Chapter 5 - Stereoisomers - Chapter 5: Stereoisomers...

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Chapter 5: Stereoisomers Constitutional isomers describes compounds that have identical molecular formulas but differ in the order in which the individual atoms are connected Stereoisomerism refers to isomers whose atoms are connected in the same order but differ in their spatial arrangement; mirror-image stereoisomerism refers to molecules that are not superimposable, molecules are “mirror images” of each other 5.1. Chiral Molecules Chiral molecules cannot be superimposed on their mirror images o Each isomer of the image is called an enantiomer o Chiral molecules can be converted to the other by breaking bonds o Achiral molecules have structures that are superimposable on their mirror images o Chiral molecules must have four different substituent groups; the nucleus is called asymmetric atom or a stereocenter o Molecules with one stereocenter is always chiral; if there are more, it may or may not be chiral The symmetry in molecules helps to distinguish chiral structures from achiral ones o Test for chirality: is there a plane of symmetry (mirror plane) ? Chiral molecules cannot have a plane of symmetry 5.2. Optical Activity Enantiomers have same physical properties, such as boiling points, melting points, and densities but differ in optical activity : when a plane-polarized light is passed through a sample of one of the enantiomers, plane of polarization is rotated in one direction while the other enantiomer rotates it the other direction by the same magnitude Dextrorotatory is an enantiomer that rotates the plane of light in a clockwise sense as the viewer faces the light source; called the + enantiomer Levorotatory is an enantiomer that rotates the plane of light in a counterclockwise sense; called the – enantiomer Enantiomers are often called optical isomers Optical rotation is measured with a polarimeter o Plane-polarized light is light passed through a polarizer, resulting beam oscillates in only one plane o Electric field of nucleus of molecules interacts with the electric field of light beam, resulting in rotation of plane of polarization called optical rotation ; sample is optically active o Optical rotations are measured using a polarimeter , which is oriented so as to maximize the transmittance of light beam to the eye of observer; measured rotation (in degrees) is the observed optical rotation
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Chapter 5 - Stereoisomers - Chapter 5: Stereoisomers...

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