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Organic ChemistryDemystified

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Organic ChemistryDemystifiedDANIEL R. BLOCHMcGRAW-HILLNew YorkChicago San Francisco Lisbon LondonMadrid Mexico City Milan New DelhiSan Juan Seoul Singapore Sydney Toronto

Copyright 2006 by The McGraw-Hill Companies, Inc. Inc. All rights reserved. Manufactured in the United States of America.Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed inany form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher.0-07-148710-7The material in this eBook also appears in the print version of this title: 0-07-145920-0.All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. Where such designations appear in this book, they have been printed with initial caps.McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporatetraining programs. For more information, please contact George Hoare, Special Sales, at george [email protected] or (212)904-4069.TERMS OF USEThis is a copyrighted work and The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and its licensors reserve all rights in and to thework. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store andretrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative worksbased upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw-Hill’s prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Yourright to use the work may be terminated if you fail to comply with these terms.THE WORK IS PROVIDED “AS IS.” McGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIESAS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THEWORK, INCLUDING ANY INFORMATION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OROTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill andits licensors do not warrant or guarantee that the functions contained in the work will meet your requirements or that its operationwill be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccuracy,error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill has no responsibility forthe content of any information accessed through the work. Under no circumstances shall McGraw-Hill and/or its licensors be liablefor any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use thework, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claimor cause whatsoever whether such claim or cause arises in contract, tort or otherwise.DOI: 10.1036/0071459200

To Nan for her assistance, patience, and helpful comments.

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For more information about this title, click hereCONTENTSPrefacexviiAcknowledgmentsxixCHAPTER 1Structure and BondingAtomic StructureAtomic Number and Atomic Mass (Weight)Electron Energy LevelsThe Octet RuleValencesLewis StructuresOrbital ShapesQuantum MechanicsBond FormationValence Bond TheoryMolecular OrbitalsBonding and Antibonding MOsBonding and 3-D Molecular ShapeCurved ArrowsElectronegativity and Bond PolarityDipole MomentsFormal ChargesResonance StructuresIntermolecular ER 2Families and Functional Groups43vii

CONTENTSviiiCHAPTER 3Acids and BasesIntroductionArrhenius DefinitionBrønsted-Lowry DefinitionConjugate Acids and BasesEquilibrium ReactionsWeak Hydrocarbon AcidsLewis Acids and BasesQuiz464647474749535456CHAPTER 4Alkanes and CycloalkanesIntroductionSources of AlkanesAcyclic and Cyclic l IsomersCycloalkanesAlkyl GroupsNomenclature—Naming Compoundsby the IUPAC SystemPhysical PropertiesChemical PropertiesConformations of AlkanesConformations of CylcoalkanesQuiz59596060616262646667CHAPTER 5StereochemistryIsomersChiral CompoundsStereocentersChirality CentersEnantiomersRacemic Mixtures7074757684949999100101102105109

CONTENTSixThe R/S SystemWhen the Lowest Priority Group Is Notin the BackMolecules with Multiple AsymmetricCentersEnantiomersDiastereomersMeso CompoundsFisher ProjectionsRotating Fisher Projections/StructuresCyclic StereoisomersNaming Cyclic StereoisomersProchiral Carbons PTER 6Structure and Properties of AlkenesIntroductionStructureNaming AlkenesCommon NamesCis and Trans IsomersThe E/Z (Easy) SystemDegrees of UnsaturationStability of AlkenesPhysical Properties of AlkenesChemical PropertiesThe Curved APTER 7Reaction ationsStereochemistry145145145148150151113

CONTENTSxCHAPTER 8CHAPTER 9The Hammond PostulateRegiochemical ReactionsThe Markovnikov RuleStereochemistryRearrangement Reactions of CarbocationsQuiz152154155156157159Reactions of AlkenesReaction with Hydrogen Halides in InertSolventsReaction with Hydrogen Halides in ProticSolventsOxymercuration-Demercuration ReactionsHydroboration-ReductionHalogenation in Inert SolventsStereochemistry HalogenationHalogenation in Reactive SolventsRadical BrominationFormation of DiolsDouble Bond lkynesIntroductionStructureNomenclaturePhysical PropertiesChemical PropertiesReactions with Brønsted-Lowry 177178180180181182

