an introduction to alcohols - Chemguide

In a secondary (2°) alcohol, the carbon with the -OH group attached is joined directly to two alkyl groups, which may be the same or different. ... In a tertiary ... INTRODUCINGALCOHOLS Thispageexplainswhatalcoholsare,andwhatthedifferenceisbetweenprimary,secondaryandtertiaryalcohols.Itlooksinsomedetailattheirsimplephysicalpropertiessuchassolubilityandboilingpoints.Detailsofthechemicalreactionsofalcoholsaredescribedonseparatepages. Whatarealcohols? Examples Alcoholsarecompoundsinwhichoneormorehydrogenatomsinanalkanehavebeenreplacedbyan-OHgroup.ForthepurposesofUKAlevel,wewillonlylookatcompoundscontainingone-OHgroup. Forexample: Note: Ifyouaren'tconfidentaboutnamingorganiccompounds,thenyoureallyoughttofollowthislinkbeforeyougoon. UsetheBACKbuttononyourbrowsertoreturntothispage. Thedifferentkindsofalcohols Alcoholsfallintodifferentclassesdependingonhowthe-OHgroupispositionedonthechainofcarbonatoms.Therearesomechemicaldifferencesbetweenthevarioustypes. Primaryalcohols Inaprimary(1°)alcohol,thecarbonwhichcarriesthe-OHgroupisonlyattachedtoonealkylgroup. Note: Analkylgroupisagroupsuchasmethyl,CH3,orethyl,CH3CH2.Thesearegroupscontainingchainsofcarbonatomswhichmaybebranched.AlkylgroupsaregiventhegeneralsymbolR. Someexamplesofprimaryalcoholsinclude: Noticethatitdoesn'tmatterhowcomplicatedtheattachedalkylgroupis.IneachcasethereisonlyonelinkagetoanalkylgroupfromtheCH2groupholdingthe-OHgroup. Thereisanexceptiontothis.Methanol,CH3OH,iscountedasaprimaryalcoholeventhoughtherearenoalkylgroupsattachedtothecarbonwiththe-OHgrouponit. Secondaryalcohols Inasecondary(2°)alcohol,thecarbonwiththe-OHgroupattachedisjoineddirectlytotwoalkylgroups,whichmaybethesameordifferent. Examples: Tertiaryalcohols Inatertiary(3°)alcohol,thecarbonatomholdingthe-OHgroupisattacheddirectlytothreealkylgroups,whichmaybeanycombinationofsameordifferent. Examples: Physicalpropertiesofalcohols BoilingPoints Thechartshowstheboilingpointsofsomesimpleprimaryalcoholswithupto4carbonatoms. Theyare: Theyarecomparedwiththeequivalentalkane(methanetobutane)withthesamenumberofcarbonatoms. Noticethat: Theboilingpointofanalcoholisalwaysmuchhigherthanthatofthealkanewiththesamenumberofcarbonatoms. Theboilingpointsofthealcoholsincreaseasthenumberofcarbonatomsincreases. Thepatternsinboilingpointreflectthepatternsinintermolecularattractions. Note: Ifyouaren'thappyaboutintermolecularforces(includingvanderWaalsdispersionforcesandhydrogenbonds)thenyoureallyoughttofollowthislinkbeforeyougoon.Thenextbitwon'tmakemuchsensetoyouifyouaren'tfamiliarwiththevarioussortsofintermolecularforces. UsetheBACKbuttononyourbrowsertoreturntothispage. Hydrogenbonding Hydrogenbondingoccursbetweenmoleculeswhereyouhaveahydrogenatomattachedtooneoftheveryelectronegativeelements-fluorine,oxygenornitrogen. Inthecaseofalcohols,therearehydrogenbondssetupbetweentheslightlypositivehydrogenatomsandlonepairsonoxygensinothermolecules. Thehydrogenatomsareslightlypositivebecausethebondingelectronsarepulledawayfromthemtowardstheveryelectronegativeoxygenatoms. Note: Ifyouwanttobefussy,thediagramisslightlymisleadinginthatitsuggeststhatallofthelonepairsontheoxygenatomsareforminghydrogenbonds.Inanalcoholthatcan'thappen.Takingthealcoholasawhole,thereareonlyhalfasmanyslightlypositivehydrogenatomsastherearelonepairs.Atanyonetime,halfofthelonepairsinthetotalliquidalcoholwon'thavehydrogenbondsfromthembecausetherearen'tenoughslightlypositivehydrogenstogoaround. Inthediagram,toshowthe3-dimensionalarrangement,thewedge-shapedlinesshowbondscomingoutofthescreenorpapertowardsyou.Thedottedbonds(otherthanthehydrogenbonds)showbondsgoingbackintothescreenorpaperawayfromyou. Inalkanes,theonlyintermolecularforcesarevanderWaalsdispersionforces.Hydrogenbondsaremuchstrongerthantheseandthereforeittakesmoreenergytoseparatealcoholmoleculesthanitdoestoseparatealkanemolecules. That'sthemainreasonthattheboilingpointsarehigher. TheeffectofvanderWaalsforces... ...ontheboilingpointsofthealcohols: