10.1 Structure and Classification of Alcohols - Chemistry ...

Primary alcohols · Secondary alcohols · Tertiary alcohols · Physical properties of alcohols · Hydrogen bonding · The effect of van der Waals forces. Skiptomaincontent PrimaryalcoholsSecondaryalcoholsTertiaryalcoholsPhysicalpropertiesofalcoholsBoilingPointsHydrogenbondingTheeffectofvanderWaalsforcesSolubilityofalcoholsinwaterContributorsAdditionalResources Thispagedefinesanalcohol,andexplainsthedifferencesbetweenprimary,secondaryandtertiaryalcohols.Itexaminesinsomedetailtheirsimplephysicalpropertiessuchassolubilityandboilingpoints.Alcoholsarecompoundsinwhichoneormorehydrogenatomsinanalkanehavebeenreplacedbyan-OHgroup.Alcoholsfallintodifferentclassesdependingonhowthe-OHgroupispositionedonthechainofcarbonatoms.Therearesomechemicaldifferencesbetweenthevarioustypes. Primaryalcohols Inaprimary(1°)alcohol,thecarbonatomthatcarriesthe-OHgroupisonlyattachedtoonealkylgroup.Someexamplesofprimaryalcoholsareshownbelow: Noticethatthecomplexityoftheattachedalkylgroupisirrelevant.IneachcasethereisonlyonelinkagetoanalkylgroupfromtheCH2groupholdingthe-OHgroup.Thereisanexceptiontothis.Methanol,CH3OH,iscountedasaprimaryalcoholeventhoughtherearenoalkylgroupsattachedtothethe-OHcarbonatom. Secondaryalcohols Inasecondary(2°)alcohol,thecarbonatomwiththe-OHgroupattachedisjoineddirectlytotwoalkylgroups,whichmaybethesameordifferent.Examplesincludethefollowing: Tertiaryalcohols Inatertiary(3°)alcohol,thecarbonatomholdingthe-OHgroupisattacheddirectlytothreealkylgroups,whichmaybeanycombinationofthesameordifferentgroups.Examplesoftertiaryalcoholsaregivenbelow: Physicalpropertiesofalcohols BoilingPoints Thechartbelowshowstheboilingpointsofthefollowingsimpleprimaryalcoholswithupto4carbonatoms: Theseboilingpointsarecomparedwiththoseoftheequivalentalkanes(methanetobutane)withthesamenumberofcarbonatoms. Noticethat: Theboilingpointofanalcoholisalwayssignificantlyhigherthanthatoftheanalogousalkane. Theboilingpointsofthealcoholsincreaseasthenumberofcarbonatomsincreases. Thepatternsinboilingpointreflectthepatternsinintermolecularattractions. Hydrogenbonding Hydrogenbondingoccursbetweenmoleculesinwhichahydrogenatomisattachedtoastronglyelectronegativeelement:fluorine,oxygenornitrogen.Inthecaseofalcohols,hydrogenbondsoccurbetweenthepartially-positivehydrogenatomsandlonepairsonoxygenatomsofothermolecules. Thehydrogenatomsareslightlypositivebecausethebondingelectronsarepulledtowardtheveryelectronegativeoxygenatoms.Inalkanes,theonlyintermolecularforcesarevanderWaalsdispersionforces.Hydrogenbondsaremuchstrongerthanthese;therefore,moreenergyisrequiredtoseparatealcoholmoleculesthantoseparatealkanemolecules.Thisisthemainreasonforhigherboilingpointsinalcohols. Compound IUPACName CommonName MeltingPoing(oC) BoilingPoint(oC) SolubilityinH2Oat23oC PhysicalPropertiesofAlcholsandSelectedAnalogousHaloalkanesandAlkanes CH3OH Methanol Methylalcohol -97.8 65.0 Infinite CH3Cl Chloromethane Methylchloride -97.7 -24.2 0.74g/100mL CH4 Methane -182.5 -161.7 3.5mL(gas)/100mL CH3CH2OH Ethanol Ethylalcohol -114.7 78.5 Infinite CH3CH2Cl Chloroethane Ethylchloride -136.4 12.3 0.447g/100mL CH3CH3 Ethane -183.3 -88.6 4.7mL(gas)/100mL CH3CH2CH2OH 1-Propanol Propylalcohol -126.5 97.4 Infinite CH3CH2CH3 Propane -187.7 -42.1 6.5mL(gas)/100mL CH3CH2CH2CH2OH 1-Butanol Butylalcohol -89.5 