CONTENTSxiReactions with HBr and PeroxidesReaction with HalogensHydration ReactionsHydroboration-Oxidation ReactionsHydrogenation/Reduction ReactionsOxidation ReactionsAcidity of AlkynesAlkylation ReactionsPreparation of AlkynesQuiz193195195198199200201202204204CHAPTER oscopyUV SpectroscopyNuclear Magnetic Resonance SpectroscopyNuclear PropertiesNMR Spectrometers1 H NMR Spectroscopy13 C NMR SpectroscopyMass 51CHAPTER reparation of HalidesRadical HalogenationIsomeric ProductsAllylic HalogenationReactions of OrganohalidesQuiz256256256257258258260263265266

CONTENTSxiiCHAPTER 12CHAPTER 13Nucleophilic Substitution andElimination ReactionsIntroductionProperties of NucleophilesProperties of BasesProperties of Substrates/ElectrophilesProperties of Leaving GroupsProperties of SolventsSecond-Order Nucleophilic Substitution(SN 2) ReactionsFirst-Order Nucleophilic Substitution(SN 1) ReactionsSummary of SN 1 and SN 2 ReactionsSecond-Order Elimination (E2)ReactionsFirst-Order Elimination (E1) ReactionsSummary of E1 and E2 ReactionsCompetition between Substitution andElimination ReactionsQuizAlcoholsIntroductionProperties of AlcoholsNomenclatureAcidity and Basicity of AlcoholsReactions with Active MetalsPreparation of AlcoholsOrganometallic CompoundsPreparation of Alcohols UsingOrganometallic ReagentsReactions of AlcoholsConversion of Alcohols to Alkyl HalidesDehydration 91291292295295296297299301301306307310311313313

CONTENTSxiiiCHAPTER ion of EthersReactions of EthersThree-Membered Ether RingsQuiz317317317318319321323327CHAPTER 15Sulfur CompoundsNomenclaturePropertiesReactions of Sulfur CompoundsQuiz330330331333335CHAPTER 16Conjugated SystemsIntroductionStability of DienesElectrophilic Addition to Conjugated DienesAllylic Cations, Radicals, and AnionsDiels-Alder ReactionsQuiz337337337344347349355CHAPTER 17Aromatic Compounds358Introduction358Reactivity of Aromatic Compounds358Nomenclature359Kekulé Structures361Stability of Benzene362The Resonance Model364Molecular Orbital Description of Aromaticity 365Properties of Aromatic, Nonaromatic, andAntiaromatic Compounds369Hückel’s Rule370Heterocyclic Compounds371

CONTENTSxivAromatic IonsPolycyclic Aromatic CompoundsQuiz372374375CHAPTER 18Reactions of Benzene and otherAromatic Compounds378Introduction378Electrophilic Aromatic Substitution379Nucleophilic Aromatic Substitution386More Derivatives of Benzene388Multiple Substitution Reactions392Electrophilic Substitution in DisubstitutedBenzenes399Quiz400CHAPTER 19Aldehydes and KetonesIntroductionNomenclaturePhysical PropertiesChemical PropertiesPreparation of Aldehydes and KetonesHydration of AlkynesReduction of Acid ChloridesReactions of Aldehydes and Ketones withNucleophilesWittig ReactionsOxidation and Reduction ReactionsQuiz404404405408408410412414Carboxylic AcidsIntroductionNomenclatureThree-Dimensional StructureMolecular Orbital (MO) DescriptionPhysical Properties430430431434434435CHAPTER 20418424425427

CONTENTSxvAcidity of Carboxylic AcidsPreparation of Carboxylic AcidsDerivatives of Carboxylic AcidsQuiz437439444444CHAPTER 21Derivatives of Carboxylic AcidsIntroductionCarboxylic Acid HalidesCarboxylic Acid AnhydridesCarboxylic EstersAmidesCyclic 8470CHAPTER 22Alpha-Substitution Reactions inCarbonyl CompoundsIntroductionEnol and Enolate AnionsAlpha Monohalogenation of Aldehydesand Ketonesα,β Unsaturated Ketonesα-Bromination of Carboxylic AcidsAcidity of α-Hydrogen AtomsMalonic Ester SynthesisAcetoacetic Ester SynthesisAdditional Condensation ReactionsQuizCHAPTER 23Carbonyl Condensation ReactionsIntroductionAldol ReactionsDehydration of Aldol CompoundsMixed or Crossed Aldol 1492493

