Acyclic to Cyclic Carbohydrates: A Focus on Glucose Having trouble seeing carbohydrates in their acyclic and cyclic form? Confused at the connectivity and conformation? Well, you’ve come to the right place. We’ll be looking at the acyclic form of glucose in particular and how the acyclic form converts to the cyclic.
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AcyclictoCyclicCarbohydrates:AFocusonGlucose
Havingtroubleseeingcarbohydratesintheiracyclicandcyclicform?
Confusedattheconnectivityandconformation?
Well,you’vecometotherightplace.
We’llbelookingattheacyclicformofglucoseinparticular
andhowtheacyclicformconvertstothecyclic.
Togoaboutthistopic,thebestideaisprobablytofirstbuildamodel.Wewillbeginbybuildingthestructurebasedontheimagebelow.Whenlookingatthegeneralandcommonlyseenacyclicstructure,youmaynoticethatboththehydrogenatomsandthealcoholgroupsprojecttowardstheviewer.Thismayconfusesomepeoplewhoareusedtothepresenceofonewedgeandonegroupconnectedwithdashedlines.Italsomayappeardeceivingbecausetheimagepresentsthemoleculealmostasifitwereplanar.BuildthemodelsothatthecarbonylisatthetopandtheCH2OHgroupisclosesttothebottom.
Youcanseethat
thecarbonylgroupandtheCH2OHgroup
collideintooneanother
Thewedgesshowthatthehydrogen
atomsandthealcoholgroupspointtowardtheviewer.
Iffocusingonanindividualcarbon,let’ssaythefirstafterthecarbonyl,youwillseethatthecarbonsconnectedtoitwill
pointwayfromtheviewerasrepresentedwithdashedlinesandtheothergroupsconnectedwillpoint
towardstheviewer.Thiswillbeseeninthebuiltmodel.
Afterbuildingthemodel,youfindthattheconnectedcarbonatomsformaringshapedstructure,withtheCH2OHgroupandthecarbonylcollidingintooneanother.Besuretorotatetheatomssothatthehydrogenatomsandthealcoholgroupsofthecarbonstereocenterspointtowardtheviewer.Ifyouweretolookateachindividualcarbonthatisastereocenter(thecarbonsexcludingthoseinthecarbonylandintheCH2OHgroup)youcanseethatitfitstheimageabove.Thecarbonsconnectedtotheparticularcarbonthatyouchoosetofocusonpointawayfromtheviewerwhiletheotherconnectedpointtowardtheviewer.
Hereisanotherviewofthemoleculefromits
side.Fromthisangle,whichisadifferentanglethantheoriginalimagefromwhichwebuiltthemodelbasedoffof,wecanseea
clearerimageofthecollidingcarbonylandCH2OHgroup.Already,youcanstarttoseehowthecyclicformwilltakeplace.
Switchingtothisview,wewillnowconverttheacyclicformtothecyclicform.Twistthemoleculeas
neededsothatthecarbonylandCH2OHgroupnolongercollideinordertoseethemoleculemoreclearly.
Glucoseisapyranose,whichmeansitformsasix‐memberedring.Inthecaseofglucose,thecarbonof
thecarbonylwillbondwiththealcoholofthecarbonjustabovetheCH2OHgroupinordertomakeasixmemberedring.Rememberthatthoughglucosedoes
indeedcontainsixcarbons,itisnotexactlyasixcarbonring.Theringwillconsistoffivecarbonatomsandoneoxygenatom.Thesixthcarbonwillbeanequatorial
branchconnectedtothering.
Thisbondingwillnowbeshownthroughimagesofthemodelforclarity.Inthetwoimagesshownabove,thedoublebondfromtheoxygenofthecarbonylhasbeenremovedandreplacedwithayellow“stick”orbond.Thechoiceofadifferentcoloredstickisonlysothatitwillbeeasiertodistinguishandfindtheanomericcarbonandkeeptrackofitslocation.ThealcoholgroupofthecarbonnearesttotheCH2OHgrouphasalsobeenrotatedsothattheoxygenpointstowardtheanomericcarbon.Youmaynoticethatthealcoholhaslostitshydrogenatom.Donotworryaboutthisdetail,forwearefocusingonstructureratherthanreactivity.Youmayalsonoticethatastick,representativeofabond,hasbeenplacedonthegroundbetweentheanomericcarbonandtheoxygenatom.Thisiswherethejoiningwilltakeplacetoconverttheacyclicmoleculetothecyclicform.
TheoxygenofwhatwasthealcoholgroupofthecarbonabovetheCH2OHgrouphasnowbeenjoinedtoformabondwiththeanomericcarbon.
Thisisthetopviewofthecyclicmolecule.
Bytiltingthemolecule,youcanseethe
familiarchairconformationthatisoftendrawninChem14C.Noticethatallthealcoholgroupsareequatorial.Thisisaunique
characteristicofglucose.Theoxygenoftheanomericcarbonshouldbeanalcoholgroup,butthatchangeisnotshownhereandthe
hydrogenatomhasbeenomitted.
WorksCited
http://www.chem.ucla.edu/harding/index.html
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