DOCSLIB.ORG

Chapter 15 Benzene and Aromaticity Aromatic Compounds

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Chapter 15 Benzene and Aromaticity Aromatic Compounds Chapter 15 Benzene and Aromaticity Aromatic Compounds • Aromatic – Originally used to describe fragrant substances – Refers to a class of compounds that meets Hückel criteria for aromaticity 2 Aromatic Compounds • Aromatic – Originally used to describe fragrant substances – Refers to a class of compounds that meets Hückel criteria for aromaticity 3 Aromatic Compounds • The Hückel 4n + 2 Rule – Developed by Erich Hückel in 1931 – States that a molecule can be aromatic only if: • It has a planar, monocyclic system of conjugation • It contains a total of 4n + 2 molecules – n = 0,1,2,3… • 4n electrons are considered antiaromatic 4 Aromatic Compounds: Source • Coal and petroleum are the major sources of simple aromatic compounds • Coal primarily comprises Coal of large arrays of conjoined benzene-like rings • When heated to 1000°C, coal thermally breaks down to yield coal tar A representative structure of bituminous coal Proc. Natl. Acad. Sci. USA 79, 3365 (1982) https://grist.files.wordpress.com/2013/09/lump-o-coal.jpg 5 Aromatic Compounds: Source Fractional distillation of coal tar yields many aromatic compounds © 2016 Cengage Learning. 6 Aromatic Compounds: Source • Petroleum primarily comprises alkenes and few aromatic compounds • Formation of more aromatic molecules occur when alkanes are passed over a catalyst Petroleum at high pressure and temperature http://www.investigroup.com/wp-content/uploads/2014/07/5.jpg 7 Aromatic Compounds: Nomenclature • Aromatic compounds naming system uses: – Nonsystematic names © 2016 Cengage Learning. 8 Aromatic Compounds: Nomenclature • Aromatic compounds naming system uses: – Nonsystematic names – International Union of Pure and Applied Chemistry (IUPAC) Rules • Allows use of widely used names 9 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Monosubstituted benzenes have systematic names with –benzene being the parent name © 2016 Cengage Learning. 10 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Arenes are alkyl-substituted benzenes • Alkyl-substituent benzenes are smaller than the ring (<6 carbons) – Phenyl-substituted benzenes • Phenyl-substituted benzenes are larger than the ring (>7 carbons) – The term phenyl (Ph or Φ) is used in substituent benzene ring – C6H5 – The term benzyl is used for the C6H5CH2– group 11 © 2016 Cengage Learning. Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Disubstituted Benzenes • Names based on the placement of substituents – Ortho- is 1,2 disubstituted – Meta- is 1,3 disubstituted – Para- is 1,4 disubstituted – Provides clarity in the discussion of reactions © 2016 Cengage Learning. 12 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Disubstituted Benzenes • Names based on the placement of substituents – Ortho (o), meta (m) , and para (p) » Provide clarity in the discussion of reactions © 2016 Cengage Learning. 13 Aromatic Compounds: Nomenclature • International Union of Pure and Applied Chemistry (IUPAC) Rules – Benzenes +2 or more substituents • Numbers with the lowest possible values are chosen • List substituents alphabetically with hyphenated numbers • Common names, such as toluene can serve as root name(as in TNT) © 2016 Cengage Learning. 14 Worked Example • Provide the IUPAC name for the following compound • Solution: – The compound is 1-Ethyl-2,4-dinitrobenzene • Substituents on trisubstituted rings receive the lowest possible numbers © 2016 Cengage Learning. 15 STRUCTURE AND STABILITY OF BENZENE 16 Aromatic Compounds: Stability of Benzene • The reactivity of benzene is much lesser than that of alkenes despite having six fewer hydrogens – Benzene - C6H6 – Cycloalkane - C6H12 © 2016 Cengage Learning. 17 Aromatic Compounds: Stability of Benzene Comparison of the heats of hydrogenation proves the stability of benzene Remember • Heat of Hydrogenation is the heat produced when alkene is reduced to an alkane – Alkene with lower (less negative) value is more stable – Reduction is exothermic (converting weaker pi bond to stronger sigma bond) – Depends on degree of substitution of double bond (greater substitution, lower heat of hydrogenation) – Trans alkene is lower than cis alkene © 2016 Cengage Learning. 