CONTENTSxviIntramolecular Aldol ReactionsClaisen Condensation Reactionsα-Substitution ReactionsQuiz494495499502Final Exam504Quiz and Exam Solutions531Appendix A / Periodic Tableof the Elements537Bibliography539Index541

PREFACEOrganic chemistry is the chemistry of carbon-containing compounds. Everyliving organism, plant and animal, is composed of organic compounds. Anyonewith an interest in life and living things needs to have a basic understanding oforganic chemistry.Articles continue to appear in newspapers and magazines describing thedevelopment of new medicines and diagnostic tests. These new products andtechnologies are results of a better understanding of the structure and functionof DNA, proteins, and other organic biological molecules. The reactions andinteractions of these complex molecules are the same reactions and interactionsthat occur in more simple organic molecules.This text was written to help those who are intimidated by the words organicchemistry. Those who have never had a formal course in organic chemistry andstudents currently taking or planning to take a formal course will find this textan easy-to-read introduction and supplement to other texts.The chapters are written in the same general order as found in most collegetextbooks. It would be helpful, but not necessary, if the reader had a course inintroductory chemistry. The first three chapters cover the background materialtypically covered in general chemistry courses. It is not necessary that chaptersbe read sequentially, but since material tends to build on previous concepts it willbe easier to understand the material if the chapters are read in sequential order.Key terms and concepts are italicized. Be sure you understand these conceptsas they will continue to appear in other sections of this book. Questions (andanswers) are given within each chapter to help you measure your understanding.Each chapter ends with a quiz covering the material presented. Use each quizto check your comprehension and progress. The answers to quizzes are givenin the back of the text. Review those problems (immediately) you did not getcorrect. Be sure you understand the concepts before going to the next chapteras new material often builds upon previous concepts.xviiCopyright 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use.

xviiiPREFACEAs you read each chapter, take frequent breaks (you can munch on the extragum drops used to make models in Chapter 5). The book contains a lot of figuresand diagrams. Follow these as you read the text. It is often easier to understanda reaction mechanism in a diagram than to describe it in words.Yes, there is some memorization. New terms will appear that you probablyhave never heard before. For a series of terms I recommend making a mnemonicand I suggested a few. Reaction mechanisms are not as difficult as they mayappear. You can predict most reactions in that negative species will be attractedto positive species (opposites attract). Atoms with electrons to share will beattracted to species that want more electrons—it is just that simple.There is a multiple-choice final exam at the end of the text. The final examhas more general, but similar, questions than those in the quizzes. Answers aregiven in the back of the book. If you are able to answer 80% of the final examquestions correctly (the first time), you will have a good understanding of thematerial.I hope you will enjoy reading about organic chemistry as much as I haveenjoyed writing about it.Daniel R. Bloch

ACKNOWLEDGMENTSThe author expresses his appreciation to Nan for her assistance, patience, andhelpful comments during the preparation of this book.The following individuals were kind enough to review various chapters inthis book:Vaughn Ausman, Marquette UniversityKate Bichler, University of Wisconsin Center—ManitowocPeter Conigliaro (retired), S.C. JohnsonSheldon Cramer (emeritus), Marquette UniversityTimothy Eckert, Carthage CollegeSharbil Firson, Sigma-AldrichKevin Glaeske, Wisconsin Lutheran CollegeBruce HolmanShashi Jasti, Sigma-AldrichSteven Levsen, Mount Mary CollegeJulie Lukesh, University of Wisconsin—Green BayKevin Morris, Carthage CollegePatt Nylen, University of Wisconsin—MilwaukeeStephen Templin, Cardinal Stritch UniversityA special thanks to Priyanka Negi and Judy Bass who assisted with thetechnical editing of this book.xixCopyright 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use.

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Organic ChemistryDemystifiedxxiCopyright 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use.

