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Chapter 3

CH 320M Chapter 3 Textbook Notes

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CH 320M
John A.Colapret

Stereoisomerism and Chirality Saturday, August 31, 2013 6:37 PM Stereochemistry - study of moleculesas 3D objects 3.1 Chirality - The Handedness of Molecules Monday,September 09,2013 11:52PM Mirror image - reflectionofan objectin a mirror Chiral - objectsthat are notsuperposable[identical]ontheirmirrorimages Achiral- whenan objectand its mirrorimageare superposableex: perfectsphere Planeof symmetry- an imaginaryplanepassingthroughan objectdividingit so that onehalfis themirrorimageofthe otherhalf Center of symmetry- a pointso situatedthat identicalcomponentsofthe objector moleculeare located equidistantandon oppositesidesfrom thepointalonganyaxis passingthroughthat point Ex: a cube Chapter 3 Page 2 3.2 Stereoisomerism Monday,September 09,2013 11:56PM Isomersare differentcompoundswith the samemolecularformula. • 3 types ofisomers ○ Constitutionalisomers- same molecularformulabuta differentconnectivityof atomsin their molecules  ○ Stereoisomers- same molecularformulaandthe same connectivitybutdifferentorientationsof their atomsin space  Ex: cis,transisomersin cycloalkanes ○ Configurationalisomers- isomersthatdiffer by the configurationof substituentson anatom   Positionsof atoms can'tinterchange Chiralitycan arise from the abilityof a moleculeto existas configurationalisomers. Enantiomers- nonsuperposablemirrorimages • Refers to therelationshipbetweena pairofobjects ○ Unlikechiralandachiralthatrefer to the objects itself Commoncause of chiralityis a tetrahedralatom (mostcommonlyC) • Chiralcenter- a tetrahedralatomthat isbondedto 4 differentgroups ○ Notlimited to carbon  Like nitrogen • Stereocenter- an atomaboutwhichexchangeof two groupsproducesa stereoisomer ○ Chiralcenter is one type of stereocenter Diastereomers- stereoisomersthataren'tmirrorimagesof eachother • Refers to a relationshipbetweena pairofobjects Conformationalisomerism - anotherform ofstereoisomerism • Conformationalchangesoccur withinmoleculesvia rotationsalongsinglebonds ○ Ex: butane--- gaucheandantiforms  Newmanprojections Atropisomers- enantiomersthatlacka chiralcenter anddifferbecauseof hinderedrotation Example 3.1 Tuesday, September10, 2013 12:04 AM 3.3 NamingChiralCenters - The R,S System Tuesday, September 10, 2013 12:07 AM Absoluteconfiguration - which of the two possibleisomers an enantiomer (whether it is the right-or left-handed isomer) R,S system- a set of rules for specifying absolute configurationabout a chiral center (also called the Cahn-Ingold-Prelogsystem) • R = order of priorityof groups ona chiralcenter is CLOCKWISE • S = orderof priorityof groups ona chiral center is COUNTERCLOCKWISE PRIORITYRULES: 1. Each atom bonded to the chiral center is assigned a priority a. Priorityis based on ATOMIC NUMBER i. The higher the atomicnumber, the higher the priority! ii. 2. If prioritycan't be assigned based on the atomic#, lookat the next set of atoms and continue until a prioritycan be assigned a. 3. Atoms participating in a double ortriple bond are considered to be bonded to an equivalent # of similar"phantom"atoms by single bonds a. 4. Priorityis made at the first pointof difference between groups Chapter 3 Page 5 Example 3.2 Thursday, September12, 2013 10:31 PM How to Assign R or S Configuration to a Chiral Center Thursday, September12, 2013 10:32 PM 1. Locate the chiral center a. Identifyits 4 substituents and assigna priority from 1 (highest)to 4 (lowest)to each substituent 2. Orient the moleculein space so that the group of lowest priorityis directed away from you 3. Read the three groups projecting towards you in order from highest priorityto lowest priority 4. If the groups are read in a clockwise direction, the configuration is designated as R a. If it's read counterclockwise direction, the configuration is S Example 3.3 Thursday, September12, 2013 10:38 PM 3.4 Acyclic Molecules with Two or More Stereocenters Thursday, September 12, 2013 10:42 PM When 2< stereocenters exist, multiple stereoisomers are possible! Ex: ENANTIOMERS AND DIASTEREOMERS: A molecule with n chiral centers, the maximum number of stereoisomers possible is2 Ex: four stereoisomers of 2,3,4-trihydroxybutanal (2 chiral centers) • Stereoisomers (a) and (b) are NONSUPERPOSABLE mirrorimages [a pair ofenantiomers] • Stereoisomers (c) and (d) are nonsuperposable mirror images [second pair of enantiomers] • (a) and (c) arediastereomers • (b) and (d) are diastereomers MESO COMPOUNDS: Meso compounds - molecules containing two or more chiral centers with special symmetry properties that reduce the number of stereoisomers to fewer than the maximum number predicted by the 2 rule • Contain 2 or more chiral centers but isachiral • They have internal mirror plane (or center of symmetry) FISCHER PROJECTION FORMULAS: Fischer projections - a 2D projection of a molecule • Groups on the right and left are by convention in front, while those at the top and bottom are to the rear • To write a Fischer projection: ○ Draw a 3D representation of the molecule oriented so that the vertical bonds from the chiral center are directed away from yo u and the horizontal bonds from the chiral center are directed toward you ○ Then writethe molecule as 2D with the chiral center indicated
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