Chemistry
Resonance Chemistry
Resonance in chemistry refers to the concept where multiple Lewis structures can be drawn for a molecule, and the actual structure is a hybrid of these resonance forms. It helps explain the stability and reactivity of molecules, particularly those with delocalized electrons. Resonance is often depicted using curved arrows to show the movement of electrons.
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4 Key excerpts on "Resonance Chemistry"
- L Christophorou(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
The molecular excited states with which inner-shell resonances are as-sociated contain an atomic inner-shell electron that has been promoted to an unfilled valence or Rydberg orbital of the molecule. These states may be identified using photoabsorption or electron energy loss spectroscopy, and the resonance states give rise to structure in the electron-impact ionization functions (King et a/., 1980) Table II summarizes the molecular inner-shell resonances reported. More recently, five positive-ion decay channels for the (lsC) -1 (2 ) 2 state have been studied by Ziesel et al. (1979) and branching ratios have been determined. The decay of the resonance occurs mainly by dissociation of CO + by double Auger transitions in which an extra electron is ejected by the shakeoff process (see Kay et al, 1977). F. Resonances and Theoretical Chemistry The importance of resonances to theoretical chemistry is becoming increasingly clear, and is likely to stimulate continued research. The tasks of theoretical chemistry include not only the achievement of the ability to predict the stability of molecules by quantitative understanding of the nature and the energies of all molecular orbitals, but also the ability to predict reactivities in a wide variety of situations. One of the factors which influences reactivity is the electrophilicity of a molecule—that is, its ability to attract electrons. 5 Electron-Molecule Resonances 423 The energies of the occupied orbitals in a molecule are determined experimentally by the methods of photoelectron spectroscopy. The first ionization potential provides the energy of the highest occupied molecular orbital (HOMO), and the excitation energies of the first positive ion are those of deeper occupied orbitals. The first electron affinity represents the energy of the lowest unoccupied molecular orbital (LUMO), and the temporary states of the negative ion correspond to the energies of the higher unoccupied orbitals.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
Resonance Stabilization We developed the concept of resonance using the allyl cation as an example, and we saw that the two π electrons are spread out over the three carbon atoms of the allylic system. This spreading of electrons, called delocalization, is a stabilizing factor. That is, molecules and ions are stabilized by the delocalization of electrons. This stabilization is often referred to as resonance stabilization, and the allyl cation is said to be resonance stabilized. Resonance stabilization plays a major role in the outcome of many reactions, and we will invoke the concept of resonance in almost every chapter of this textbook. The study of organic chemistry therefore requires a thorough mastery of drawing resonance structures, and the following sections are designed to foster the necessary skills. 2.8 Curved Arrows In this section, we will focus on curved arrows, which are the tools necessary to draw resonance structures properly. Every curved arrow has a tail and head: Tail Head Curved arrows used for drawing resonance structures do not represent the motion of electrons— they are simply tools that allow us to draw resonance structures with ease. These tools treat the elec- trons as if they were moving, even though the electrons are actually not moving at all. In Chapter 3, we will encounter curved arrows that actually do represent the flow of electrons. For now, keep in mind that all curved arrows in this chapter are just tools and do not represent a flow of electrons. It is essential that the tail and head of every arrow be drawn in precisely the proper location. The tail shows where the electrons are coming from, and the head shows where the electrons are going (remember, the electrons aren’t really going anywhere, but we treat them as if they were for the purpose of drawing the resonance structures). We will soon learn patterns for drawing proper curved arrows. But, first, we must learn where not to draw curved arrows.