18 Aromatic Compounds: Stability of Benzene Comparison of the heats of hydrogenation proves the stability of benzene 19 © 2016 Cengage Learning. Aromatic Compounds: Structure of Benzene • All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds • Electron density in all six C-C bonds is identical • Structure is planar, hexagonal 1.39 Å © 2016 Cengage Learning. 20 Aromatic Compounds: Structure of Benzene • Carbon atoms and p orbitals in benzene are equivalent – Impossible to define three localized bonds in which a given p orbital overlaps only one neighboring p orbital • All electrons move freely in the entire ring due to equal overlap of all p orbitals 1.39 Å © 2016 Cengage Learning. 21 Aromatic Compounds: Structure of Benzene • Structure is in resonance – Resonance influences its rate of reactivity 1.39 Å © 2016 Cengage Learning. 22 Aromatic Compounds: Structure of Benzene • Benzene resonance forms can be represented by a single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds – The ring does not indicate the number of electrons in the ring but is a reminder of the delocalized structure © 2016 Cengage Learning. 23 Aromatic Compounds: Structure of Benzene • Molecular orbital description of benzene – The 6 p-orbitals combine to give: • Three bonding orbitals with 6 electrons • Three antibonding with no electrons • Orbitals with the same energy are degenerate © 2016 Cengage Learning. 24 Aromatic Compounds • Observations about benzene and benzene like aromatic compounds – Unusually stable - Heat of hydrogenation 150 kJ/mol less negative than a hypothetical cyclic triene – Planar hexagon - Bond angles are 120°, carbon- carbon bond length is 139 pm – Undergoes substitution rather than electrophilic addition – Resonance hybrid with structure between two line-bond structures 25 AROMATICITY AND THE HÜCKEL 4N+2 RULE 26 Aromatic Compounds: Hückel Rule • States that a molecule can be aromatic only if: – It has a planar, monocyclic system of conjugation – It contains a total of 4n + 2 electrons • n = 0,1,2,3… • Antiaromatic if 4n electrons are considered 27 Aromatic Compounds: Hückel Rule • Does molecule contain (4n+2) or 4n pi electrons – Cyclobutadiene • Four pi electrons • Antiaromatic – It reacts readily and exhibits none of the properties corresponding to aromaticity – It dimerizes by a Diels-Alder reaction at –78 °C © 2016 Cengage Learning. 28 Aromatic Compounds: Hückel Rule • Does molecule contain (4n+2) or 4n pi electrons – Benzene possesses six electrons (4n + 2 = 6 when n = 1) – Aromatic © 2016 Cengage Learning. 29 Aromatic Compounds: Hückel Rule • Does molecule contain (4n+2) or 4n pi electrons – Cyclooctatetraene possesses eight electrons – Not aromatic – Comprises four double bonds © 2016 Cengage Learning. 30 Aromatic Compounds: Stability and Molecular Orbital Theory • Molecular orbitals for cyclic conjugated molecules – Always contain a single lowest-lying MO – Above lowest MO, MOs come in Energy Levels of the Six Benzene degenerate pairs Molecular Orbitals © 2016 Cengage Learning. 31 Worked Example • To be aromatic, a molecule must have 4n + 2 electrons and must have a planar, monocyclic system of conjugation – Explain why cyclodecapentaene has resisted all attempts at synthesis though it has fulfilled only one of the above criteria 32 Worked Example • Solution: – Cyclodecapentaene possesses 4n + 2 (n = 2) but is not flat – If cyclodecapentaene were flat, the starred hydrogen atoms would crowd each other across the ring • To avoid this interaction, the ring system is distorted from planarity © 2016 Cengage Learning. 33 Aromatic Ions • The 4n + 2 rule applies to ions as well as neutral substances – Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic © 2016 Cengage Learning. 34 Aromatic Ions • How are ions aromatic? – Starting with a neutral saturated hydrocarbon – Remove one hydrogen from the saturated CH2 – Rehybridize the carbon from sp3 to sp2 – Result is a fully conjugated product with a p orbital on every product © 2016 Cengage Learning. 