule, the most favorable 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 (see also the electrostatic potential map in the margin).
34
Estruturas de Ressonância
Como saber qual a estrutura mais importante?
Regra 4: A regra 1 tem prioridade sobre as outras.