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Stereoisomers (Ochem_chap5) Flashcards

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2870716354constitutional isomerismcompounds with identical molecular formulas but differ in order in which the individual atoms are connected0
2870724114Isomers1
2870724115stereoisomerismisomers whose atoms are connected in the same order but differ in their spatial arrangement examples include (1) relatively stable and isolable cis-trans isomers (2) the rapidly equilibrating (and usually not isolable) conformational ones -cis/trans, anti/gauche, equatorial/axial, mirror-image stereoisomerism2
2870742522chirala molecule that is not superimposable on tis mirror image -molecules with one stereo center are always chiral -many chiral objects do not have stereo centers -only criterion for chirality is the non superimposable nature of object and mirror image3
2870746960enantiomereach isomer of the image-mirror image pair4
2870763228test for chiralitythe presence of absence of a plane of symmetry (mirror plane)-- one that bisects the molecule so that the part of the structure lying on one side of the plane mirrors the part on the other side chiral molecules cannot have a plan of symmetry5
2870770336optical activity of enantiomers-special interaction with light: optical activity -enantiomers are frequently called optical isomers -when plane-polarized light is passed through a sample of one of the enantiomers, the plan of polarization of the incoming light is rotated in one direction (either clockwise or counterclockwise)6
2870773068dextrorotatory-(dexter, Latin, right) -an enantiomer that rotates the plane of light in a clockwise sense as the viewer faces the light source -referred to as the (+) enantiomer7
2870774422levorotatory-laevus, Latin, left -counterclockwise rotation -(-) enantiomer8
2870777423ordinary light and plane-polarized light-bundles of electromagnetic waves that oscillate simultaneously in all planes perpendicular to the direction of the light beam -when such light is passed through a material called a polarizer, all but one of these light waves are "filtered" away, and the resulting beam oscillates in only one plane: plane-polarized light9
2870803664optical rotation-when light travels through a molecule, the electrons around the nuclei and in the various bonds interact with the electric field of the light beam. -If a beam of plane-polarized light is passed through a chiral substance, the electric field interacts differently with, say, the "left" and "right" halves of the molecule -this interaction results in a rotation of the plane of polarization is called optical rotation -the sample giving rise to it is referred to as optically active10
2870805895polarimeteroptical rotations are measured by using a polarimeter11
2870811074racemic mixture (racemate)a compound containing one stereo center is chiral and exists as a pair of enantiomers -A 1:1 mixture of enantiomers12
2870816599Absolute configuration: R-S sequence rules-there is no straightforward correlation between the sign of rotation and the structure of the particular enantiomer -x-ray diffraction can establish the absolute configuration13
2870831228stereocenters are labeled R or S-rank all four substituents in the order of decreasing priority -place the lowest-priority substituent farthest -if the progression from a to b to c is counterclockwise, the configuration at the stereocenter is named S (sinister, Latin, left) -if the progression is clockwise, the center is R (rectus, Latin, right) ex: (R)-2-bromobutane and (S)-2-bromobutanee the sign of the rotation of plane-polarized light maybe added if it is known, i.e. (S)-(+)-2-bromobutane and (R)-(-)-2-bromobutane14
2870858677sequence rules assign priorities to substituents(1) look at atoms directly attached to the stereocenter. Higher atomic number has higher priority. In regard to isotopes, the atom of higher atomic mass receives higher priority (2) look for the first point of difference (3) double and triple bonds are treated as if they were single, and the atoms in them are duplicated or triplicated at each end by the respective atoms at the other end of the multiple bond.15
2870878009Fischer projectionsimplified way of depicting tetrahedral carbon atoms and their substituents in two dimensions16
2870899416conversion of hawed-wedged line structures into fischer projections17
2870902935Fischer projections tell us the absolute configuration18
2870906416diastereomers-presence of more than one stereocenter stereoisomers that are not related to each other as object to mirror image. -Cis and trans isomers of cyclic compounds are examples of disastereomers -because they are not mirror images of each other, diastereomers are molecules with different physical and chemical properties -a compound with n stereocenters can have a maximum of 2^n stereoisomers19
2870971364meso compounda compound that contains two stereocenters but is superimposable with its mirror image (mess, Greek, middle) -the presence of an internal mirror plane20
2871091978In general, the formation of chiral compounds (e.g., 2-bromobutane) from chiral reactants (e.g., butane and bromine) yieldsracemates. Or, optically inactive starting materials furnish optically inactive products21
2871095756stereoselectivethe preference for the formation of one stereoisomer, when several are possible22
2871096027resolutionthe separation of enantiomers -It is achieved by the reaction of the racemate with the pure enantiomer of a chiral compound to yield separable diastereomers -chemical removal of the chiral reagent frees both enantiomers of the original racemate.23
2871098396chiral chromatographyanother way of separating enantiomers is by chiral chromatography on an optically active support -the sample is applied to the top of a column filled with an enantiopure chiral support. One enantiomer (green) interacts more strongly with the support than the other (red) and is relatively slow to pass through the column -the red enantiomer is eluted before its green mirror image. Commercial columns often use the glucose polymer cellulose as the chiral stationary phase.24

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