Stereogenic Centers PDF

Stereogenic Centers Organic chemistry Stereogenic Centers Labeling Stereogenic Centers with R or S Naming enantiomers with the prefixes R or S is called the Cahn Ingold–Prelog system. Because enantiomers are two different compounds, we need to distinguish them by name. This is done by adding the pre...

Stereogenic Centers Organic chemistry Stereogenic Centers Labeling Stereogenic Centers with R or S Naming enantiomers with the prefixes R or S is called the Cahn Ingold–Prelog system. Because enantiomers are two different compounds, we need to distinguish them by name. This is done by adding the prefix R or S to the IUPAC name of the enantiomer. To designate an enantiomer as R or S, first assign a priority (1, 2, 3, or 4) to each group bonded to the stereogenic center, and then use these priorities to label one enantiomer R and one S. Stereogenic Centers Rules Needed to Assign Priority Rule 1: Assign priorities (1, 2, 3, or 4) to the atoms directly bonded to the stereogenic center in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1). In CHBrClF, priorities are assigned as follows: Br (1, highest) → Cl (2) → F (3) → H (4, lowest). In many molecules the lowest priority group will be H. Stereogenic Centers Rules Needed to Assign Priority Rule 2: 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 a higher priority. With 2-butanol, the O atom gets highest priority (1) and H gets lowest priority (4) using Rule 1. To assign priority (either 2 or 3) to the two C atoms, look at what atoms (other than the stereogenic center) are bonded to each C. Stereogenic Centers Rules Needed to Assign Priority The order of priority of groups in 2-butanol is: –OH (1), –CH2CH3 (2), –CH3 (3), and –H (4). If priority still cannot be assigned, continue along a chain until a point of difference is reached. Stereogenic Centers Rules Needed to Assign Priority Rule 3: If two isotopes are bonded to the stereogenic center, assign priorities in order of decreasing mass number. In comparing the three isotopes of hydrogen, the order of priorities is: Stereogenic Centers Rules Needed to Assign Priority Rule 4: 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 bonded to a stereogenic center here. Stereogenic Centers Rules Needed to Assign Priority Other common multiple bonds are drawn below. Stereogenic Centers Rules Needed to Assign Priority Stereogenic Centers Fischer's projection of enantiomers Stereogenic Centers How to Assign R or S to a Stereogenic Center Step Assign priorities from 1 to 4 to each group bonded to the stereogenic center. The priorities for the four groups around the stereogenic center in 2-butanol Stereogenic Centers How to Assign R or S to a Stereogenic Center Step We fix one group for example the higher priority group (1) and rotate the remaining three groups, so that the lowest priority group (4) is in the down position always. Step Trace a circle from priority group Stereogenic Centers How to Assign R or S to a Stereogenic Center If tracing the circle goes in the clockwise direction—to the right —the isomer is named R. If tracing the circle goes in the counterclockwise direction—to the left —the isomer is named S. Stereogenic Centers How to Assign R or S to a Stereogenic Center The other enantiomer is Diastereomers Stereogenic Centers Diastereomers The situation is more complex for compounds with two stereogenic centers, because more stereoisomers are possible. Moreover, a molecule with two stereogenic centers may or may not be chiral. For n stereogenic centers, the maximum number of stereoisomers is 2n. When n = 1, 21 = 2. With one stereogenic center there are always two stereoisomers and they are enantiomers. Stereogenic Centers Diastereomers When n = 2, 22 = 4. With two stereogenic centers, the maximum number of stereoisomers is four, although sometimes there are fewer than four. [* = stereogenic center] Maximum number of stereoisomers = 4 This compound contains two stereogenic centers, C-2 and C-3 Stereogenic Centers Diastereomers Let us first make structure I and its mirror image II, and see if these are superimposable. We find that I and II are not superimposable, and hence must be enantiomers. Stereogenic Centers Diastereomers Are there any other stereoisomers of 2,3-dichloropentane? We can make structure III. Which we find to be non-superimposable on either I or II: it is not the mirror image of either I or II. Diastereomers are stereoisomers that are not mirror images of each other. Stereogenic Centers Diastereomers What is the relationship between III and I ? Between III and II? They are stereoisomers but not enantiomers, Stereoisomers that are not mirror images of each other are called diastereomers. Now, is III chiral? Using models, we make its mirror image, structure IV, and find that this is not superimposable on III. Structures III and IV represent a second pair of enantiomers. Like III, compound IV is a diastereomer of I and of II. Stereogenic Centers Diastereomers How to assign R or S to the stereogenic centers for the stereoisomers of 2,3-dichloropentane. We start with structure (I): Stereogenic Centers Diastereomers We take structure (II): Stereogenic Centers Diastereomers The structure (III): Stereogenic Centers Diastereomers The structure (IV): Meso Compounds Stereogenic Centers Meso Compounds Now let us look at 2,3-dichlorobutane, which also has two stereogenic centers. Does this compound exist in four stereoisomeric forms? Using models as before, we arrive first at the two structures V and VI. These are mirror images that are not superimposable or interconvertible; they are therefore enantiomers, and each should be capable of optical activity Stereogenic Centers Meso Compounds Next, we make VII, which we find to be a diastereomer of V and of VI We now have three stereoisomers; is there a fourth? No. If we make VIII, the mirror image of VII, We find the two to be superimposable; turned end-forend. In spite of its stereogenic centers, VII is not chiral. It cannot exist in two enantiomeric forms, and it cannot be optically active. It is called a meso compound. Stereogenic Centers Meso Compounds A meso compound is one whose molecules are superimposable on their mirror images even though they contain stereogenic centers. A meso compound is optically inactive. We can often recognize a meso structure on sight by the fact that (in at least one of its conformations) one half of the molecule is the mirror image of the other half. This can be seen for meso-2,3-dichlorobutane by imagining the molecule to be cut by a plane lying where the dotted line is drawn. The molecule has a plane of symmetry, and cannot be chiral. Stereogenic Centers Meso Compounds