Organic Chemistry Stereochemistry PDF

Organic Chemistry, Second Edition Janice Gorzynski Smith University of Hawai’i Introduction to Stereochemistry (Ch. 5) Prepared by Rabi Ann Musah State University of New York at Albany Copyrig...

Organic Chemistry, Second Edition Janice Gorzynski Smith University of Hawai’i Introduction to Stereochemistry (Ch. 5) Prepared by Rabi Ann Musah State University of New York at Albany Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1  Stereochemistry The Two Major Classes of Isomers: Recall that isomers are different compounds with the same molecular formula. The two major classes of isomers are constitutional isomers and stereoisomers. Constitutional/structural isomers have different IUPAC names, the same or different functional groups, different physical properties and different chemical properties. Stereoisomers differ only in the way the atoms are oriented in space. They have identical IUPAC names (except for a prefix like cis or trans). They always have the same functional group(s). A particular three-dimensional arrangement is called a configuration. Stereoisomers differ in configuration. 2  Stereochemistry Stereoisomers may be geometric (cis/trans) or optical. Optical isomers are chiral and exhibit optical activity. 3  Stereochemistry Chiral and Achiral Molecules: Although everything has a mirror image, mirror images may or may not be superimposable. A molecule or object that is superimposable on its mirror image is said to be achiral (lacking-chirality). A molecule or object that is not superimposable on its mirror image is said to be chiral. 4  Stereochemistry Chiral and Achiral Molecules: A carbon atom with four different groups is a chiral center. The case of 2-butanol. A and its mirror image labeled B are not superimposable. Thus, 2-butanol is a chiral molecule and A and B are isomers. Non-superimposable mirror image stereoisomers like A and B are called enantiomers. 5  Stereochemistry Chiral and Achiral Molecules: With one stereogenic center, a molecule will always be chiral. With two or more stereogenic centers, a molecule may or may not be chiral. Achiral molecules usually contain a plane of symmetry but chiral molecules do not. A plane of symmetry is a mirror plane that cuts the molecule in half, so that one half of the molecule is a reflection of the other half. 6  Stereochemistry Stereogenic Centers: 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. Always omit from consideration all C atoms that cannot be tetrahedral stereogenic centers. These include CH2 and CH3 groups Any sp or sp2 hybridized C 7  Stereochemistry Identifying of Stereogenic Centers: Stereogenic centers may also occur at carbon atoms that are part of a ring. To find stereogenic centers on ring carbons, always draw the rings as flat polygons, and look for tetrahedral carbons that are bonded to four different groups. Contains a plane of symmetry 8  Stereochemistry Drawing Stereogenic Centers - the wedge diagram: To draw both enantiomers of a chiral compound such as 2-butanol, 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. Then, to form the first enantiomer, arbitrarily place the four groups—H, OH, CH3 and CH2CH3—on any bond to the stereogenic center. Then draw the mirror image. 9  Stereochemistry Drawing Stereogenic Centers - the wedge diagram: 10  Stereochemistry Labeling Stereogenic Centers with R or S: The three dimensional arrangement about a tetrahedral carbon atom is referred to as its configuration. Since enantiomers are two different compounds, they need to be distinguished by name. This is done by adding the prefix R or S to the IUPAC name of the enantiomer. Naming enantiomers with the prefixes R or S is called the Cahn- Ingold-Prelog system. To designate enantiomers as R or S, priorities must be assigned to each group bonded to the stereogenic center, in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1). 11  Stereochemistry Labeling Stereogenic Centers with R or S: If two atoms on a stereogenic center are the same, assign priority based on the atomic number of the atoms bonded to these atoms. One atom of higher atomic number determines the higher priority. 