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1CHAPTERStructure andBondingIntroductionThe study of organic chemistry involves the reactions and interactions ofmolecules. Since molecules are composed of atoms, it is necessary to reviewthe structure of atoms and how they contribute to the properties of molecules.Atomic StructureAtoms are composed of a nucleus surrounded by electrons, as shown in Fig. 1-1.The nucleus consists of positively charged protons and neutral neutrons. Although the nucleus consists of other subatomic particles, the proton, neutron,and electron are the only subatomic particles that will be discussed in this text.1Copyright 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use.

CHAPTER 12Structure and BondingNucleus containsprotons and neutrons and isabout 0.0001 angstrom in diameterElectron cloud1ÅdiameterFig. 1-1. Structure of an atom.The atom is extremely small. It has a diameter of about 10 10 m(0.000,000,000,1 m or 0.000,000,004 in.). These small dimensions are usually expressed in angstroms (Å), where 1 Å equals 1 10 10 m, or pm where1 pm equals 1 10 12 m. The nucleus is about 1/10,000th the diameter of theatom, or about 10 4 Å. A key point: most of the volume of an atom is occupiedby the electrons. To put this in terms that are easier to understand, if the atomwas magnified so that the nucleus was the size of a marble, the area occupied bythe electrons would be the size of a football stadium. Take a minute to visualizethat. The area occupied by electrons is huge relative to that of the nucleus. Thearea occupied by electrons is referred to as the electron cloud.MASSES OF ATOMSThe mass of an atom is concentrated in the nucleus. A proton and a neutroneach have a mass of about 1.66 10 24 g. An electron has a mass of 1/1800ththat of a proton. Since these are such very small numbers, it is more convenientto give the mass of a proton and a neutron in atomic mass units (amu). Oneamu is equal to 1.66 10 24 g. The mass of individual atoms is also given ina.m.u. The mass of 1 mole of atoms (a mole is a specific number, approximately6.022 1023 ) is the atomic mass, which we usually call the atomic weight, ofan element. The atomic weight is expressed in grams/mol.ELECTRON CLOUDSStructures of molecules are usually written as shown in Fig. 1-2. Structure 1-2aimplies that atoms are quite far apart, relative to their size. This is certainly truefor the nuclei, but not for the electron clouds. The distance between a hydrogennucleus and a carbon nucleus in a carbon-hydrogen bond is about 1.14 Å. The

CHAPTER 1Structure and Bonding1.14 ÅHHCElectron cloudoverlapHHH1-2aHC3HH1-2bFig. 1-2. Bond formation resulting from electron cloud overlap.radius of the electron cloud of an isolated hydrogen atom is calculated to be 0.79Å and the radius of an isolated carbon atom is calculated to be 0.91 Å. Since thesum of the radii of the two atoms is 1.70 Å and the actual bond length is only1.14 Å, the electron clouds must overlap to form the C H bond. Generally, thegreater the electron cloud overlap, the greater the electron density in the bondand the stronger the bond. The area occupied by the electrons, the electron cloud,is much greater than implied in the structures typically drawn (such as Structure1-2a) in this book and other organic chemistry textbooks. The area occupied byelectrons in a molecule is more accurately represented by Structure 1-2b.Why do we need to be so concerned with electrons and electron clouds?Organic chemistry involves physical interactions and chemical reactions between molecules. Electrons are primarily responsible for these interactions andreactions.QUESTION 1-1Atoms consist of which three subatomic particles?ANSWER 1-1Protons, neutrons, and electrons.Atomic Number and Atomic Mass (Weight)The atomic number (Z ) for an element is equal to the number of protons in thenucleus of an atom of a given element. The sum of the number of protons andneutrons is the mass number (A). If the number of protons changes (a nuclearreaction), a new element results. There are no changes in the number of protonsin an atom in chemical reactions.An element is identified with a symbol. The symbol is an abbreviation for anelement: H stands for hydrogen, C for carbon, He for helium, and Na for sodium.Symbols are not always the first letters of a current name as some symbols are