Hydrogenbondingisn'ttheonlyintermolecularforceinalcohols.TherearealsovanderWaalsdispersionforcesanddipole-dipoleinteractions. Thehydrogenbondingandthedipole-dipoleinteractionswillbemuchthesameforallthealcohols,butthedispersionforceswillincreaseasthealcoholsgetbigger. Theseattractionsgetstrongerasthemoleculesgetlongerandhavemoreelectrons.Thatincreasesthesizesofthetemporarydipolesthataresetup. Thisiswhytheboilingpointsincreaseasthenumberofcarbonatomsinthechainsincreases.Ittakesmoreenergytoovercomethedispersionforces,andsotheboilingpointsrise. ...onthecomparisonbetweenalkanesandalcohols: Eveniftherewasn'tanyhydrogenbondingordipole-dipoleinteractions,theboilingpointofthealcoholwouldbehigherthanthecorrespondingalkanewiththesamenumberofcarbonatoms. Compareethaneandethanol: Ethanolisalongermolecule,andtheoxygenbringswithitanextra8electrons.BothofthesewillincreasethesizeofthevanderWaalsdispersionforcesandsotheboilingpoint. Ifyouweredoingareallyfaircomparisontoshowtheeffectofthehydrogenbondingonboilingpointitwouldbebettertocompareethanolwithpropaneratherthanethane.Thelengthwouldthenbemuchthesame,andthenumberofelectronsisexactlythesame. Solubilityofalcoholsinwater Thesmallalcoholsarecompletelysolubleinwater.Whateverproportionsyoumixthemin,youwillgetasinglesolution. However,solubilityfallsasthelengthofthehydrocarbonchaininthealcoholincreases.Onceyougettofourcarbonsandbeyond,thefallinsolubilityisnoticeable,andyoumaywellendupwithtwolayersinyourtesttube. Thesolubilityofthesmallalcoholsinwater Considerethanolasatypicalsmallalcohol.Inbothpurewaterandpureethanolthemainintermolecularattractionsarehydrogenbonds. Inordertomixthetwo,youwouldhavetobreakthehydrogenbondsbetweenthewatermoleculesandthehydrogenbondsbetweentheethanolmolecules.Itneedsenergytodobothofthesethings. However,whenthemoleculesaremixed,newhydrogenbondsaremadebetweenwatermoleculesandethanolmolecules. Theenergyreleasedwhenthesenewhydrogenbondsaremademoreorlesscompensatesforthatneededtobreaktheoriginalones. Inaddition,thereisanincreaseinthedisorderofthesystem-anincreaseinentropy.Thatisanotherfactorindecidingwhetherthingshappenornot. Note: Ifyouhaven'tcomeacrossentropybefore,don'tworryaboutit.Imentionitbecausetheenergyreleasedwhenthenewbondsaremadeisn'tquiteenoughtocompensateforbreakingtheoldones,meaningthatthemixingprocessisendothermic.Ifitweren'tfortheincreaseinentropy,thesolutionwouldn'tbeformed. Toreallyunderstandthis,youneedtohavestudiedentropyandfreeenergy.Ifyoushouldknowaboutthis,butaren'thappyaboutthecalculationsinvolved,youmightliketohavealookatchapter11ofmychemistrycalculationsbook. Thelowersolubilityofbiggeralcohols Imaginewhathappenswhenyouhavegot,say,5carbonatomsineachalcoholmolecule. Thehydrocarbonchainsareforcingtheirwaybetweenwatermoleculesandsobreakinghydrogenbondsbetweenthosewatermolecules. The-OHendofthealcoholmoleculescanformnewhydrogenbondswithwatermolecules,butthehydrocarbon"tail"doesn'tformhydrogenbonds Thatmeansthatquitealotoftheoriginalhydrogenbondsbeingbrokenaren'treplacedbynewones. AllyougetinplaceofthoseoriginalhydrogenbondsarevanderWaalsdispersionforcesbetweenthewaterandthehydrocarbon"tails".Theseattractionsaremuchweaker.Thatmeansthatyoudon'tgetenoughenergybacktocompensateforthehydrogenbondsbeingbroken.Evenallowingfortheincreaseindisorder,theprocessbecomeslessfeasible. Asthelengthofthealcoholincreases,thissituationjustgetsworse,andsothesolubilityfalls. Questionstotestyourunderstanding Ifthisisthefirstsetofquestionsyouhavedone,pleasereadtheintroductorypagebeforeyoustart.YouwillneedtousetheBACKBUTTONonyourbrowsertocomebackhereafterwards. questionsontheintroductiontoalcohols answers Wherewouldyouliketogonow? Tothealcoholsmenu... Tothemenuofotherorganiccompounds... ToMainMenu... ©JimClark2003(modifiedOctober2015)