Lewis Structures and Resonance Forms

Electron flow paths are written in the language of Lewis dot structures and curved arrows. Lewis dot structures are used to keep track of all electrons, and curved arrows are used to symbolize electron movement. You must be able to draw a proper Lewis structure complete with formal charges accurately and quickly. Your command of curved arrows must also be automatic. These two points cannot be overemphasized, since all explanations of reactions will be expressed in the language of Lewis structures and curved arrows. A Lewis structure contains the proper number of electrons, the correct distribution of those electrons over the atoms, and the correct formal charge. We will show all valence electrons lone pairs are shown as darkened dots and bonds by lines. 

An atom in a molecule is most stable if it can achieve the electronic configuration of the nearest noble gas, thus having a completely filled valence shell. Hydrogen with two electrons around it, a duet, achieves the configuration of helium. Second-row elements achieve the configuration of neon with an octet of valence electrons. Third-row elements achieve an octet but may also expand their valence shell for example SFg is a stable molecule with six single bonds to sulfur (12 bonding electrons total). 

Count the number of bonds used in the structure and multiply by two electrons per bond to get the number of electrons shared in bonds. Subtract the number of shared electrons from the number of valence electrons to get the number of unshared 

Assign formal charges. Formal charge is a comparison of the number of electrons an atom owns in the Lewis structure with the number it would have if it were free. The atom is assigned only half of the electrons that it shares in a bond, but all of its unshared electrons. 

Formal chaise = free atom valence - ( of bonds + unshared of electrons) 

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In this chapter we have explored the structure of organic compounds. This is important since structure determines reactivity. We have seen that weak bonds are a source of reactivity. Strong bonds are made by good overlap of similar-sized orbitals (same row on periodic table). Bends or twists that decrease orbital overlap weaken bonds. Lewis structures and resonance forms along with electron flow arrows allow us to keep track of electrons and explain the changes that occur in reactions. VSEPR will help us predict the shape of molecules. Next we must review how bonds are made and broken, and what makes reactions favorable. Critical concepts and skills from this chapter are ... 

PROBLEM 1.19 Write Lewis structures and resonance forms for the following compounds. If you have problems visualizing the structure of some of these molecules, see the inside front cover of this book. 

The double arrow indicates that these Lewis structures are resonance forms. It does not mean that the molecule vibrates back and forth between these two forms but rather that there is just one form of ozone. The reason for writing the two structures is that there is a limitation in the ability of Lewis structures to describe the electron distribution in some molecules. 

Often, one Lewis structure is not sufficient to describe the electron distribution in a molecule. In many cases, the use of a resonance hybrid is necessary. All resonance forms are valid Lewis structures. In resonance forms, only the electrons move and not the atomic nuclei. Each resonance form does not have a separate existence but is a part of a hybrid whole. The use of a double-headed arrow, between the forms reinforces the notion of a hybrid representation of a single structure. The forms are not in equilibrium with each other (equilibrium is shown by two opposing arrows ). 

Both satisfy the octet rule. However, experimental evidence shows no double bond in SO2. The two sulfur-oxygen bonds are equivalent. Apparently, neither structure accurately represents the structure of SO2, and neither actually exists. The actual structure is said to be an average or hybrid of these two Lewis structures. When a compound has two or more Lewis structures that contribute to the real structure, we say that the compound displays the property of resonance. The contributing Lewis structures are resonance forms. The true structure, a hybrid of the resonance forms, is known as a resonance hybrid and may be represented as ... 

A molecule or ion for which two or more valid Lewis structures can be drawn, differing only in the placement of the valence electrons. These Lewis structures are called resonance forms or resonance structures. Individual resonance forms do not exist, but we can estimate their relative energies. The more important (lower-energy) structures are called major contributors, and the less important (higher-energy) structures are called minor contributors. When a charge is spread over two or more atoms by resonance, it is said to be delocalized and the molecule is said to be resonance stabilized, (pp. 14-18)... 

Resonance forms must be valid Lewis structures and obey normal rules of valency. 

