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CHEM 2331H
T.Andrew Taton

Chapter 5 Book Notes Stereochemistry 5.1) Introduction • Stereochemistry: study of 3D structure of molecules • Constitutional (structural) isomers: differ in bonding sequence; atoms connected differently • Stereoisomers: same bonding sequence but different orientation in space o Geometric isomers (cis-trans isomers): specific type of stereoisomers 5.2) Chirality • Chiral: when objects have left/right “handedness” o has a mirror image that’s different than original object; can’t superimpose mirror image onto original • Achiral: not chiral; can superimpose mirror image on original object • 5.2A) Chirality and Enantiomerism in Organic Molecules o Superimposable: can be placed on top of each other and 3D positions are exactly the same o Enantiomers: nonsuperimposable mirror-image molecules   chiral compounds have enantiomers • 5.2B) Asymmetric Carbon Atoms, Chirality Centers, and Stereocenters o Asymmetric/Chiral carbon atom: when carbon is bonded to 4 different groups, they will have chirality  designated by asterisk * o Chirality/chiral center: any atom that holds set of ligands in a spatial arrangement & not superimposable on its mirror image; ex: asymmetric carbon atom o Stereocenters/stereogenic atom: broader group that includes chirality; any atom in which interchange of 2 groups gives it a stereoisomer  ex: asymmetric carbons and double-bonded carbon atoms in cis-trans isomers o Asymmetric carbons need 4 different groups attached to it o Rules for chirality  1) If compound has no asymmetric carbon atom, it’s usually achiral (but can have exceptions)  2) If compound has just 1 asymmetric C, must be chiral  3) If compound has more than 1 asymmetric C, may or may not be chiral • 5.2C) Mirror Planes of Symmetry o Internal mirror plane: can draw line through middle of molecule and halves are symmetric to each other  means that the molecule is achiral (its complete mirror image is also same as molecule) o  Any molecule w/ internal mirror plane of symmetry can’t be chiral even if it has asymmetric C atoms o BUT converse isn’t true  if can’t find internal mirror plane of symmetry, molecule’s not necessarily chiral 5.3) (R) and (S) Nomenclature ofAsymmetric CarbonAtoms • Configurations: 2 possible spatial arrangements of asymmetric C atoms • Cahn-Ingold-Prelog convention: system for naming configurations of chirality centers o 1) Assign relative “priority” to each group bonded to asymmetric C; 1 is highest, 4 is lowest  Higher atomic #  higher priority (look at atom directly bonded to C)  If tied, heavy isotopes get higher priority  In case of tie, look at next atoms along chain of each group (1 higher atomic # atom takes priority over many low atomic # atoms)  Treat double/triple bonds as multiple copies of same atom o 2) Position tetrahydral carbon so that lowest priority group faces away and other groups towards you in a circle  Draw arrow from 1 priority through 2 to 3d rd  Clockwise  (R)  Counterclockwise  (S) 5.4) OpticalActivity • Mirror image molecules have almost identical physical properties • Differences are seen when interacting w/ other chiral molecules like enzymes • Polarimetry: distinguishes enantiomers based on their ability to rotate plane of polarized light in opposite directions • 5.4A) Plane-Polarized Light o Plane-polarized light: composed of waves that vibrate in only 1 plane (unlike unpolarized light that vibrates randomly in all directions) o When unpolarized light goes through polarizing filter, only light vibrating in certain plane/direction (axis of the filter) pass through o Polarizing filters can be made from carefully cut calcite crystals/specially treated plastic sheets (lenses/sunglasses) o If light passes through 2 polarized filters st  If axes of filters = parallel (lined up)  almost all light through 1 goes through 2 nd  If axes of filters = perpendicular (crossed poles)  no light from 1 goes nd through 2  Intermediate angles….in between amts of light • 5.4B) Rotation of Plane-Polarized Light o Optical activity: plane of vibration of polarized light passing through sol’n w/ chiral compound rotates   substances that rotate plane of polarized light = optically active o Enantiomers = optical isomers, but optical isomers covers other things apart from enantiomers too o Enantiomeric compounds rotate plane of polarized light by exactly same amt, but in opposite directions o But can’t predict what direction particular enantiomer (R or S) will rotate plane of polarized light…R