35 Aromatic Ions • Methods to remove hydrogen from saturated CH2 – Removing the hydrogen with both electrons (H:–) from the C–H bond results in a carbocation – Removing the hydrogen with one electron (H·) from the C–H bond results in a carbon radical – Removing the hydrogen without any electrons (H+) from the C–H bond results in a carbanion © 2016 Cengage Learning. 36 Aromatic Ions: Cyclopentadienyl Anion • Disadvantages of the four--electron cyclopentadienyl cation and the five-- cyclopentadienyl radical – Highly reactive – Difficult to prepare – Not stable enough for aromatic systems • Advantages of using the six--electron cyclopentadienyl cation – Easily prepared – Extremely stable – pKa =16 • Acidicty of a hydrogen atom © 2016 Cengage Learning. 37 Worked Example • Cyclooctatetraene readily reacts with potassium metal to form the stable 2– cyclooctatetraene dianion, C8H8 – Explain why this reaction occurs so easily – Determine the geometry for the cyclooctatetraene dianion © 2016 Cengage Learning. 38 Worked Example • Solution: – When cyclooctatetrene accepts two electrons,
Load more

Recommended publications
  • 69-4845 BEGLAND, Robert Walter, 1941
    This dissertation has been microfilmed exactly as received 69-4845 BEGLAND, Robert Walter, 1941- PARTICIPATION AND STERIC EFFECTS OF NEIGHBORING DIVALENT OXYGEN. The Ohio State University, Ph.D., 1968 Chemistry, organic University Microfilms, Inc., Ann Arbor, Michigan PARTICIPATION AND STEPJ.C EFFECTS OF NEIGHBORING DIVALENT OXYGEN DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Robert Walter Beglnnd, B.S., M.S. * it * * it it * The Ohio State University 1968 Approved by Adviser \J Departmeirt of Chemistry DEDICATION This dissertation is dedicated to my w ife June, ny con Michael Brian and my son Douglas Samuel. i i ACKNOWLEDGMENT The author wishes to express his appreciation to Dr. L. A. Paquette for his guidance, encouragement and considerable time spent in the course of this research. The author also wishes to express his appreciation to his parents, Mr. and Mrs. Walter C. Bcgland, for their assistance and encouragement throughout his college training, and to lir. and Mrs. Lester Berndt. i i i VITA July 23, 1941 Born - Oak Park, Illinois 1963 B.S. in Education, The Ohio State University Columbus, Ohio 1965 M.S., The Ohio State University Columbus, Ohio 1968 Ph.D., The Ohio State University Columbus, Ohio PUBLICATIONS 1. Transannular Participation of Ether Oxygen in the Hydrolysis of a Mesocyclic Dienamine, J. Am. Chen. Soc. 87, 3784 (1965). 2. Stabilized Derivatives of cis,cis,cis-1,3.5-Cyclodecatricne Keto-Enol Tautomerism in 2,3-DicarboiEeth.oxy-cis,c is -3,5~ cyclodecadienones and c.is-3-Cycloalkenones.
    [Show full text]
  • 8.3 Bonding Theories >
    8.3 Bonding Theories > Chapter 8 Covalent Bonding 8.1 Molecular Compounds 8.2 The Nature of Covalent Bonding 8.3 Bonding Theories 8.4 Polar Bonds and Molecules 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals Molecular Orbitals How are atomic and molecular orbitals related? 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals • The model you have been using for covalent bonding assumes the orbitals are those of the individual atoms. • There is a quantum mechanical model of bonding, however, that describes the electrons in molecules using orbitals that exist only for groupings of atoms. 3 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals • When two atoms combine, this model assumes that their atomic orbitals overlap to produce molecular orbitals, or orbitals that apply to the entire molecule. 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals Just as an atomic orbital belongs to a particular atom, a molecular orbital belongs to a molecule as a whole. • A molecular orbital that can be occupied by two electrons of a covalent bond is called a bonding orbital. 5 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 8.3 Bonding Theories > Molecular Orbitals Sigma Bonds When two atomic orbitals combine to form a molecular orbital that is symmetrical around the axis connecting two atomic nuclei, a sigma bond is formed. • Its symbol is the Greek letter sigma (σ).
