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Queen's University
CHEM 281
John Carran

Chirality & Stereochemistry • an object is achiral (not chiral) if the object and its mirror image are identical • a chiral object is one that cannot be superposed on its mirror image Chiralcarbon The Biological Significance of Chirality • chiral molecules: molecules that cannot be superimposable w/ their mirror images • one enantiomer causes birth defects, the other cures morning sickness • one enantiomer is a bronchodilator, the other inhibits platelet aggregation → only way to figure out: see how inteacts w/ another chiral compound → enzymes are chiral compounds : for transient diasteroisomers • 66% of all drugs in development are chiral, 51% are being studied as a single enantiomer • of the $475 billion in world-wide sales of formulated pharmaceutical products in 2008, $205 billion was attributable to single enantiomer drugs (mostly b/c of thalidomide) Isomerism: Constitutional Constitutional Isomers • Isomers: different compounds that have the same molecular formula ◦ Constitutional isomers: isomers that have the same molecular formula but different connectivity – their atoms are connected in a different order Examples: Stereoisomers • Stereoisomers are NOT constitutional isomers • Stereoisomers have their atoms connected in the same sequence but they differ in the arrangement of their atoms in space • the consideration of such spatial aspects of molecular structure is called stereochemistry Enantiomers & Diastereomers • Stereoisomers can be subdivided into two general categories: enantiomers & diasteromers ◦ enantiomers – stereoisomers whose molecules are nonsuperposable mirror images of each other (no different properties except for way polar light passes through and how interact w/ other enantiomers) ◦ diastereomers – stereoisomers whose molecules are not mirror images of each other Geometrical isomers • (cis & trans isomers) are: ◦ Diastereomers Subdivision of Isomers Enantiomers and Chiral Molecules • enantiomers occur only with compounds whose molecules are chiral • a chiral molecule is one that is NOT superposable on its mirror image • the relationship between a chiral molecule and its mirror image is one that is enantiomeric • a chiral molecule and its mirror image are said to be enantiomers of each other Midtermstop point A Single Chirality Center Causes a Molecule to Be Chiral • the most common type of chiral compounds that we encounter are molecules that contain a carbon atom bonded to four different groups • such a carbon atom is called an asymmetric carbon or a chiral center and is usually designated with an asterisk (*) (III) and (IV) are nonsuperposable mirror images ofeachother → enantiomers (V) and (VI) are superposable ⇒ not enantiomers ⇒ achiral Tetrahedral vs. Trigonal Stereogenic Centers • chirality centers are tetrahedral stereogenic centers → stereogenic center is an atombearinggroups suchthat aninterchangingofanytwo groups leads to a stereoisomer Cis and trans alkene isomers contain trigonal stereogenic centers Tetrahedral vs. Trigonal Stereogenic Centers • chirality centers are tetrahedral stereogenic centers • cis and trans alkene isomers contain trigonal stereogenic centers More about the Biological Importance of Chirality Enantiomers → + and - :from experimentalresults (way theyrotate plane polarized light) Thalidomide • the activity of drugs containing chirality centers can vary between enantiomers, sometimes with serious or even tragic consequences • for several years before 1963 thalidomide was used to alleviate the symptoms of morning sickness in pregnant women • in 1963 it was discovered that thalidomide (sold as a mixture of both enantiomers) was the cause of horrible birth defects in many children born subsequent to the use of the drug Enzymes + drugform transient diasteriosomers → whydifferent enantiomers ofdrugreact differently How to Test for Chirality: Planes of Symmetry • a molecule will not be chiral if it possesses a plane of symmetry • a plane of symmetry (mirror plane) is an imaginary plane that bisects a molecule such that the two halves of the molecule are mirror images of each other • all molecules with a plane of symmetry in their most symmetric conformation are achiral Naming Enantiomers: R ,S -System • recall: • using only the IUPAC naming that we have learned so far, these two enantiomers will have the same name: 2-Butanol • this is undesirable because each compound must have its own distinct name How to Assign (R) and (S) Configurations • rule 1: assign priorities to the four different groups on the stereocenter from highest to lowest ◦ priority bases on atomic number, the higher the atomic number, the higher the priority • rule 2: when a priority cannot be assigned on the basis of the atomic number of the atoms that are directly attached to the chirality center, then the next set of atoms in the unassigned groups is examined ◦ this process is continued until a decision can be made. • rule 3: visualize the molecule so that the lowest priority group is directed away from you, then trace a path from highest to lowest priority ◦ if the path is a clockwise motion, then the configuration at the asymmetric carbon is (R) ◦ if the path is a counter-clockwise motion, then the configuration is (S) • rule 4: for groups containing double or triple bonds, assign priorities as if both atoms were duplicated or triplicated Properties of Enantiomers: Optical Activity • Enantiomers: mirror images that are not superposable • enantiomers have identical physical properties (e.g. melting point, boiling point, refractive index, solubility etc.) racemate:mixture oftwo enantiomers Enantiomers • have the same chemical properties (except reaction/interactions with chiral substances) • show different behavior only when they interact with other chiral substances (form diasteriomers w/ different properties) • turn plane-polarized light on opposite direction • Optical activity: the property possessed by chiral substances of rotating the plane of polarization of plane-polarized light Plane-Polarized Light • the electric field (like the magnetic field) of light is oscillating in all possible planes • when this light passes through a polarizer (Polaroid lens), we get plane- polarized light (oscillating in only one plane) The Polarimeter • a device for measuring the optical activity of a chiral compound Specific Rotation • the value of α depends on
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