and S are just names to distinguish • 5.4C) Polarimetry o Polarimeter: measures rotation of polarized light  Tubular cell filled w/ sol’n of optically active material & system for passing polarized light through sol’n & measuring rotation as light emerges  Light from sodium lamp filtered  1 wavelength (1 color) (b/c most compounds rotate different wavelengths of light by diff amts)   Sodium D line: yellow emission line in spectrum of sodium; most commonly used for polarimetry o Monochromatic (1 color) light  polarizing filter  sample cell w/ sol’n of optically active compound  another polarizing filter (rotated until maximum light passes through  reads observed rotation (α) on protractor) o Dextrorotatory (d or +): compounds that rotate plane of polarized light clockwise o Levorotatory (l or -): …… counterclockwise • 5.4D) Specific Rotation o Rotation (α) observed in polarimeter depends on concentration of sample sol’n, length of cell, and optical activity of compound o Specific rotation: rotation found using 10 cm cell, and 1 g/mL concentration  If using other measurements, make sure to divide out length and concentration  [α] = α(observed)/c*l where c = concentration, and l = length of sample cell in decimeters (dm) o Rotation depends on wavelength of light used and temperature o R and S and + and – are ways of distinguishing enantiomers, but + and – are based on tests w/ interactions w/ light, while R and S are artificial way just to look at them  + and – are for the lab  R and S are for drawing on paper 5.5) Biological Discriminations of Enantiomers • Chiral probe: anything that can distinguish between enantiomers • Enzymes in living systems = chiral: only 1 of the enantiomers can fit into chiral active site • One of the enantiomers has desired effect, other has no effect or different effect • Whether or not biological, enantiomers don’t interact identically w/ other chiral molecules 5.6) Racemic Mixtures • Racemic mixture: sol’n that contains equal amts of 2 enantiomers  net rotation of polarized light = 0  mixture = optically inactive o AKAracemate, ( ± ) pair, or (d,l) pair (can pu± () or (d,l) in front of compound name) •  Actually not that unusual; many rxns have racemic products, esp when achiral molecule → chiral molecule • Arxn that uses optically inactive reactants/catalysts can’t produce optically active product  any chiral product must be formed as racemic mixture • You can get racemic mixtures b/c it’s equally probable for atoms to add in both possible spatial locations of a molecule during a rxn (ex on p. 191); dextrorotatory and levorotatory favored equally  form in equal amts 5.7) Enantiomeric Excess and Optical Purity • Optical purity (o.p.): ratio of rotation of mixture to rotation of pure enantiomer o o.p. = (observed rotation)/(rotation of pure enantiomer) * 100% o b/c we might not always have mixtures that = optically pure (all 1 enantiomer) nor racemic (equal amts of both enantiomers) • Enantiomeric excess (e.e.): similar method to express relative amts of enantiomers in mixture o calculate excess of predominant enantiomer as % of entire mixture o for chemically pure compound, e.e. generally = o.p. o o.p. = e.e. = (excess of one over the other)/(entire mixture)*100% = |d-l|/(d+l) * 100% o Units cancel out, so concentrations, grams, or percentages…any can be used 5.8) Chirality of Conformationally Mobile Systems • For cyclic cis hexanes, might have internal plane of symmetry when drawn as flat hexagon, but in chair conformation, has nonsuperimposable mirror image…but still not chiral b/c undergoes chair-chair interconversions @ fast rate  sample has same amts of 2 mirror images b/c both have same energies  most achiral compounds can exist in transient chiral conformations that = in equilibrium w/ their mirror image conformations • Molecule can’t be optically active if its chiral conformations = in equilibrium w/ their mirror images  such molecules = achiral •  Different from racemic mixture b/c in racemic mixture, might be able to separate out isomers to make optically active sample; impossible to create optically active sample for achiral molecule w/ chiral conformations  use flat ring to predict chirality, not chair conformation • To determine whether conformationally mobile molecule can be optically active, consider its most symmetric conformation • AKA molecule can’t be optically active if = in equilibrium w/ achiral structure/conformation b/c inherently chiral compounds have NO achievable achira
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