IB Chemistry Power Points Topic 20 Organic Chemistrywww.pedagogics.ca Stereoisomers
Isomers – different compounds with the same molecularformula.Structural Isomers – isomers that differ in which atoms arebonded to which atoms. CH3eg. C4H10 CH3CH2CH2CH3 CH3CHCH3 n-butane isobutane
Stereoisomers – isomers that differ in the way the atoms areoriented in space, but not in which atoms are bonded to whichatoms.Geometric Isomers - alkenes eg. cis-2-butene trans-2-butene H 3C CH3 H 3C H C C C C H H H CH3
Stereochemistry: importance: biochemicals medicine tool for studying mechanisms
optical isomers - mirror-image stereoisomers.The physical and chemical properties of enantiomers areidentical, except 1) the direction of rotation of the plane ofplane polarized light and 2) how they react with opticallyactive reagents.chiral center– is a carbon that is bonded to four differentgroups of atoms. *CH3CH2CHBrCH3 * *CH3CHBrCHBrCH3
configuration – the arrangement in space of the four differentgroups about a chiral center.How do we show configurations?“wedge” formulas
optical activity – when a substance rotates the plane ofplane polarized light. (1815 by Biot)plane polarized light – light that has been passed througha nicol prism or other polarizing medium so that all of thevibrations are in the same plane. non-polarized polarized
polarimeter – an instrument used to measure optical activity. polarizer analyzerlight source sample tube
dextrorotatory – when the plane of polarized light is rotatedin a clockwise direction when viewed through a polarimeter. (+) or (d)levorotatory – when the plane of polarized light is rotated in acounter-clockwise direction when viewed through apolarimeter. (-) or (l)The angle of rotation of plane polarized light by an opticallyactive substance is proportional to the number of atoms in thepath of the light.
Why are some substances optically active and others not? Canwe predict which ones will be and which ones won’t?Louis Pasteur (1848) recrystallized sodium ammonium tartrate(optically inactive). He noticed that the crystals were of twotypes which he physically separated. The two types of crystalswere optically active, but rotated the plane of polarized light inopposite directions. He proposed that the molecules came intwo forms, “left handed” and “right handed”. Together, themixture of the two forms is optically inactive.
chiral – not superimposeable on the mirror image(“handedness”)achiral – superimposeable on the mirror image; not chiral.Test for optical activity: chiral molecules are opticallyactive.racemic modification – equimolar molar mixture ofenantiomers (will be optically inactive) (+).
- compounds with one chiral center will show optical activity- compounds without chiral centers do not normally showoptical acitivity- compounds with more than one chiral center may or maynot show optical activity depending on whether or not theyare non-superimposable on their mirror image (chiral) orsuperimposable (achiral).