lecture 8 - CH EM 231 L ECT U RE 8 Ster eochemistr y I L...

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Unformatted text preview: CH EM 231 L ECT U RE 8 Ster eochemistr y I L ECT U RE 8 1 Consi der ati ons of ster eoi somer s Our life in chemist r y has been pr et t y simple t hus far I n t he full consider at ion of or ganic chemist r y we ar e ver y concer ned wit h connect ivit y and spat ial or ient at ion. L ect ur e 8 2 Consi der ati ons of ster eoi somer s Just as we can t ell our left shoe fr om our r ight . And am open pair of scissor s fr om closed , we must lear n how t o det er mine and descr ibe t he differ ence of molecules t hat have t he same par t s, but t he par t s ar e ar r anged differ ent ly in space. L ect ur e 8 3 Ster eoi somer s i n fl at mol ecul es: Gener al CH3 H H CH3 CH3 H CH3 H CH3 H F Br CH3 H Br F L ect ur e 8 4 Ster eoi somer s i n fl at mol ecul es: Al kenes CH3 H F CH3 H Br F Br Even flat molecules can get complex r apidly, and t o communicat e well in or ganic we need some naming r ules t o help L ect ur e 8 5 E-Z notati on for al kenes The EZ nomenclat ur e divides alkenes int o quadr ant s CH3 H Br F Rank the 2 gr oups on the r ight Rank the 2 gr oups on the left Fir st we r ank and t hen we compar e L ect ur e 8 6 E-Z notati on for al kenes Since t her e ar e only 2 gr oups per side we us simple r anking H ighe r CH3 H Br F H ighe r L ow e r L ow e r L ect ur e 8 7 Zussamen = together = Z We now compar e t he higher r ank ed gr oups(or lower if pr efer r e H ighe r CH3 H Br F H ighe r L ow e r L ow e r Gr oups t hat have same r ank together have a name t hat means t oget her in Ger man: Zusammen L ect ur e 8 8 Entgegen = apar t = E We now compar e t he higher r ank ed gr oups(or lower if pr efer r e H ighe r CH3 H F Br L ow e r L ow e r H ighe r Compounds t hat don't have similar r anked gr oups t oget her have a name t hat means apar t in Ger man: Entgegen L ect ur e 8 9 Cahn-I ngol d-Pr el og Rul es: Atomi c Number Fir st compar ison in r anking is at omic number H igher 6 CH 3 L ow er 1 H 9 L ow er F Br 35 H igher I n t his case t he at omic number s ar e enough t o allow r anking A compar ison of t he 2 gr oups on t op indicat es L ect ur e 8 10 Cahn-I ngol d-Pr el og Rul es: Atomi c mass Somet imes gr oups have t he same at omic number but not t he same mass H igher 6 CH3 2 3 1 H H 3higher L ow er 2 H 1 low er This is a pr et t y subt le example but t his is legit mat e. L ect ur e 8 11 Cahn-I ngol d-Pr el og Rul es: fi r st poi nt of di ffer ence Oft en 2 gr oups on t he alkene ar e ident ical. To solve t his follow bonds away fr om alkene t o det er mine r ank 61,1,1 low er CH3 F H 9 higher 68,1,1 HO CH2 higher 1 low er L ect ur e 8 12 Cahn-I ngol d-Pr el og Rul es: M ul ti pl e bonds M ult iple bonds get count ed as mult iple single bonds 68,1,1 low er HO CH2 68,8,1 higher F 9 higher O CH NH2 7 low er L ect ur e 8 13 Pr acti ce questi ons for E-Z notati on H Cl F Br H H3C H Cl L ect ur e 8 14 A qui ck l ook at a `compl ex' or gani c mol ecul e CH3 CH3 Br F F CH3 CH3 Br F CH3 CH3 Br L ect ur e 8 15 L ook at your hands Ar e t hey t he same- or not t he same? L ect ur e 8 16 Enanti omer s Object s t hat t ake up t hr ee dimensional space can have H igh or L ow symmet r y A cube has high symmet r y a foot or hand has low symmet r y Enant iomer s ar e 1.pair s of const it ut ional isomer s t hat ar e r elat ed t o each ot her t hr ough r eflect ion 2.They cannot be super imposed on each ot her . A macr oscopic example ar e your hands. While (for t he most par t ) t hey ar e ofident ical const it ut ion t he r ight hand does not super impose on t he left hand (can't stack lik e paper cups) L ect ur e 8 17 Enanti omer s H ave t he exact same const it ut ion H ave t he exact same physical pr oper t ies, like boiling pt solubilit y and so on Enant iomer s ar e ident ical in ever y way except t hat t hey ar e mir r or images of each ot her L ect ur e 8 18 Exer ci se on Enanti omer s F Br Cl CH3 H H CH3 H F D T H F T F D Cl Br L ect ur e 8 19 Defi ni ti on of chi r al i ty An object t hat is chir al is non super imposable on it s mir r or image I n t he wor ld of car bon, a chi r a l i ty center will have 