CHAPTER 14Structure and Bondingderived from historical or non-English names. A symbol is often shown with asuperscript indicating the atomic weight and a subscript indicating the atomicnumber, e.g., ZA He or 42 He.ISOTOPESThe number of neutrons may vary for a particular element. About 99% ofcarbon atoms have six neutrons, about 1% have seven neutrons, and a very smallpercentage contain eight neutrons. Atoms with the same atomic number (andthus the same number of protons) but different mass numbers (the sum of protonsand neutrons) are called isotopes. The average mass of carbon is 12.0107 g/mol,the element’s atomic weight. Note that the atomic weights (we should really sayatomic masses, but organic chemists usually use the term weight) of elementsin the periodic table (Appendix A) are not whole numbers as they represent theaverage of the isotopic composition. The number of electrons in a neutral atom,one without a charge, equals the number of protons. Electrons also contributeto an atom’s molecular weight, but an electron’s total weight is about 1/2000ththat of a proton and their weight contribution is usually ignored.QUESTION 1-2A sample consists of three atoms of chlorine: one has a mass of 35.0 amu andtwo have masses of 36.0 amu. How many protons, neutrons, and electrons arein each atom? What is the average mass of the sample?ANSWER 1-2The atom of 35.0 amu has 17 electrons, 17 protons, and 18 neutrons. The otheratoms have 17 electrons, 17 protons, and 19 neutrons. The average mass is(1 35.0 2 36.0)/3 35.7 amu.Electron Energy LevelsElectrons occupy concentric shells and subshells around a nucleus. The shellsare given numbers called principle quantum numbers of 1, 2, 3, etc., to identifythe levels. The energy of each shell and distance between the electrons in a shelland the nucleus increases with increasing principle quantum number. Level 1 isthe lowest energy level and the electrons in that shell are nearest to the nucleus.Level 2 is higher in energy and the electrons in this level are found further fromthe nucleus than are the electrons in Level 1. Shells are composed of subshells.Subshells have designations s, p, d, and f. The energy of the shells and subshellsincreases as shown in Fig. 1-3. The electron configurations for hydrogen, helium,

CHAPTER 1Structure and sFig. 1-3. Energy levels of shells and subshells.carbon, nitrogen, and oxygen atoms are shown in Fig. 1-4. Electrons prefer tooccupy the lowest energy levels available to them. This represents their moststable state called their ground state.AUFBAU PRINCIPLEFigure 1-4 is a more concise method showing how electrons fill the subshells asthe atomic number of the element increases. Each additional electron goes intothe lowest energy subshell available to it. This is called the aufbau (buildingup) principle. Figure 1-4 shows the electron lowest-energy configuration of sixcommon elements. Each s subshell consists of one orbital. Each p subshellconsists of three orbitals. Note the term orbital, not orbit, is used. An orbital isdefined in a following section.Each orbital can hold a maximum of two electrons. When the orbitals ina subshell are filled, electrons go into the next higher-energy subshell. Eachprinciple shell has only one s orbital: 1s, 2s, 3s, etc. Each principle shell ofLevel 2 and higher has three p orbitals, px , p y , and pz . All p orbitals in the samesubshell (2px , 2p y , and 2pz ) are of equal energy. Orbitals of equal energy are2p2p2p2s2s2s1s1s1sHydrogen, 1HHelium, 2HeCarbon, 6C2p2p2p2s2s2s1s1s1sNitrogen, 7NOxygen, 8ONeon, 10NeFig. 1-4. Electron configuration of elements.

