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Chapter 20

Organic Chemistry

Figure 20.1All organic compounds contain carbon and most are formed by living things, although they are also

formed by geological and artificial processes. (credit left: modification of work by Jon Sullivan; credit left middle:

modification of work by Deb Tremper; credit right middle: modification of work by "annszyp"/Wikimedia Commons;

credit right: modification of work by George Shuklin)

Chapter Outline

20.1Hydrocarbons

20.2Alcohols and E thers

20.3Aldehydes, K etones, Carboxylic Acids, and Esters

20.4Amines and A mides

Introduction

Allliving thingsonearthareform edmostlyofca rboncom pounds.Theprevalenceofc arboncompoundsi nliving

thingshasled totheepi thet"carbon-based" life.T hetruthis weknowofnootherkindoflife.E arlychemistsregarded

substancesisolatedfromorganisms(plantsandanimals) asadiffe renttypeofmatterthatcould notbesynthesiz ed

artificially,andthesesubstanceswerethusknownasorganiccompounds.The widespreadbe liefcalledvitali smheld

thatorganiccom poundswereformedbyavital forcepresentonlyi nlivingor ganisms.TheGerma nc hemi stFriedrich

Wohlerwasoneofthee arlychem ists tore futethis aspectofvitalism, when,in1828,hereportedthesynthesisof urea,

acom ponentofmanybodyfluids,fromnonl iving materials. Since then,ithasbee nrec ognizedthatorganic molecules

obeythesam enaturall awsasinorganic substances,andtheca tegoryoforganicc ompoundshasevol vedtoinclude

bothnatura landsyntheticcompoundst hatcontai ncarbon.Somecarbon-containing compoundsarenotclassifiedas

organic,forexample,carbona tes andcyanides,andsimpleoxides,suc hasCOandCO 2 .Alt houghasingle,preci se

definitionhasyettobeidentifi edbythec hemistry community ,most agreethatadefiningtraitoforganic mole cules

is the presence of carbon as the principal element, bonded to hydrogen and other carbon atoms.

Today,organiccompoundsa rekeycomponentsofplast ics,soaps,perfume s,sweetene rs,fabrics,pha rmaceuticals,and

manyothersubstanc esthatweusee veryday.Thevaluetousofor ganiccompoundsensure stha torga nicchemistry

isanim portantdisci plinewithinthegeneralfi eldofchemistry.Inthischapt er,wediscusswhythee lementc arbon

givesrisetoava stnumbera ndvariety ofcom pounds,howthosecom poundsareclassifi ed,andthe roleoforganic

compounds in representative biological and industrial settings.

Chapter 20 | Organic Chemistry1097

20.1Hydrocarb ons

By the end of this section, you will be able to:

•Explain the importance of hydrocarbons and the reason for their diversity •Name saturated and unsaturated hydrocarbons, and molecules derived from them •Describe the reactions characteristic of saturated and unsaturated hydrocarbons •Identify structural and geometric isomers of hydrocarbons

Thelarge stdatabase

[1] oforga niccompoundslistsabout10 millionsubsta nces,whichincludecom poundsoriginati ng fromli vingorganismsandthosesynt hesizedbychemists.The numberofpote ntialorga niccompoundshasbeen estimated [2] at10 60
- anastronomi callyhighnumber.Theexistenceofsomanyorga nicmolec ulesisaconsequence

ofthe ability ofcarbonatomstoformuptofourstrong bondsto othercarbonatom s,resultinginchains andringsof

many different sizes, shapes, and complexities. Thesimple storganiccompoundscontainonlytheelement scarbonand hydrogen,anda recalledhydrocarbons. Eventhoughtheya recomposed ofonlytwotypesof atoms,the reisawidevariety ofhydroca rbonsbecauset hey mayconsistofvaryi nglengthsofchai ns,branc hedchains,and ringsofcarbonatoms,orc ombina tionsofthese

structures.Inaddition,hydrocarbons may differinthetypesofcarbon-c arbonbondspresentin theirmolecul es.

Manyhydrocarbonsa refoundinplants,a nimals, andt heirfossils; otherhydrocarbonshavebeenpre paredinthe

laboratory.Weusehydrocarbonseveryday, mainly asfue ls,suchasnatura lgas,acetyle ne,propane,butane ,andthe

principalcomponentsofgasoline ,dieselfuel,andheati ngoil .Thefamiliarplastic spolyethylene,polypropyle ne,and

polystyrenearealsohydrocarbons. Wecandi stinguishseveral typesofhydrocarbonsbydi fferences inthebonding

between carbon atoms. This leads to differences in geometries and in the hybridization of the carbon orbitals.