    [Show full text]
  • Molecular Orbital Theory
    Molecular Orbital Theory The Lewis Structure approach provides an extremely simple method for determining the electronic structure of many molecules. It is a bit simplistic, however, and does have trouble predicting structures for a few molecules. Nevertheless, it gives a reasonable structure for many molecules and its simplicity to use makes it a very useful tool for chemists. A more general, but slightly more complicated approach is the Molecular Orbital Theory. This theory builds on the electron wave functions of Quantum Mechanics to describe chemical bonding. To understand MO Theory let's first review constructive and destructive interference of standing waves starting with the full constructive and destructive interference that occurs when standing waves overlap completely. When standing waves only partially overlap we get partial constructive and destructive interference. To see how we use these concepts in Molecular Orbital Theory, let's start with H2, the simplest of all molecules. The 1s orbitals of the H-atom are standing waves of the electron wavefunction. In Molecular Orbital Theory we view the bonding of the two H-atoms as partial constructive interference between standing wavefunctions of the 1s orbitals. The energy of the H2 molecule with the two electrons in the bonding orbital is lower by 435 kJ/mole than the combined energy of the two separate H-atoms. On the other hand, the energy of the H2 molecule with two electrons in the antibonding orbital is higher than two separate H-atoms. To show the relative energies we use diagrams like this: In the H2 molecule, the bonding and anti-bonding orbitals are called sigma orbitals (σ).
    [Show full text]
  • Inorganic Chemistry/Chemical Bonding/MO Diagram 1
    Inorganic Chemistry/Chemical Bonding/MO Diagram 1 Inorganic Chemistry/Chemical Bonding/MO Diagram A molecular orbital diagram or MO diagram for short is a qualitative descriptive tool explaining chemical bonding in molecules in terms of molecular orbital theory in general and the Linear combination of atomic orbitals molecular orbital method (LCAO method) in particular [1] [2] [3] . This tool is very well suited for simple diatomic molecules such as dihydrogen, dioxygen and carbon monoxide but becomes more complex when discussing polynuclear molecules such as methane. It explains why some molecules exist and not others, how strong bonds are, and what electronic transitions take place. Dihydrogen MO diagram The smallest molecule, hydrogen gas exists as dihydrogen (H-H) with a single covalent bond between two hydrogen atoms. As each hydrogen atom has a single 1s atomic orbital for its electron, the bond forms by overlap of these two atomic orbitals. In figure 1 the two atomic orbitals are depicted on the left and on the right. The vertical axis always represents the orbital energies. The atomic orbital energy correlates with electronegativity as a more electronegative atom holds an electron more tightly thus lowering its energy. MO treatment is only valid when the atomic orbitals have comparable energy; when they differ greatly the mode of bonding becomes ionic. Each orbital is singly occupied with the up and down arrows representing an electron. The two AO's can overlap in two ways depending on their phase relationship. The phase of an orbital is a direct consequence of the oscillating, wave-like properties of electrons.
    [Show full text]
  • Structure of Benzene, Orbital Picture, Resonance in Benzene, Aromatic Characters, Huckel’S Rule B
    Dr.Mrityunjay Banerjee, IPT, Salipur B PHARM 3rd SEMESTER BP301T PHARMACEUTICAL ORGANIC CHEMISTRY –II UNIT I UNIT I Benzene and its derivatives 10 Hours A. Analytical, synthetic and other evidences in the derivation of structure of benzene, Orbital picture, resonance in benzene, aromatic characters, Huckel’s rule B. Reactions of benzene - nitration, sulphonation, halogenationreactivity, Friedelcrafts alkylation- reactivity, limitations, Friedelcrafts acylation. C. Substituents, effect of substituents on reactivity and orientation of mono substituted benzene compounds towards electrophilic substitution reaction D. Structure and uses of DDT, Saccharin, BHC and Chloramine Benzene and its Derivatives Chemists have found it useful to divide all organic compounds into two broad classes: aliphatic compounds and aromatic compounds. The original meanings of the words "aliphatic" (fatty) and "aromatic" (fragrant/ pleasant smell). Aromatic compounds are benzene and compounds that resemble benzene in chemical behavior. Aromatic properties are those properties of benzene that distinguish it from aliphatic hydrocarbons. Benzene: A liquid that smells like gasoline Boils at 80°C & Freezes at 5.5°C It was formerly used to decaffeinate coffee and component of many consumer products, such as paint strippers, rubber cements, and home dry-cleaning spot removers. A precursor in the production of plastics (such as Styrofoam and nylon), drugs, detergents, synthetic rubber, pesticides, and dyes. It is used as a solvent in cleaning and maintaining printing equipment and for adhesives such as those used to attach soles to shoes. Benzene is a natural constituent of petroleum products, but because it is a known carcinogen, its use as an additive in gasoline is now limited. In 1970s it was associated with leukemia deaths.