4 differ ent gr oups at t ached t o it . Any chir al compound will have 1 enant iomer (nonsuper imposable mir r or image) A compound is chir al if it has no mir r or plane or cent er of symmet r y L ect ur e 8 20 Exer ci se on chi r al i ty H CH3 CH3 H H CH3 CH3 H k CH3 H CH3 Br l F H CH3 Br m n L ect ur e 8 21 Opti cal acti vi ty M any r eact ions we will lear n in t his cour se will allow us t o cr eat e chir al compounds Unless t her e is some CH I RAL AGENT we will cr eat e an equal mixt ur e of enant iomer s An equal mixt ur e of enant iomer s is known as a r acemic mixt ur e For example, Dr . K issling has a r acemic mixt ur e of hands as well as a r acemic mixt ur e of feet . L ect ur e 8 22 Opti cal acti vi ty The abilit y of a subst ance t o r ot at e plane polar ized light . Opt ical act ivit y is a par amet er t hat a scient ist can measur e for a chir al subst ance When an or ganic chemist says t hat a chir al compound is opt ically act ive, t hey ar e saying t hat t he amount of t he 2 enant iomer s is not equal Chemist s descr ibe t he level of opt ical act ivit y in differ ent ways. L ect ur e 8 23 Pol ar i metr y Polar imet r y is t he inst r ument al t echnique for det er mining opt ical act ivit y L ight of a specific wavelengt h is passed t hr ough a polar izing filt er The polar ized light t hen passes t hr ough a solut ion of our chir al analyt e Since wavelenght and amplit ude of light is close t o t he scale of t he molecule, t he 4 differ ent gr oups on t he chir alit y cent er ser ve t o r ot at e t he light by a small amount . A solut ion pr ovides millions of t hese int er act ions, t hus t he effect of t he minut e r ot at ion is scaled. L ect ur e 8 24 Schemati c of a pol ar i meter L ect ur e 8 25 The key i s the detai l s We must know: t he of light sour ce Pat h lengt h t hr ough sample Concent r at ion of solut ion Solvent used Temper at ur e Rot at ion dur ing exper iment , L ect ur e 8 26 Usi ng data fr om pol ar i metr y Clockwise r ot at ion is known as dext r or ot at or y Ant iclockwise r ot at ion is known as levor ot at or y Clockwise r ot at ion is known as (+) Ant iclockwise r ot at ion is known as (-) Specific r ot at ion is a pr oper t y inher ent t o a specific chir al molecule = 100 cl I f you ar e given any 3 of t he 4 par amet er s in t his equat ion you should be able t o calculat e t he four t h L ect ur e 8 27 Pol ar i metr y data-enanti omer i c excess I f only one enant iomer pr esent sample is opt ically pur e I f a r acemic mixt ur e has no exess of one enant iomer over anot her , an opt ically pur e sample would have complet e excess Chemist s say t hat a sample t hat is opt ically pur e has 100% enant iomer ic excess (ee) A r acemic mixt ur e is 0% ee Enzymes make compounds t hat ar e ver y close t o 100% ee L ect ur e 8 28 Absol ute confi gur ati on Exact spat ial ar r angement in 3 dimensions Cr yst al st r uct ur es ar e t he best means of obt aining t his. Polar imet r y dat a does not give us absolut e configur at ion I f you ar e given t he absolut e configur at ion of a chir al compound, you can easily det er mine t he configur at ion of it s enant iomer . H H HO (-)-2-butanol For example OH (+)-2-butanol L ect ur e 8 29 Rel ati ve confi gur ati on Relat ive configur at ion is det er mined t hr ough logic. Relat ive configur at ion can not be det er mined t hr ough looking at opt ical r ot at ion values Opt ical r ot at ion values ar e ver y good for det er mining ee of a subst ance, but t he physics of t he pr ocess does not allow us ext r apolat ion int o absolut e or r elat ive configur at ions. We can t alk about absolut e configur at ion if we ar e given it via a pict ur e We can pr edict r elat ive configur at ion t hr ough logic Or ganic chemist s have come up wit h L ect ur e 8 30 R S notati on somethi ng to l ook for war d to We can t alk about absolut e configur at ion if we ar e given it via a pict ur e We can pr edict r elat ive configur at ion t hr ough logic Or ganic chemist s have come up wit h R-S not at ion t o discuss absolut e configur at ion fr om dr awing dat a. We'll t alk about t his next t ime. L ect ur e 8 31 ...
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