Alkanes

Alkanes,orsaturatedhydrocarbons,cont ainonlysinglecovalentbonds betweenc arbonatoms.Eac hofthecarbon atomsinanalkane hassp 3 hybridorbita lsandisbondedtofourothera toms,eac hofwhich iseithe rcarbonor

hydrogen.TheLe wisstructure sandmodelsofmet hane,ethane,andpentanea reillustra tedinFigure20.2.Ca rbon

chainsareusuallydra wnasstrai ghtlinesinLewisst ructures,but onehastore memberthatLewis structuresarenot

intendedtoindicatethe geometryof molecules.Noticethatthec arbonatoms inthestructuralmodels(t hebal l-and-

stickandspace-fill ingmodel s)ofthepentanemoleculedonotlieinastra ightline.Because ofthe sp 3 hybridization,

the bond angles in carbon chains are close to 109.5°, giving such chains in an alkane a zigzag shape.

Thestructure sofalkanesandotherorga nicm oleculesmayalsobere presentedi nalessdetai ledmannerbycondense d

structuralformulas(orsimply, condensedformulas).Inste adoftheusualformatfor chem icalformul asinwhic heach

elementsymbolappearsjustonce,a condensedformul aiswrittentosugge stthebondingi nthemolecule .T hese formulashavetheappea ranceofaL ewisstruct urefromwhichmostorallofthebondsymbol shave been removed.

Condensedstructuralformula sforethaneandpentaneare shownatthebottomofFigure20.2,and severala dditional

examples are provided in the exercises at the end of this chapter.

1.Thi s is the Beilstein database, now available through the Reaxys site (www.elsevier.com/online-tools/reaxys).

2.Pepl ow, Mark. "Organic Synthesis: The Robo-Chemist,"Nature512 (2014): 20-2.

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Figure 20.2Pictured are the Lewis structures, ball-and-stick models, and space-filling models for molecules of

methane, ethane, and pentane. Acom monmethodusedbyorgani cchemiststosim plifythedra wingsof largermolecules istouseaskeletal

structure(alsocalled aline-anglestructure).Inthist ypeofst ructure,carbonatomsarenotsymboli zedwithaC,but

representedbyeachendofaline orbend inaline.Hydrogenat omsarenotdra wnif they areattache dto acarbon. Otheratomsbesi descarbonandhydrogenarere presentedbytheire lementalsymbol s.Figure20.3showsthre e different ways to draw the same structure.

Figure 20.3The same structure can be represented three different ways: an expanded formula, a condensed

formula, and a skeletal structure.

Chapter 20 | Organic Chemistry1099

Example 20.1

Drawing Skeletal Structures

Draw the skeletal structures for these two molecules:

Solution

Eachcarbonatomis convertedinto theendofaline ortheplac ewherelinesintersect. Allhydrogenatoms attachedtothecarbonatomsarel eftoutoft hestructure(a lthoughwestillne edtorecognizethe yarethere ):

Check Your Learning

Draw the skeletal structures for these two molecules:

Answer:

Example 20.2

Interpreting Skeletal Structures

Identify the chemical formula of the molecule represented here:

Solution

Thereareeightpl aceswhere linesinterse ctorend,meaningthatthe reareeight carbonatomsinthe molecule.Sinceweknowthatcarbonat omstendtoma kefourbonds, ea chcarbonatomwi llhavethe numberofhydrogenat omst hatarerequi redforfourbonds.Thiscompoundc ontains16hydrogena tomsfor

1100Chapter 20 | Organic Chemistry

This OpenStax book is available for free at http://cnx.org/content/col11760/1.9 a molecular formula of C 8 H 16 .

Location of the hydrogen atoms:

Check Your Learning

Identify the chemical formula of the molecule represented here:

Answer:C

9 H 20

Allalkane sarecomposedofcarbonandhydrogen atoms,a ndhavesimilarbonds,struc tures, andformulas;noncyc lic

alkanesallhaveaformula ofC n H 2n+2 .The numberofca rbonatomspresenti nan alkanehasnol imit.Greaternumbe rs

ofat omsinthemolecule swillle adtost rongerintermolecularattractions(di spersionforces)and correspondingly

differentphysicalpropertiesof themolecules.Propert iessuchasmeltingpoint andboi lingpoint( Table20.1)usual ly

change smoothly and predictably as the number of carbon and hydrogen atoms in the molecules change.

