sof chiral and achiral molecules 罗caay H 8bsehrgonesereocenterar 5-2 Optical Activity Optical rotation is measured with a polarimeter. 出a5tm2 a ates perpe oooehh said to b ote degrees)sthe of the chiral molecul Temperature 2 2 Compounds whose mirror images are superimposable are called achiral. Examples of chiral and achiral molecules: Above all, the chiral examples contain an atom that is connected to 4 different substituent groups. This atom is called an asymmetric atom or a stereocenter. Often, asymmetric atoms are marked with an asterisk. Molecules having one stereocenter are always chiral. The symmetry in molecules helps to distinguish chiral structures from achiral ones. For most organic molecules, a sufficient test for chirality is absence of a plane of symmetry (mirror plane). A mirror plane is one that bisects the molecule in such a way that the half of the molecule on one side of the plane is the mirror image of the half on the other side of the plane. Methane has 6 planes of symmetry, chloromethane has 3, dichloromethane 2, bromochloromethane 1, and bromochlorofluoromethane none: 5-2 Optical Activity Enantiomers cannot be distinguished on the basis of their physical properties, such as boiling points, melting points, and densities. Enantiomers can be distinguished by the way they interact with plane-polarized light. When plane-polarized light is passed through a sample of one of the enantiomers, the plane of polarization is rotated either clockwise or counterclockwise. When the experiment is repeated with the other enantiomer, the plane-polarized light is rotated an equal amount, but in the opposite direction. If facing the light source: •Clockwise rotation: enantiomer is dextrorotary (+) •Counterclockwise rotation: enantiomer is levorotary (-) This interaction with light is called optical activity and enantiomers are often called optical isomers. Optical rotation is measured with a polarimeter. Light is electromagnetic radiation that oscillates perpendicular to its direction of motion. The oscillation of light can be resolved into two perpendicular components. When light is passed through a polarizer, only one of the two perpendicular components of light is passed through. This light is referred to as plane-polarized light. When plane-polarized light interacts with a chiral molecule, the plane of polarization of the light is rotated to the left or right. This effect is called optical rotation and the molecule is said to be optically active. Optical activity is measured using a polarimeter. This device contains a light source, a polarizer to produce the plane-polarized light, a sample cell, and an analyzer to determine the amount of rotation. The measured rotation (in degrees) is the observed optical rotation, α, of the sample. The value of α depends upon: •Structure of the chiral molecule •Concentration of the chiral molecule •Length of the sample cell •Wavelength of the light •Solvent used •Temperature