CHAPTER 16Structure and Bondingcalled degenerate orbitals. The maximum number of electrons in a main shellis 2n 2 , where n is the principle quantum number, 1, 2, 3, etc.PAULI EXCLUSION PRINCIPLESince electrons have negative charges, there is some resistance for two electronsto occupy the same orbital, that is, to pair up. Species of like charge (two negativecharges) repel each other. The helium atom has two electrons to be placed inorbitals. (See the electron configuration of helium in Fig. 1-4.) One electron canbe put into the lowest energy orbital, the 1s orbital. The second electron cango into the 1s orbital or the 2s orbital. The energy required to put the secondelectron into the higher energy 2s orbital is greater than the energy required(electron-electron repulsion) to pair the electrons in the 1s orbital. Thereforethe second electron goes into the 1s orbital. Each electron is said to have a spin,like a top, and the spin can be clockwise or counterclockwise. The spin directionis indicated by an arrow pointing up or down. Two electrons in the same orbitalmust have opposite spins (Pauli exclusion principle). Helium’s two electronsare shown with opposite spins ( ) in Fig. 1-4.HUND’S RULEConsider the carbon atom with six electrons. The electron configuration is shownin Fig. 1-4. Using the aufbau principle, the first two electrons go into the 1sorbital. The next two electrons go into the next higher energy 2s orbital. Thenthe last two electrons go into the higher energy 2p orbitals. The last two electronscould go into one p orbital or each could go into two different p orbitals. Fordegenerate (equal energy) orbitals, it is more energy efficient for electrons togo into different degenerate orbitals until they must pair up (Hund’s rule).Now consider oxygen with eight electrons. When seven electrons are addedby the aufbau principle, the electron configuration will be the same as shown fornitrogen (see Fig. 1-4). The last electron added pairs with an electron already ina 2p orbital. Their spins must be opposite (Pauli exclusion principle) as shownin Fig. 1-4.ELECTRON CONFIGURATIONSA short-hand method for writing the electron configurations of atoms is shownfor oxygen as 1s2 2s2 2p4 . Verbally one would say one s two, two s two, twop four. The prefix indicates the principle energy level (1, 2, 3, etc.), the letter(s, p, d, or f) gives the type of orbital, and the exponent is the number of electronsin that orbital.

CHAPTER 1Structure and Bonding7QUESTION 1-3Draw the short-hand electron configuration for the sodium atom.ANSWER 1-3The sodium atom has 11 electrons: 1s2 2s2 2p6 3s1 .VALENCE ELECTRONSThe electrons in the outermost shell are called the valence electrons. Elementsin the first row (period) in the periodic table, hydrogen and helium, have only a1s orbital. The maximum number of electrons these two elements can accommodate is 2. A 2-electron configuration will be called a duet. When hydrogenhas 2 valence electrons in its 1s orbital it will be called duet happy. Elements inthe second row (period) in the periodic table, from lithium to neon, can hold amaximum of 10 electrons. The outermost shell, the valence shell, has a principlequantum number of 2 and can hold a maximum of 8 electrons, 2s2 , 2p6 . Whenthe valence shell orbitals are filled, the atom will be called octet happy. Thenumber of valence electrons in the elements in the first three rows of theperiodic table is equal to their group number (see the periodic table in AppendixA). Hydrogen in Group IA has 1 valence electron, carbon in Group IVB has4 valence electrons, and fluorine in Group VIIB has 7 valence electrons. Anatom can gain valence electrons from, or loose electrons to, other atoms. Valenceelectrons are important since they are involved in forming chemical bonds.QUESTION 1-4How many electrons does a nitrogen atom have? How many valence electronsdoes it have?ANSWER 1-4It has seven electrons and five valence electrons.The Octet RuleNeon, argon, and the other elements in column VIIIB in the periodic table arecalled the noble gases. They have eight electrons in their valence shell. Heliumis an exception since its valence shell (1s) can hold only two electrons. Noblegases are so called because they are, of course, gases and tend to be unreactiveor inert. There is a special stability associated with atoms with eight electrons intheir valence shell (except for the elements in row 1). The octet rule states thatelements will gain, lose, or share electrons to achieve eight electrons in their

CHAPTER 18Structure and Bondingoutermost (valence) shell. An explanation for this special stability is beyond thescope of this book.There are some exceptions to the octet rule. Third row elements (such assulfur and phosphorus) can hold up to 18 electrons in their outermost valenceshell (3s, 3p, and 3d orbitals). Beryllium and boron atoms can have less than8 electrons in their valence shells. An example of a boron compound will bediscussed in a following section.ValencesThe bonding capacity or the number of bonds to an atom is called its valence.(It would be helpful to look at the periodic table in Appendix A as you readthis paragraph.) The valence of atoms in Groups IA to IVA is the same asthe group number. Lithium (Group IA) has a valence of one and will have asingle bond to another atom. Carbon (Group IVB) has a valence of four andthere will be four bonds to each carbon atom. Carbon is called tetravalent. Thevalence of elements in Groups VB to VIIB is 3, 2, and 1 (or eight minus thegroup number) respec

Organic chemistry is the chemistry of carbon-containing compounds. Every living organism, plant and animal, is composed of organic compounds. Anyone with an interest in life and living things needs to have a basic understanding of organic chemistry. Articles cont