LEWIS FORMULAS, STRUCTURAL ISOMERISM,
AND RESONANCE STRUCTURES
CHARACTERISTICS OF LEWIS FORMULAS : Lewis formulas are structures that show the connectivity , or bonding quence of the atoms, indicating single, double, or triple bonds . They should also show any formal charges and unshared electrons that might be prent in the molecule. Additional examples of Lewis formulas follow.
H C
C H H
H C C
H H H
Cl
H
O C 3H 5
ClO C 3H 5ClO
C 3H 5ClO
H C
H H
C Cl
H H
Cl
H H
The examples were deliberately chon becau all three molecules shown have the same molecular formula,but different connectivities, or bonding quences. Such substances are called structural isomers , or sometimes constitutional isomers .
Notice that only the first structure shows the unshared electrons of chlorine. In Lewis formulas of organic compounds,it is customary to omit the lone electron pairs on the halogens unless there is a reason to show them explicitly.Lewis formulas are mostly ud for covalent substances, but occasionally they also show ionic bonds that might be prent in certain compounds.
H
roar歌词N H H Cl
The bond between nitrogen and
chlorine is ionic.All others are covalent.
H
obayC
H
O
Na
The bond
between oxygen and
sodium is ionic.All others are covalent.
COMMON BONDING PATTERNS FOR FIRST AND SECOND ROW ELEMENTS : Once we write enough Lewis formulas containing the elements of interest in organic chemistry, which are mostly the cond row elements, we find that certain bonding patterns occur over and over. Learning the patterns is uful when trying to write Lewis formulas becau they provide a convenient starting point. For example, in veral of the structures given in the previous ction, we find that the carbon bonded to three hydrogens is a unit that occurs quite frequently. It is called the methyl group, repres
ented by CH 3. It is so common that it is valid to write it as such in Lewis formulas, even though it is in fact an abbreviated form, becau everybody knows what it stands for.
H
C
H O
It is equally valid to reprent the acetate
ion by either of the formulas.广西公务员考试真题
CH 3
O
or
LEARNING OBJECTIVES : To understand the us and limitations of Lewis formulas, to introduce structural isomerIsm, and to learn the basic concept of resonance structures.
H
H
HYDROGEN : Usually forms only one bond.
什么是diy
H
F
H
C H
H
Cl
CARBON : Forms four bonds when neutral, but it can also have only three bonds by bearing a positive or a negative charge. When it bears a negative charge it should also carry a pair of unshared electrons.
H
C H
H Cl C
C H
H
H
H
Neutral carbon
H 3
3CH 3
A carbocation has a central carbon with an incomplete octet and a formal +1charge.
H 3
容易的近义词
3
CH 3
A carbanion has a central carbon with an
unshared electron pair and a formal -1charge.
ginkgo
NITROGEN : Forms three bonds and carries a lone pair of electrons when neutral. It can also form four bonds by bearing a positive charge, in which ca it carries no unshared electrons. Finally, it can also form two bonds as it carries two unshared electron pairs and a negative charge.
C
N H 3H 3C
H
Neutral nitrogen
N
H N H H
H
H
C H 3H 3C
锻炼口才Positively charged nitrogen
N H H Negatively charged nitrogen
Other common bonding patterns are shown below.
OXYGEN : Forms two bonds and carries two lone pairs when neutral. It can form three bonds with a positive charge,or one bond with a negative charge. In each ca it must carry the appropriate number of unshared electron pairs to complete the octet.
O
H
O
H
water hydronium ion hydroxide ion
休息室英文
HALOGENS : Form one bond and carry three electron pairs when neutral. Can carry a negative charge with no bonds. They are rarely en with positive charges.
Cl
THIRD ROW ELEMENTS : They behave like their cond-row counterparts, except that they can expand their valence shells if needed.
S H O S O
儿童科普知识H O
Electron pairs on oxygen
are not shown for clarity.Cl Cl
Br P Br
Br
ELECTRON DEFICIENCY IN SECOND ROW ELEMENTS : One thing worth noting is that, in the cond row, only boron and carbon can form relatively stable species in which they bond with an incomplete octet. Examples have already been discusd. Boron has no choice but to be electron deficient. Carbon can bond with a complete octet or with an incomplete octet. Obviously bonding with a complete octet provides higher stability.
F H 33
CH 3Boron has no choice but to have an incomplete octet An electron deficient carbon in a carbocation
It is however very rare to obrve species where nitrogen or oxygen bond with incomplete octets. Their high
electronegativity renders such situation high energy, and therefore very unstable. For all intents and purpos, avoid writing formulas where oxygen or nitrogen are shown with incomplete octets, even if they carry a positive charge.
CH
O
3
CH 3
To write this structure without the lone pair of electrons on oxygen is unacceptable.