Properties of Some Alkanes

[3]

AlkaneMolecular

Formula

Melting Point

(°C)

Boiling Point

(°C)

Phase at

STP [4]

Number of Structural

Isomers

methaneCH 4 -182.5-161.5gas1 ethaneC 2 H 6 -183.3-88.6gas1 propaneC 3 H 8 -187.7-42.1gas1 butaneC 4 H 10 -138.3-0.5gas2 pentaneC 5 H 12 -129.736.1liquid3 hexaneC 6 H 14 -95.368.7liquid5 heptaneC 7 H 16 -90.698.4liquid9 octaneC 8 H 18 -56.8125.7liquid18 nonaneC 9 H 20 -53.6150.8liquid35 decaneC 10 H 22
-29.7174.0liquid75 tetradecaneC 14 H 30

5.9253.5solid1858

octadecaneC 18 H 38

28.2316.1solid60,523

Table 20.1

Hydrocarbonswiththesa meformula,incl udingalka nes,canhavedif ferentstructures.Forexam ple,twoalkanes

havetheformul aC 4 H 10 :The yarecalled n-butaneand2-methyl propane(orisobut ane),andhavethefollowingLewis

3.Physic al properties for C

4 H 10 and heavier molecules are those of thenormal isomer,n-butane,n-pentane, etc.

4.STP i ndicates a temperature of 0 °C and a pressure of 1 atm.

Chapter 20 | Organic Chemistry1101

structures: Thecompoundsn-butaneand2-methyl propanearestruc turalisomers(thetermconstitut ionalisome rsisalso

commonlyused).Constitutiona lisomersha vethesamemolecularformulabutdifferentspa tialarrangementsof the

atomsintheirmol ecules.T hen-butanemolecule containsanunbranchedchain,me aningthatnocarbonatomi s bondedtomore thantwoot hercarbona toms.Weusethe termnormal,ort hepre fixn,to referto achainofcarbon atomswithoutbranching.T hecompound2-methylpropa nehasabranchedchai n(thecarbon atominthec enterofthe Lewis structure is bonded to three other carbon atoms)

IdentifyingisomersfromLewisst ructuresisnotasea syas itlooks.Lewisstructure sthatlookdifferentmay actuall y

representthesameisom ers.Forexam ple,thethreest ructuresinFigure20.4allrepresentthe samemolecule,

n-butane,andhenceare notdifferent isomers.Theyare identic albe causeeachcontainsanunbra nchedchainoffour

carbon atoms.

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Figure 20.4These three representations of the structure of n-butane are not isomers because they all contain the

same arrangement of atoms and bonds. The Basics of Organic Nomenclature: Naming Alkanes TheInternat ionalUnionofPureandAppliedChemistry(IUP AC)hasdevi seda system ofnomencla turethatbegins withthenames oftheal kanesandcanbeadjust edfrom thereto accountformorecomplicate dstructures. The nomenclature for alkanes is based on two rules:

1.Tonamea nalkane,firsti dentifythe longestchainofcarbonatom sinits structure.Atwo-carbonchain is

calledethane;athree-carbon chain,propane;andafour-carbon chai n,butane.Longerchainsarenamed as follows:pentane(five -carbonchain),hexane(6), heptane(7),octane(8),nonane(9),and decane(10).These prefixes can be seen in the names of the alkanes described inTable 20.1.

2.Addprefi xestothenameoft helongestcha intoindi catethepositi onsa ndnamesof substituents.Substi tuents

arebranches orfunctionalgroupsthatrepl ace hydrogenatomsonachain.The positi onofasubstituent or branchisidenti fiedbythe numberofthecarbonatomitisbondedto int hechain.W enumbertheca rbon atomsinthechai nbycounting fromtheendof thechainnearestt hesubsti tuents.Multiplesubstit uents are named individually and placed in alphabetical order at the front of the name. Whenmorethanone substituent ispresent,eit heron thesamecarbonatomorondi fferentca rbonatoms,the

substituentsarelistedalphabet icall y.Becausethecarbonatom numberingbeginsattheendclosesttoasubstituent,

thelongestc hainofcarbona tomsisnumberedinsuc hawa yastoproducet helowestnumberforthesubsti tuents.The

ending-oreplaces-ideatthee ndofthenam eofanel ect ronegativesubst ituent(inioni ccom pounds,thenegat ively

chargedionendswith-idelikechloride;i norganiccompounds,suchatomsa retreat edassubstituentsand the-o

Chapter 20 | Organic Chemistry1103

endingisused).The numberofsubsti tuentsof thesamet ypeisindicate dbyt heprefixesdi-(two),tri-(three),tetra-

(four), and so on (for example,difluoro-indicates two fluoride substituents).