Resonance forms must he valid Lewis structures and obey normal rules of valency. A resonance form is like any other structure The octet rule still applies.. For example, one of the following structures for the acetate ion is noV a valid resoriance fViv because the carbon atom has five bonds and ten valence electrons ... 

Some molecules can t be adequately represented by a single Lewis structure. For example, two valid Lewis structures can be drawn for the anion (HCONH). One stmcture has a negatively charged N atom and a C - O double bond the other has a negatively charged O atom and a C - N double bond. These structures are called resonance structures or resonance forms. A doubleheaded arrow is used to separate two resonance structures. 

RULE 4 Resonance forms must be valid Lewis structures and obey normal rules of valency. A resonance form is like any other structure The octet... 

SAMPLE PROBLEM 10.5 Writing Lewis Structures for Octet-Rule Exceptions Problem Write Lewis structures for (a) H3PO4 (pick the most likely structure) (b) BFCI2. Plan We write each Lewis structure and examine it for exceptions to the octet rule. In (a), the central atom is P, which is in Period 3, so it can use d orbitals to have more than an octet. Therefore, we can write more than one Lewis structure. We use formal charges to decide if one resonance form is more important. In (b), the central atom is B, which can have fewer than an octet of electrons. 

Problem Draw a Lewis structure and identify the octet-mle exception for (a) H3PO4 (draw two resonance forms and select the more important) (b) BFClj. 

However, in this case, swapping the single for double bonds and vice versa leads to an alternative arrangement for the Lewis structure, and the resonance model uses the idea that the real structure is made up of a combination of the two resonance structures, which is represented by a double-headed arrow. By extension, the more resonance forms that can be drawn, the more thermodynamically stable is the system. 

Such an extension requires that we relax our definitions of correct and incorrect Lewis structures and broadly regard all resonance forms as potential contributors to the true picture of a molecule. The task is then to recognize which resonance form is the most important one. In other words, which one is the major resonance contributor Here are some guidelines. 

When two or more Lewis structures differing only in the positions of the electrons are needed to describe a molecule, they are called resonance forms. None correctly describes the molecule, its true representation being an average (hybrid) of all its Lewis structures. If the resonance forms of a molecule are unequal, those which best satisfy the rules for writing Lewis structures and the electronegativity requirements of the atoms are more important. 

This approach does not, however, account for a special type of heteroatoms that possesses expanded valence electron shells, i.e., those in their hypervalent state. A long-known and amply discussed example of the heteroaromatic compounds containing such a heteroatom is given by l,6,6aA -trithiapental-ene and related compounds 1. The structures of the parent trithiapentalene 1 (X = Y = S, A = B = CH, Z = C) and its symmetrically substituted derivatives and analogues have symmetry. Therefore, they cannot be represented by a single Lewis structure and have to be treated as either no bond resonance la la or resonance of the two forms lb lb containing three-center, four-electron Y—X—Y bonds. 

The Lewis structure and structural formula of one resonance form of N03 are shown in Structures Q.16 and Q.17. The ion has three repulsion axes and is planar with a C3 axis. There are three C2 axes perpendicular to the C3 axis, and so the nitrate ion has D3j, symmetry. 

It IS good chemical practice to represent molecules by their most stable Lewis structure The ability to write alternative resonance forms and to compare their relative stabilities however can provide insight into both molecular structure and chemical behavior This will become particularly apparent m the last two thirds of this text where the resonance concept will be used regularly... 

The table gives the computed spin densities for each atom (the value in parenthese following the substituent is its electronegativity). The illustrations are Lewis doi structures showing the primary resonance form for each structure and indicatinj unpaired electrons and lone pairs. 

Draw the most important Lewis structure for each of the following molecules. Show all lone pairs and formal charges. Draw all equivalent resonance forms, (a) HONCO (b) H2CSO (c) H2CNN (d) ONCN. 

C09-0108. Carbon, nitrogen, and oxygen form two different polyatomic ions cyanate ion, NCO, and isocyanate ion, CNO". Write Lewis stmctures for each anion, including near-equivalent resonance structures and indicating formal charges. 

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