117.3 8.0g/100mL CH3(CH2)4OH 1-Pentanol Pentylalcohol -79 138 2.2g/100mL Thistableshowsthatalcohols(inred)havehigherboilingpointsandgreatersolubilityinH2Othanhaloalkanesandalkaneswiththesamenumberofcarbons.Italsoshowsthattheboilingpointofalcoholsincreasewiththenumberofcarbonatoms. TheeffectofvanderWaalsforces Boilingpointsofalcohols:Hydrogenbondingisnottheonlyintermolecularforcealcoholsexperience.TheyalsoexperiencevanderWaalsdispersionforcesanddipole-dipoleinteractions.Thehydrogenbondinganddipole-dipoleinteractionsaresimilarforallalcohols,butdispersionforcesincreaseasthesizeofthealcoholsincrease.Theseattractionsbecomestrongerasthemoleculeslengthenandcontainmoreelectrons.Thisincreasesthesizesofthetemporarydipolesformed.Thisiswhytheboilingpointsincreaseasthenumberofcarbonatomsinthechainsincreases.Ittakesmoreenergytoovercomethedispersionforces;thus,theboilingpointsrise. Comparisonbetweenalkanesandalcohols:Evenwithoutanyhydrogenbondingordipole-dipoleinteractions,theboilingpointofthealcoholwouldbehigherthanthecorrespondingalkanewiththesamenumberofcarbonatoms. Compareethaneandethanol: Ethanolisalongermolecule,andtheoxygenatombringswithitanextra8electrons.BothoftheseincreasethesizeofthevanderWaalsdispersionforces,andsubsequentlytheboilingpoint.Amoreaccuratemeasurementoftheeffectofthehydrogenbondingonboilingpointwouldbeacomparisonofethanolwithpropaneratherthanethane.Thelengthsofthetwomoleculesaremoresimilar,andthenumberofelectronsisexactlythesame. Solubilityofalcoholsinwater Smallalcoholsarecompletelysolubleinwater;mixingthetwoinanyproportiongeneratesasinglesolution.However,solubilitydecreasesasthelengthofthehydrocarbonchaininthealcoholincreases.Atfourcarbonatomsandbeyond,thedecreaseinsolubilityisnoticeable;atwo-layeredsubstancemayappearinatesttubewhenthetwoaremixed. Considerethanolasatypicalsmallalcohol.Inbothpurewaterandpureethanolthemainintermolecularattractionsarehydrogenbonds. Inordertomixthetwo,thehydrogenbondsbetweenwatermoleculesandthehydrogenbondsbetweenethanolmoleculesmustbebroken.Energyisrequiredforbothoftheseprocesses.However,whenthemoleculesaremixed,newhydrogenbondsareformedbetweenwatermoleculesandethanolmolecules. Theenergyreleasedwhenthesenewhydrogenbondsformapproximatelycompensatesfortheenergyneededtobreaktheoriginalinteractions.Inaddition,thereisanincreaseinthedisorderofthesystem,anincreaseinentropy.Thisisanotherfactorindecidingwhetherchemicalprocessesoccur.Considerahypotheticalsituationinvolving5-carbonalcoholmolecules. Thehydrocarbonchainsareforcedbetweenwatermolecules,breakinghydrogenbondsbetweenthosewatermolecules.The-OHendsofthealcoholmoleculescanformnewhydrogenbondswithwatermolecules,butthehydrocarbon"tail"doesnotformhydrogenbonds.Thismeansthatmanyoftheoriginalhydrogenbondsbeingbrokenareneverreplacedbynewones. InplaceofthoseoriginalhydrogenbondsaremerelyvanderWaalsdispersionforcesbetweenthewaterandthehydrocarbon"tails."Theseattractionsaremuchweaker,andunabletofurnishenoughenergytocompensateforthebrokenhydrogenbonds.Evenallowingfortheincreaseindisorder,theprocessbecomeslessfeasible.Asthelengthofthealcoholincreases,thissituationbecomesmorepronounced,andthusthesolubilitydecreases. Contributors JimClark(Chemguide.co.uk) AdditionalResourcesKhanAcademyIntroductiontoalcoholsCliffsNotesAlcohols,structureandnamingPracticeProblemsIntrotoalcoholswithpracticeproblems