    [Show full text]
  • Reactions of Aromatic Compounds Just Like an Alkene, Benzene Has Clouds of  Electrons Above and Below Its Sigma Bond Framework
    Reactions of Aromatic Compounds Just like an alkene, benzene has clouds of electrons above and below its sigma bond framework. Although the electrons are in a stable aromatic system, they are still available for reaction with strong electrophiles. This generates a carbocation which is resonance stabilized (but not aromatic). This cation is called a sigma complex because the electrophile is joined to the benzene ring through a new sigma bond. The sigma complex (also called an arenium ion) is not aromatic since it contains an sp3 carbon (which disrupts the required loop of p orbitals). Ch17 Reactions of Aromatic Compounds (landscape).docx Page1 The loss of aromaticity required to form the sigma complex explains the highly endothermic nature of the first step. (That is why we require strong electrophiles for reaction). The sigma complex wishes to regain its aromaticity, and it may do so by either a reversal of the first step (i.e. regenerate the starting material) or by loss of the proton on the sp3 carbon (leading to a substitution product). When a reaction proceeds this way, it is electrophilic aromatic substitution. There are a wide variety of electrophiles that can be introduced into a benzene ring in this way, and so electrophilic aromatic substitution is a very important method for the synthesis of substituted aromatic compounds. Ch17 Reactions of Aromatic Compounds (landscape).docx Page2 Bromination of Benzene Bromination follows the same general mechanism for the electrophilic aromatic substitution (EAS). Bromine itself is not electrophilic enough to react with benzene. But the addition of a strong Lewis acid (electron pair acceptor), such as FeBr3, catalyses the reaction, and leads to the substitution product.
    [Show full text]
  • 18. Nucleophilic Sigma Bonds
    Professor David L. Van Vranken Chemistry 201: Organic Reaction Mechanisms I Topic 18: Nucleophilic Sigma Bonds E+ E+ E+ R Li R C R H References: Literature cited Recall the Six Types of Canonical Frontier Orbitals ■ We’ve already discussed the interactions of 3 types of filled orbitals and 3 types of unfilled orbitals. If all things are equal then C-C and C-H sigma bonds should be the least reactive type of nucleophiles but if we replace C with Li to make a C-Li bond then the bonds became exquisitely reactive. σ* π* Li E+ p H E+ - E n B MO H H π H H E+ σ -O ■ We need to discuss the nucleophilicity of sigma bonds more broadly. Periodic Trends - Electronegativity ■ Here are some periodic trends you should know… electronegative rapid atom transfer pi character weaker, more nucleophilic bonds electropositive http://www.green-planet-solar-energy.com/electronegativity-values.html Frontier Molecular Orbital Energies Predict Reactivity ■ What happens when you replace carbon with an electropositive atom? Bonds to electropositive atoms can be highly nucleophilic. E+ faster .. nO(Li) O Li faster nO(Be) E E+ E MO MO nO(B) E+ slower σC-Li H3C Li .. nO(C) O CH3 σC-Be σC-B E+ E+ σC-C H3C CH3 σO-Li O Li σC-N ■ For bonds to O and N, the non-bonding FMO (i.e., the lone pair) is still more reactive than the sigma bond FMO. Alkyllithiums: Frontside vs. Backside Attack ■ Frontside attack is most common for R-Li. Et Et Still, W.
