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