Stereochemistry PDF

Stereochemistry Organic chemistry Stereochemistry Is the part of the science which deals with structure in three dimensions. Isomers: are different compounds that have the same molecular formula. There are two major classes of isomers: constitutional isomers and stereoisomers. Stereochemistry Consti...

Stereochemistry Organic chemistry Stereochemistry Is the part of the science which deals with structure in three dimensions. Isomers: are different compounds that have the same molecular formula. There are two major classes of isomers: constitutional isomers and stereoisomers. Stereochemistry Constitutional (or structural) isomers differ in the way the atoms are connected to each other. Constitutional isomers have: Different IUPAC names. The same or different functional groups. Different physical properties. Different chemical properties. Stereochemistry Stereoisomers differ only in the way atoms are oriented in space. Stereoisomers have identical IUPAC names (except for a prefix like cis or trans). Because they differ only in the three dimensional arrangement of atoms, stereoisomers always have the same functional group(s). Stereochemistry A comparison of constitutional isomers and stereoisomers Chiral and Achiral Molecules Stereochemistry Chiral and Achiral Molecules Some molecules are like hands. Left and right hands are mirror images of each other, but they are not identical. They are not superimposable. A molecule (or object) that is not superimposable on its mirror image is said to be chiral Stereochemistry Chiral and Achiral Molecules Other molecules are like socks. Two socks from a pair are mirror images that are superimposable. One sock can fit inside another. A sock and its mirror image are identical. A molecule (or object) that is superimposable on its mirror image is said to be achiral Stereochemistry Chiral and Achiral Molecules We can now consider several molecules to determine whether or not they are chiral. Stereochemistry Chiral and Achiral Molecules With CHBrClF, the molecule (labeled A) and its mirror image (labeled B) are not superimposable. No matter how you rotate A and B, all the atoms never align. Stereochemistry Chiral and Achiral Molecules CHBrClF is thus a chiral molecule, and A and B are different compounds. A and B are stereoisomers because they are isomers differing only in the three-dimensional arrangement of substituents. These stereoisomers are called enantiomers. Stereochemistry Enantiomers are mirror images that are not superimposable CHBrClF contains a carbon atom bonded to four different groups, A carbon atom bonded to four different groups is called a tetrahedral stereogenic center, most chiral molecules contain one or more stereogenic centers. A carbon atom with four different groups is a tetrahedral stereogenic center, because the interchange of two groups converts one enantiomer into another. A molecule that is not superimposable on its mirror image is a chiral molecule. Plane of symmetry Stereochemistry Plane of symmetry A plane of symmetry is a mirror plane that cuts a molecule in half, so that one half of the molecule is a reflection of the other half. Achiral molecules usually contain a plane of symmetry but chiral molecules do not. The achiral molecule CH2BrCl has a plane of symmetry, but the chiral molecule CHBrClF does not. Stereochemistry Plane of symmetry Any carbon atom bonded to four different groups is a tetrahedral stereogenic center. To locate a stereogenic center, examine each tetrahedral carbon atom in a molecule, and look at the four groups—not the four atoms—bonded to it. Stereochemistry Plane of symmetry Always omit from consideration all C atoms that can’t be tetrahedral stereogenic centers. These include: CH2 and CH3 groups (more than one H bonded to C) any sp or sp2 hybridized C (less than four groups around C) Stereochemistry Drawing a Pair of Enantiomers Any molecule with one tetrahedral stereogenic center is a chiral compound and exists as a pair of enantiomers. 2-Butanol, for example, has one stereogenic center. To draw both enantiomers, use the typical convention for depicting a tetrahedron: place two bonds in the plane, one in front of the plane on a wedge, and one behind the plane on a dash. Stereochemistry Stereogenic Centers in Cyclic Compounds Does methylcyclopentane have a stereogenic center? Stereochemistry Stereogenic Centers in Cyclic Compounds With 3-methylcyclohexene, the result is different. C3 is bonded to different alkyl groups in the ring. C3 is therefore bonded to four different groups, making it a stereogenic center. Because 3-methylcyclohexene has one tetrahedral stereogenic center it is a chiral compound and exists as a pair of enantiomers. The Polarimeter Stereochemistry The polarimeter It consists of a light source, two lenses (Polaroid or Nicol), and between the lenses a tube to hold the substance that is being examined for optical activity. These are arranged so that the light passes through one of the lenses (polarizer), then the tube, then the second lens (analyzer), and finally reaches our eye. When the tube is empty, we find that the maximum amount of light reaches our eye when the two lenses are so arranged that they pass light vibrating in the same plane. Stereochemistry The polarimeter Stereochemistry The polarimeter Let us adjust the lenses so that a maximum amount of light is allowed to pass. Now let us place the sample to be tested in the tube. If the substance does not affect the plane of polarization, light transmission is still at a maximum and the substance is said to be optically inactive. Stereochemistry The polarimeter If the substance rotates the plane of polarization, then the lens nearer our eye must be rotated to conform with this new plane if light transmission is again to be a maximum, and the substance is said to be optically active. If the rotation of the plane, and hence our rotation of the lens, is to the right (clockwise), the substance is dextrorotatory (Latin: dexter, right); if the rotation is to the left (counterclockwise), the substance is levorotatory (Latin: laevus. left). The symbols (+) and (-) are used to indicate rotations to the right and to the left, respectively. Stereochemistry The polarimeter Stereochemistry Racemic modification A mixture of equal parts of enantiomers is called a racemic modification. A racemic modification is optically inactive: when enantiomers are mixed together, the rotation caused by a molecule of one isomer is exactly canceled by an equal and opposite rotation caused by a molecule of its enantiomer. The prefix (±) is used to specify the racemic nature of the particular sample, as, for example, (±)-lactic acid or (±)-2-methyl-l-butanol.