    [Show full text]
  • 6902206.PDF (3.699Mb)
    This dissertation has been microfilmed exactly as received 69-2206 IRE LAN, John Ralph Smiley, 1935- 9, lO-DlSUBSTlTUTED-9, lO-DlHYDRONAPHTHALENE CHEMISTRY. The University of Oklahoma, Ph.D., 1968 Chemistry, organic University Microfilms, Inc., An n Arbor, Michigan THE UNIVERSITY OF OKLAHOMA GRADUATE COLLEGE 9,10-DISUBSTITUTED-9,10-DIHYDR0NAPHTHALENE CHEMISTRY A DISSERTATION SUBMITTED TO THE GRADUATE FACULTY in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY BY JOHN RALPH SMILEY IRELAN Norman, Oklahoma 1968 9,10-DISUBSTITUTED-9,lO-DIHYDROWAPHTHALENE CHEMISTRY APPRCWED BY DISSERTATION COMMITTEE To Nancy and Our Parents ACKNOWLEDGMENTS The author wishes to express his appreciation to Dr. J. J. Bloomfield, who suggested these problems, for his council and assistance during the course of this work. Ap­ preciation is extended to the University of Oklahoma for financial aid in the form of teaching assistantships and for research assistantships on National Science Foundation Grants GP 260 and GP 44^9. Appreciation is also extended to Dr. Tom Karns, Mr. A1 Peters, and Dr. Francis J. Schmitz for their aid in the absence of Dr. Bloomfield. To many members of the faculty and my fellow graduate students are given my thanks for their assistance and fellowship. Finally, the author wishes to express his love and thanks to his wife, Nancy, whose patience, effort, and en­ couragement made this work possible. IV TABLE OF CONTENTS Page LIST OF TABLES .............................. vi INTRODUCTION................................. 1 Recent Literature ....................... 20 RESULTS AND DISCUSSION ....................... 26 Part I, Five-Membered Lactones ........... 26 Summary ................................ 36 Part II, Five-Membered Bridges ........... 37 Summary ................................ 46 Part III, Four-Membered Rings ...........
    [Show full text]
  • I 1.9 Aromotic Compounds
    5t8 CHAPTERI I CarbonChains and Rings PRACTICE EXERCISEII.S \ Draw structural formulas for the following alkenes.If a compound has geometric isomers, draw both the cls and the transforms. (a) l-pentene (b) 2-hexene (c) 2-methyl-2-hexene (d) 2,3-dimethyl-2-butene Alkynes Organic compounds containing carbon-carbon triple bonds are called alkynes. Like alkenes,alkynes are unsaturated compounds. This is the car- bon-carbon triple bond found in alkynes: -C:C- Allqmes are quite rare in nature. The simplest alkyne is the gasethlme, C2H2. The common name for ethyne is acetylene-the fuel burned in oxyacety- lene torches used in welding. The single bonds that extend from the car- bons involved in the carbon-carbon Figurel l .l0 triple bond of ethpre are separatedby (Fig. Ball-and-stickmodel of ethyne the maximum bond angle, IB0 degrees f f .10). Ethyne is a linear (commonname acetylene). molecule. The development of an orbital picture for ethyne requires us to go through the same steps that we used for methane and ethene: hybridiza- tion of carbon atomic orbitals and orbital overlap. As with methane and ethene,remember that each orbital representsone electron.The descrip- tion of ethyne that best flts the experimental data is obtained if the 2s atomic orbital of carbon is mixed with only one of the three available 2p atomic orbitals (Fig. f 1.11).The result of this mixing is two sp atomic orbitals; two 2p orbitals remain unused. Overlap of one of the sp hybrid orbitals of one carbon with that of another carbon producesa carbon-car- bon sigma bond.
    [Show full text]
  • Valence Bond Theory
    Valence Bond Theory • A bond is a result of overlapping atomic orbitals from two atoms. The overlap holds a pair of electrons. • Normally each atomic orbital is bringing one electron to this bond. But in a “coordinate covalent bond”, both electrons come from the same atom • In this model, we are not creating a new orbital by the overlap. We are simply referring to the overlap between atomic orbitals (which may or may not be hybrid) from two atoms as a “bond”. Valence Bond Theory Sigma (σ) Bond • Skeletal bonds are called “sigma” bonds. • Sigma bonds are formed by orbitals approaching and overlapping each other head-on . Two hybrid orbitals, or a hybrid orbital and an s-orbital, or two s- orbitals • The resulting bond is like an elongated egg, and has cylindrical symmetry. Acts like an axle • That means the bond shows no resistance to rotation around a line that lies along its length. Pi (π) Bond Valence Bond Theory • The “leftover” p-orbitals that are not used in forming hybrid orbitals are used in making the “extra” bonds we saw in Lewis structures. The 2 nd bond in a double bond nd rd The 2 and 3 bonds in a triple bond /~harding/IGOC/P/pi_bond.html • Those extra bonds form only after the atoms are brought together by the formation of the skeletal bonds made by www.chem.ucla.edu hybrid orbitals. • The “extra” π bonds are always associated with a skeletal bond around which they form. • They don’t form without a skeletal bond to bring the p- orbitals together and “support” them.