Example 20.3

Naming Halogen-substituted Alkanes

Name the molecule whose structure is shown here:

Solution

Thefour-ca rbonchainisnumberedfrom theendwiththechl orineatom. Thisputsthesubstituent son positions1and2(numbering fromthe otherendwoul dput thesubstituentsonpositi ons3a nd4).Four carbonatomsm eansthatthebasenam eofthiscompoundwillbe butane.Thebromi neatpositi on2will bedesc ribedbyadding2-bromo-;thiswill come atthebeginning ofthename ,sincebromo-come sbefore chloro-alphabeti cally.Thechlorineatposition1willbedescribedbyadding1-chloro-, resultingi nthe name of the molecule being 2-bromo-1-chlorobutane.

Check Your Learning

Name the following molecule:

Answer:3,3-dibromo-2-iodopentane

Wecall asubstituentthatcont ains onelesshydrogenthanthecorresponding alkaneanal kylgroup.Thenameofan alkyl groupis obtained by dropping the suffix-aneof the alkane name and adding-yl:

The open bonds in the methyl and ethyl groups indicate that these alkyl groups are bonded to another atom.

Example 20.4

Naming Substituted Alkanes

Name the molecule whose structure is shown here:

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Solution

Thelongestc arbonchainrunshoriz ontallyacross thepageandcontainssi xca rbonatoms(thismakesthe baseofthena me hexane,butwe willalsoneed toincorporatethenam eoft hebranc h).Inthiscase, we wanttonumbe rfromrightto left(asshownbythe blue numbers)sothe branchisc onnectedtoca rbon3 (imaginethenumbersfromleftt oright - this wouldputthebranch onca rbon4,violatingour rules).The branchattache dtoposition3ofourchaincontainst wocarbonat oms(numberedinred) - sowet akeour namefortwocarbons eth-andatt ach-ylattheend tosignifywe aredescribi ngabranch.Put tingallthe pieces together, this molecule is 3-ethylhexane.

Check Your Learning

Name the following molecule:

Answer:4-propyloctane

Somehydrocarbonsc anformmorethanone typeofalkylgroupwhe nthe hydrogenatoms thatwouldberemove d havediffere nt"environments"inthemole cule.Thisdiversityofpossiblealkylgroups canbeide ntified inthe

followingway:Thefourhydrogen atomsin amethane mole culeare equivalent;theyal lhavethesa mee nvironment.

Theyareequival entbeca useeachisbondedtoacarbonatom (thesamecarbonatom )thatisbondedtothre ehydrogen

atoms.(Itmaybeea sier toseetheequiva lenc yintheballandstick modelsin Figure20.2.Re movalofanyoneofthe

fourhydrogen atomsfrom methaneformsamethyl group.Li kewise,thesixhydrogenatoms inethane aree quivalent

(Figure20.2)and removingany oneofthesehydrogenatoms producesane thylgroup. Eac hofthesixhydrogen atomsisbondedtoac arbonatomt hatisbonded totwoot herhydroge natomsandac arbonatom.However ,inboth

propaneand2-m ethylpropane, therearehydrogenatomsintwodifferent environments,disti nguishedbythe adjace nt

atoms or groups of atoms:

Chapter 20 | Organic Chemistry1105

Eachofthesixequiva lent hydrogenatoms ofthefirst typeinpropaneandeachoftheninee quivalent hydrogenatoms

oftha ttypein2-methyl propane(allshown inblack) arebondedtoacarbonatomthati sbondedtoonlyoneot her carbonatom.The twopurplehydrogenatomsi npropaneareof asecondtype .Theydif ferfromthe sixhydrogen

atomsofthefirsttypei nt hattheya rebondedtoa carbonatom bondedtot woothercarbonat oms.Thegreenhydrogen

atomin2-methyl propanediffers fromtheotherninehydrogenatomsint hatmoleculeandfrom thepurpl ehydrogen atomsinpropane.The greenhydrogena tomin2-methylpropa neisbondedt oacarbonatom bondedtothre eother carbonatoms.Two differentalkylgroupscan beformedfrom eachofthesemolecules, dependingonwhichhydroge n

atom is removed. The names and structures of these and several other alkyl groups are listed inFigure 20.5.

Figure 20.5This listing gives the names and formulas for various alkyl groups formed by the removal of hydrogen

atoms from different locations.

Notethata lkylgroupsdonotexist asstableindepe ndente ntit ies. Theyarealwa ysapartofsom elargermolecule .

The location of an alkyl group on a hydrocarbon chain is indicated in the same way as any other substituent:

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Alkanesarerelative lystable molecules,butheatorlightwillactiva tereactionsthati nvolvethebreakingofC-Hor

C-C single bonds. Combustion is one such reaction: $)  H 0  H