CH
3O
This structure is unacceptable and indeed it looks quite awkward
N
H This species might exitst in the high energy environment of a mass spectrometer,but it is not frequently obrved
in common organic reactions.
RESONANCE STRUCTURES AND SOME LIMITATIONS OF LEWIS FORMULAS : Lewis formulas are misleading in the n that atoms and electrons are shown as being static. By being esntially two-dimensional reprentations they also fail to give an accurate idea of the three-dimensional features of the molecule, such as actual bond angles and topography of the molecular frame.
Furthermore, a given compound can have veral valid Lewis formulas. For example CH 3CNO can be reprented by at least three different but valid Lewis structures called resonance forms, or resonance structures , shown below.
H C C H
H
H C C O
H
H
H C C H
H
I
II
III
However, a stable compound such as the above does not exist in multiple states reprented by structures I, or II,or III. The compound exists in a single state called a hybrid of all three structures. That is, it contains contributions of all three resonance forms, much like a person might have physical features inherited from each parent to varying degrees.
In the resonance forms shown above the atoms remain in one place. The basic bonding pattern, or connectivity ,is the same in all structures, but some electrons have changed locations. This means that there are certain rules for electron mobility that enable us to “push” electrons around to arrive from one resonance structure to another.The rules will be examined in detail in a later paper.
ALL RESONANCE STRUCTURES MUST BE VALID LEWIS FORMULAS : By convention, we u double-headed arrows to indicate that veral resonance structures contribute to the same hybrid. C
ontinuing with the example we’ve been using, the resonance structures for CH 3CNO should be written in this way if we want to emphasize that they reprent the same hybrid.
CH 3
O
CH O
CH 3
C N O
I II III
Do not confu double-headed arrows with double arrows. A double arrow indicates that two or more species are in equilibrium with each other and therefore have a parate existence. Double-headed arrows indicate resonance structures that do not exist by themlves. They simply reprent features that the actual molecule, the hybrid,posss to one extent or another.
When writing resonance structures keep in mind that THEY ALL MUST BE VALID LEWIS FORMULAS. The factors that make up valid Lewis formulas are as follows.
1. Obrve the rules of covalent bonding, including common patterns as discusd previously. Make sure to show all single, double, and triple bonds.
2. Account for the total number of valence electrons being shared (from all the elements), including bonding and nonbonding electrons. Make sure to show the nonbonding electrons.
3. Account for the net charge of the molecule or species, showing formal charges where they belong.
4.Obrve the octet rule as much as possible, but also understand that there are instances where some atoms may not fulfill this rule.
5. Avoid having unpaired electrons (single electrons with no partners) unless the total number of valence electrons for all elements is an odd number. This is not a very frequent occurrence, but the following example shows a species that could exist as a reaction intermediate in some high energy environments.
H The total number of valence electrons being shared for all atoms is 4 from carbon and 3 from the three hydrogens,for a total of 7. Becau it is an odd number, it is impossible to have all the electrons paired. Therefore the prence of a single electron cannot be avoided. Notice that there is no formal charge on carbon, since it has no surplus or deficit of valence electrons.
RELATIVE ENERGIES OF RESONANCE STRUCTURES . From the examples given so far it can be en that some resonance forms are structurally equivalent and others are not. The potential energy associated with equivalent Lewis structures is the same . If the Lewis structures are not equivalent, then the potential energy associated with them is most likely different. This means that equivalent resonance structures are also equivalent in stability and nonequivalent structures have different stabilities. This in turn means that equivalent structures contribute equally to the hybrid and nonequivalent structures do not contribute equally to the hybrid.
In the example below, the two structures are equivalent. Therefore they make equal contributions to the hybrid.
CH
O
CH O O
equivalent in energy
edinburghIt is very difficult to accurately reprent the hybrid with drawings becau it is a composite of all the resonance contributors. Some reprentations such as the first one shown below are sometimes given. In this ca the broken line reprents the electrons and the negative charge which are spread over three atoms (O-C-O). Another commonly ud reprentation of the hybrid is given on the right, showing that each oxygen atom shares a -1/2 charge.
However, none of them accurately conveys the true picture. For convenience, the unshared electrons are sometimes omitted from some reprentations, just like it’s done with hydrogen atoms. This must be kept in mind when examining the different structures.
CH
CH
-1/2
-1/2two
commonly ud reprentations
for the acetate ion hybrid
There is a third resonance form that can be drawn for the acetate ion hybrid. The structure shown below is structurally different from the ones shown above. This means that it is of different energy and therefore does not contribute to the hybrid to the same extent as the others. In this ca, it happens to be less stable than the other two and therefore does not make a significant contribution to the hybrid.
CH