12  Stereochemistry Labeling Stereogenic Centers with R or S: If two isotopes are bonded to the stereogenic center, assign priorities in order of decreasing mass number. Thus, in comparing the three isotopes of hydrogen, the order of priorities is: 13  Stereochemistry Labeling Stereogenic Centers with R or S: To assign a priority to an atom that is part of a multiple bond, treat a multiply bonded atom as an equivalent number of singly bonded atoms. For example, the C of a C=O is considered to be bonded to two O atoms. Other common multiple bonds are drawn below: 14  Stereochemistry Labeling Stereogenic Centers with R or S: Figure 5.6 Examples of assigning priorities to stereogenic centers 15  Stereochemistry Labeling Stereogenic Centers with R or S: 16  Stereochemistry Labeling Stereogenic Centers with R or S: 17  Stereochemistry Labeling Stereogenic Centers with R or S: 18  Stereochemistry Diastereomers: For a molecule with n stereogenic centers, the maximum number of stereoisomers is 2n. Let us consider the stepwise procedure for finding all the possible stereoisomers of 2,3- dibromopentane. 19  Stereochemistry Diastereomers: If you have drawn the compound and the mirror image in the described manner, you have only to do two operations to see if the atoms align. Place B directly on top of A; and rotate B 180° and place it on top of A to see if the atoms align. In this case, the atoms of A and B do not align, making A and B nonsuperimposable mirror images—i.e., enantiomers. Thus, A and B are two of the four possible stereoisomers of 2,3-dibromopentane. 20  Stereochemistry Diastereomers: Switching the positions of H and Br (or any two groups) on one stereogenic center of either A or B forms a new stereoisomer (labeled C in this example), which is different from A and B. The mirror image of C is labeled D. C and D are enantiomers. Stereoisomers that are not mirror images of one another are called diastereomers. For example, A and C are diastereomers. 21  Stereochemistry Diastereomers: Figure 5.8 Summary: The four stereoisomers of 2,3-dibromopentane 22  Stereochemistry Meso Compounds: Let us now consider the stereoisomers of 2,3-dibromobutane. Since this molecule has two stereogenic centers, the maximum number of stereoisomers is 4. To find all the stereoisomers of 2,3-dibromobutane, arbitrarily add the H, Br, and CH3 groups to the stereogenic centers, forming one stereoisomer A, and then draw its mirror image, B. 23  Stereochemistry Meso Compounds: To find the other two stereoisomers if they exist, switch the position of two groups on one stereogenic center of one enantiomer only. In this case, switching the positions of H and Br on one stereogenic center of A forms C, which is different from both A and B. A meso compound is an achiral compound that contains 24 tetrahedral stereogenic centers. C is a meso compound.  Stereochemistry Meso Compounds: Compound C contains a plane of symmetry, and is achiral. Meso compounds generally contain a plane of symmetry so that they possess two mirror image halves. Because one stereoisomer of 2,3-dibromobutane is superimposable on its mirror image, there are only three 25 stereoisomers, not four.  Stereochemistry Meso Compounds: Figure 5.9 Summary: The three stereoisomers 2,3-dibromobutane 26  Stereochemistry R and S Assignments in Compounds with Two or More Stereogenic Centers. When a compound has more than one stereogenic center, R and S configurations must be assigned to each of them. One stereoisomer of 2,3-dibromopentane The complete name is (2S,3R)-2,3-dibromopentane 27  Stereochemistry Disubstituted Cycloalkanes: Consider 1,3-dibromocyclopentane. Since it has two stereogenic centers, it has a maximum of four stereoisomers. Recall that a disubstituted cycloalkane can have two substituents on the same side of the ring (cis isomer, A) or on opposite sides of the ring (trans isomer, B). These compounds are stereoisomers but not mirror images. 28  Stereochemistry Disubstituted Cycloalkanes: To find the other two stereoisomers if they exist, draw the mirror images of each compound and determine whether the compound and its mirror image are superimposable. The cis isomer is superimposable on its mirror image, making the images identical. Thus, A is an achiral meso compound. 29  Stereochemistry Disubstituted Cycloalkanes: The trans isomer is not superimposable on its mirror image, labeled C, making B and C different compounds. B and C are enantiomers. Because one stereoisomer of 1,3-dibromocyclopentane is superimposable on its mirror image, there are only three stereoisomers, not four. 30  Stereochemistry Figure 5.10 Summary—Types of isomers 31  Stereochemistry Figure 5.11 Determining the relationship between two nonidentical molecules 32