eBook - PDF- David R. Klein(Author)
- 2016(Publication Date)
- Wiley(Publisher)
This entity, often called the resonance hybrid, is a combina- tion of the individual resonance structures. • A resonance hybrid can be drawn by using partial bonds and partial charges to illustrate the delocalization of elec- trons. SECTION 2.13 • A delocalized lone pair participates in resonance and occu- pies a p orbital. • A localized lone pair does not participate in resonance. • Whenever an atom possesses both a π bond and a lone pair, they will not both participate in resonance. SkillBuilder Review 85 SKILLBUILDER REVIEW 2.1 CONVERTING BETWEEN DIFFERENT DRAWING STYLES Draw the Lewis structure of this compound. (CH 3 ) 3 COCH 3 O CH 3 C CH 3 CH 3 H 3 C STEP 1 Draw each group separately. C C H H H O C H H H H H H C C H H H STEP 2 Draw all C H bonds. Try Problems 2.1, 2.2, 2.43, 2.44, 2.48, 2.49 2.2 READING BOND-LINE STRUCTURES STEP 1 The end of every line represents a carbon atom. STEP 2 Each carbon atom will possess enough hydrogen atoms in order to achieve four bonds. N N O Cl N N O Cl Identify all carbon atoms and hydrogen atoms. N N Cl O H H H H H H H H H H H H H Try Problems 2.3, 2.4, 2.34, 2.48, 2.50 2.4 IDENTIFYING LONE PAIRS ON OXYGEN ATOMS A neutral oxygen atom... Oxygen with a negative charge... Oxygen with a positive charge... ...has two lone pairs. O ...has three lone pairs. ⊝ ...has one lone pair. O ⊕ O Try Problems 2.8, 2.9, 2.38, 2.39 2.3 DRAWING BOND-LINE STRUCTURES Draw a bond-line drawing of this compound. H C C O H H H O H C H H C C H C C H H H H H H STEP 1 Delete all hydrogen atoms except for those connected to heteroatoms. O OH C C C C C C C STEP 2 Draw in zigzag format,keeping triple bonds linear. O C C HO C C C C C C C STEP 3 Delete all carbon atoms. HO O C C C C Try Problems 2.5, 2.6, 2.35, 2.36, 2.42, 2.51, 2.55 86 CHAPTER 2 Molecular Representations 2.8 DRAWING SIGNIFICANT RESONANCE STRUCTURES STEP 1 Identify any atoms that lack an octet. The most significant resonance forms have the greatest number of filled octets.
- Milton Orchin, Allan R. Pinhas, R. Marshall Wilson, Roger S. Macomber(Authors)
- 2005(Publication Date)
- Wiley-Interscience(Publisher)
The π molecular orbitals of 1,3-butadiene, CH 2 CH CH CH 2 . 3.4 RESONANCE STRUCTURES Two or more valence bond structures of the same compound that have identical nuclear geometries, possess the same number of paired electrons, but differ in the spatial dis- tribution of these electrons. Resonance structures are conventionally shown as related to each other by a single double-headed arrow (↔) to emphasize that such structures are not different molecules in equilibrium, that is, they do not have separate, inde- pendent existence but are different representations of the same molecule. The actual molecule has a structure that is a hybrid of all possible resonance structures. Example. Benzene is commonly used to illustrate resonance structures. The two most important resonance structures of benzene are shown in Fig. 3.4a; these two structures are equivalent, although they place the single and double bonds in differ- ent positions. Many other resonance structures are possible including structures with long bonds, the so-called Dewar structures of benzene shown in Fig. 3.4b. The three resonance structures of CO 3 –2 shown in Fig. 3.4c are likewise equivalent. The impor- tance of resonance structures becomes apparent in explaining the fact that each C C bond in every molecule of benzene is of equal length and is intermediate between a 58 DELOCALIZED (MULTICENTER) BONDING O C O O O C O O O C O O H 2 C HC CH CH 2 H 2 C HC CH CH 2 H 2 C HC CH CH 2 O C O O -2 (a) (b) (c) (d) (e) (f) (g) Figure 3.4. Resonance structures: (a) benzene, (b) Dewar benzene, (c) carbonate anion, (d ) 1,3-butadiene; the single structure representations for (e) benzene and ( f ) carbonate anion; and (g) the circle inside the hexagon that represents delocalization of the six π elec- trons of benzene. single and double bond, as is also the case with each C O bond in CO 3 2– . Resonance structures need not be equivalent; three of the resonance structures of 1,3-butadiene are shown in Fig.
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