Stereochemistry.pdf CHEM 2030 - 01
Popular in Survey of Organic Chemistry
CHEM 2030 - 01
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This 6 page Class Notes was uploaded by Shannon Z. on Sunday October 11, 2015. The Class Notes belongs to CHEM 2030 - 01 at University of Missouri - Columbia taught by Rainer Glaser in Fall 2015. Since its upload, it has received 58 views. For similar materials see Survey of Organic Chemistry in Chemistry at University of Missouri - Columbia.
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Date Created: 10/11/15
1 Light Beams a Light beams consist of waves that can oscillate through many different planes b If you pass a light beam through certain polarizing substances you can restrict that beam so that the waves are aligned only in certain planes Polarization of Light Waves Polarizer 1 Polarizer 2 Vertical Horizontal Incident Beam Unpolartzed Vertically Polarized Light Wave Figure 1 i When you do this the light is said to be planepolarized c A light beam can pass through two polarizing substances if the axes of the two substances are properly aligned 393 I quotr 1 f O The beam will not pass through both polarizing substances The beam will pass through both of the i polarizing substances 2 Polarimeter a A polarimeter is a device that measures observed rotations b Process i A polarized light is made by a polarized prism ii An analyzer rotates so that the polarized light is blocked 1 The prism axes of the analyzer and the polarizing prism are perpendicular to one another at this point 2 The angle the analyzer prism is rotated to is called the observed rotation or a 3 If the analyzer is rotated right the substance in the sample tube is classified as dextrorotary 4 If the analyzer is rotated left the substance is in the sample tube is classified as levorotary iii A sample is placed in a sample tube iv If the sample is optically inactive nothing happens v If the sample is optically active thee plane of polarization is rotated and some light will pass through the analyzer vi By turning the analyzer prism you can again block the light c The observed rotation is equal to the number of degrees the optically active substrate rotated the beam of the planepolarized light d The amount of the observed rotation of an optically active substance depends on i The sample s molecular structure ii The number of molecules in the sample tube iii The tube length iv The wavelength of the polarized light v Temperature e To compare observed rotations you must take into account all of these factors An equation is used Spevcl c rotation a l A solvent i l X r J ii I length of the tube in decimeters iii c concentration in gramsmilliliter of the optically active substance iv t temperature v A light wavelength vi a observed rotation vii The solvent used is indicated in parenthesis 3 How molecular structure affects optically active compounds a Achiral molecules are optically inactive b Chiral molecules are optically active 4 Properties of Enantiomers a Enantiomers are identical except for their chiral properties i Chiral properties include the direction the substance rotates a planepolarized light clockwise or counterclockwise b Enantiomers have identical achiral properties i Melting and boiling point ii Density iii The degrees a planepolarized light is rotated c A substance s chiral properties are most important when two chiral substances are interacting with one another 5 Diastereomers a Compounds with more than one stereogenic center These are stereoisomers that are not mirror images but are flipped b When placed in front of a mirror a diastereomer s bonds will just flip sides m i r rnr pH til l frigidH Hui avm Hi utjl Flh iIl 39l i rLj39l fl l39litlECFi rJ39ls 39ll ii Each of the chiral centers in a diastereomer can be separately R or S oriented iii When you flip a diastereomer in a mirror the R oriented chiral carbons become S oriented and the S oriented chiral carbons become R oriented c Properties i Enantiomers have the same chiral properties but different achiral properties ii Diastereomers in contrast have different chiral and achiral properties making them essentially different molecules d Forms i If a molecule has n different stereogenic centers it may have a maximum of 2quot stereoisomeric forms ii There will be a maximum of 2quot2 pairs of enantiomers iii These are maximums There can be fewer due to structural features 6 Fischer Projections a Only done with chiral molecules b Drawn so that the stereogenic centers are assumed to be in the middle where the intersection of horizontal and vertical lines representing bonds is c Rules i The largest carbon chain is arranged vertically ii Assume that all carboncarbon conformations are cis structures iii Place the most oxidized group on the top if possible iv If you were to look at the 3D structure the horizontal lings lead to groups that are above the plane protruding out at you The vertical lines lead to groups that are below the plane protruding away from you v Will be R or S oriented 7 Meso Compounds a These are diastereomers that when divided between the chiral carbon atoms will flip the R and S orientation of the chiral carbon atoms making the atom on top and bottom consist of the same atoms in the same location H Hi m m H Hi m CH3 CH3 derticani achiral i a mem irm b Meso compounds are actually achiral making them optically inactive i Meso compounds are considered aachiral even though they have chiral carbons in them c In the case of meso compounds with two chiral carbons there are a maximum of 3 three different molecules 1an mnlemle Ruth strumer BITE the same just rotated Molemlel if Moleowleg Moleowle W M l llll j s CH3 CH 3 3 3 g H1 E g H y Cl H H Cll Cll H H C1 plane oil E lll i Ci c1 II Cl I I iii 1 WWW 3 3 CH3 emanatiim nv s Eilhiral identical achiral a mem Form 8 Racemic Mixtures a A racemic mixture is a 5050 mixture of a pair of enantiomers An equal amount of enantiomers b You add two achiral reactants together and a new chiral product is made c When chiral products are made from achiral reactions both enantiomers are formed at the same rate in equal amounts d Example Com ider the following reaction EHJEHQEH Eli i HE i HHEHJZHEZHH liar Ilimtclw EI1rm1ul1utilme The magenta are both achiraljoining together to makea chiral molecule Within teaEmmi l EHEEHEEHZEHE IllEr r Therearetwo wtde that thin reaction will make in equal am um he the electrophilic addition begins ji39 llf re g thefollllawing structure The imming return canjuin either from the tag or the bottom H Jll3939J EH will 1 2quot flirty TE l39iLIlJrl JillIFquot Drive ofthe twmfullllowing EtTMtllllTEE i5 Elin 1 H E J 3 1 CH LtHi L2H H L l if EffI i hf39litilialllhlllliillalii39 if rst Hi ea 5quotquot a h i H EEi lifiu39l calmn t 2H frifl E H lrrtltltlrlii I WEALTH is Iiiar HjiJiul ruamutmune e Resolution i The process of separating a racemic mixture into two enantiomers f BUT if you have an achiral reagent and a chiral catalyst like a transition metal that form a new molecule with a chiral center the products will be formed at different rates and unequal amounts This is NOT a racemic mixture i There is an unequal amount of R and S figures are made that aren t necessarily enantiomers 1 When the molecules are not the same and not mirror images they are called diastereoisomers and are essentially different molecules 9 Misc a quotThe correctly assigned R or S configuration of a stereocenter in a molecule is called the absolute configuration of the stereocenter b A star may be used to indicate a carbon atoms is chiral 10 Take home message of this chapter a Remember these three methods of classification of stereoisomers i Enantiomers differ only in chiral properties and cannot be separated by achiral methods ii Diastereomers differ in both chiral and achiral properties iii Diastereomers that are configurational isomers can be separated by achiral methods iv Diastereomers that are conformers are not separable 11