    [Show full text]
  • BEA-Ha,5,7,9Nfentaenve PART 11 the REARRANGEMENTS of Monosuefitwuted CYCLOOCTATETRAEHEB
    PART 1 SWQIES CONCERNED WETH THE EYfoéxESES CF QCTALEEQE ANfl BICYCLE [@3‘0]BEA-ha,5,7,9nFENTAENVE PART 11 THE REARRANGEMENTS OF MONOSUEfiTWUTED CYCLOOCTATETRAEHEB Thesis 5o;- Nac Dogma of DE. D. MiCHlGAN STATE UNIVERSITY Greg A. Bullock 1968 g is r' ,_ p, Viv-f L i u .- .4 4< 1 ' ‘lichigan Sm é University This is to certify that the thesis entitled Part I: STUDIES CONCERNED WITH THE SYNTHESIS OF OCTALENE .,AND BICYCLO[6.2.0]DECA-l,3,5,7,9-PENTAENE Part II: THE REARRANGEMENTS 0F MDNOSUBSTITUTED CYCLOOCTATETRAENES presented by Greg A. Bullock ._ _.._I-_ 4 has been accepted towards fulfillment of the requirements for .-—*<7-» -.. Ph.D. degree in Chemistry Major pro sor _ Datuéymililfé? .-_ ._ 0-169 .I.‘—..._._.,._._-_.—_ -a_..-_.-__.IV.._._~_ .I ABSTRACT PART I STUDIES CONCERNED WITH THE SYNTHESIS OF OCTALENE AND BICYCLO[6.2.0]DECA-1,3,5,7,9-PENTAENE PART II THE REARRANGEMENTS OF MONOSUBSTITUTED CYCLOCCTATETRAENES by Greg A. Bullock The initial goal of this investigation concerned the synthesis of bicyclo[6.6.0]tetradeca-1,3,5,7,9,11,13-heptaene (octalene). Several synthetic approaches leading to the preparation of octalene were attempted. None of these ap- proaches, however, was successful. During the course of this investigation a new synthetic route to [2.2]paracyclo- phanes was discovered. 4.5.12,13—Tetracarbomethoxy[2.2]para— cyclophane was synthesized from dimethyl 3,6-bis(hydroxy— methyl)-1,4-cyclohexadiene-1,2-dicarboxylate diepftoluene- sulfonate by solvolysis in buffered acetic acid.
    [Show full text]
  • Chapter 1: Electronic Structure and Bonding a Brief Review of General Chemistry
    Chapter 1: Electronic Structure and Bonding Or A Brief Review of General Chemistry Part 2: Quantum Mechanics and Molecular Orbital Theory Review ideas from general chemistry: atoms, bonds, molecular geometry Hybrid Orbitals of Ethene, C2H4 Bonding in Ethene (Ethylene): A Double Bond The sigma bond is unaffected by rotation of one of the CH2 groups. The overlap of the p orbitals is disrupted by rotation of one of the CH2 groups. This would cause the double bond to break. Hybrid Orbitals of Ethene, C2H4 An sp2-Hybridized Carbon - The bond angle in the sp2 carbon is 120°. The sp2 carbon is the trigonal planar carbon. Molecular Orbital Theory So let’s look at Ethyne (Acetylene) . Draw a Lewis Dot Structure for Ethyne Hybrid Orbitals of Ethyne, C2H2 Bonding in Ethyne (Acetylene) : A Triple Bond A triple bond consists of one s bond and two p bonds. Bond angle of the sp carbon: 180°. Molecular Orbital Theory + - + Molecular Orbital Theory Summary The sharing of one pair of electrons is a single bond. The sharing of two pairs gives a double bond. The sharing of three pairs gives a triple bond. The greater the electron density in the region of orbital overlap, the stronger the bond. The more s character, the shorter and stronger is the bond. The more s character, the larger is the bond angle. A bond is weaker than a bond. Hybridization of Nitrogen and Oxygen Elements other than C can have hybridized orbitals H–N–H bond angle in ammonia (NH3) 107.3° C-N-H bond angle is 110.3 ° N’s orbitals (sppp) hybridize to form four sp3 orbitals One sp3 orbital is occupied by two nonbonding electrons, and three sp3 orbitals have one electron each, forming bonds to H and CH3.
    [Show full text]