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PROSTEREOISOMERISM Topicity of Ligands and Faces STEREOCHEMISTRY II PROSTEREOISOMERISM Topicity of Ligands and Faces SEM-2, CC-3 PART-2, PPT-6 Dr. Kalyan Kumar Mandal Associate Professor St. Paul’s C. M. College Kolkata Prostereoisomerism Topicity of Ligands and Faces Part-2 CONTENTS ❖ Introduction ❖ Homotopic Ligands ❖ Homotopic Faces Topicity of Ligands and Faces: Introduction • In certain molecules, such as propionic acid (A; Figure 1), a nonstereogenic center (here Cα) can be transformed into a stereogenic center by replacement of one or other of two apparently identical ligands by a different one. Such ligands are called “homomorphic” from Greek homos meaning same and morphe meaning form. They are identical only when separated from the rest of the molecule. • Thus the replacement of HA at Cα in propionic acid by OH generates the chiral centre of (S)-lactic acid, whereas the analogous replacement of HB gives rise to the enantiomeric (R)-lactic acid. The Cα centre in propionic acid has, therefore, been called a “prochiral” as well as “prostereogenic centre.” This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction • HA and HB at such a centre are called “heterotopic ligands” from Greek heteros meaning different and topos meaning place. Prochiral axes and planes may similarly be defined in relation to chiral axes and planes. • Substitution is one of the common ways of interconverting organic molecules, another is addition. The chiral centre in lactic acid (B and C; Figure 2) can also be generated by the addition of hydride (e.g., from sodium borohydride or lithium aluminium hydride ) to the carbonyl group of pyruvic acid (A; Figure 2). This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction • Depending on the face of the keto acid (pyruvic acid) the hydride adds to, either (S)- or (R)-lactic acid is obtained. The addition of hydride ion (H-) to the front/top face of the keto acid as depicted in Figure 2 will give rise to (R)-lactic acid (B), whereas (S)-lactic acid is obtained by addition of the nucleophile to the rear face of the C=O group. Thus the carbonyl group in pyruvic acid is also said to be prochiral and to present two heterotopic faces. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction • A prochiral axis (in chloroallene A; Figure 3) can be converted into the chiral allenes, B and C by replacement of HA and HB by C1 separately. • Ligands (atoms or groups in a molecule) and faces may be homotopic or heterotopic. Heterotopic ligands and faces may be either enantiotopic or diastereotopic. It may be pointed out that topicity describes the relationships of two or more homomorphic ligands (or faces) which together constitutes a set. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Topicity of Ligands and Faces: Introduction • In view of the interrelationship between topicity of ligands and isomerism in general, it may be instructive to draw a classification diagram (Figure 4) for topicity and to compare it with that drawn for isomerism. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands and Faces • Two criteria, namely, a substitution (or addition) criterion and (or) a symmetry criterion are employed to determine the topic relationships of homomorphic ligands and faces (only one test suffices). Substitution and Addition Criteria • Two homomorphic ligands are homotopic if substitution (or replacement) of first one and then the other by a different ligand leads to the same structure. (The replacement ligand must be different not only from the original one but also from all other ligands attached to the same atom.). Examples of homotopic ligands are shown in Figure 5. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Substitution Criterion • The two hydrogen atoms in methylene bromide (A; CH2Br2) are homotopic because replacement of either by, say, chlorine gives the same CHClBr2, molecule (B). • The three methyl hydrogen atoms in acetic acid (C; CH3CO2H) are homotopic because replacement of any one of them by, say, chlorine gives one and the same chloroacetic acid (D). • The two methine hydrogen atoms in (R)-(+)-tartaric acid (E) are homotopic because replacement of either of them, for example by deuterium, gives the same (2R,3R)-tartaric-2-d acid (F). • The three methyl hydrogen atoms in methyl chloride (CH3Cl) are homotopic because replacement of any one of them by, say, bromine gives one and the same dibromochloromethane (CHClBr2). This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Substitution Criterion Homotopic Faces: Addition Criterion • Two corresponding faces of a molecule (usually, but not invariably, faces of a double bond) are homotopic when addition of the same reagent to either face gives the same product. • Addition of HCN to acetone will give the same cyanohydrin (A; Figure 6), no matter to which face addition occurs and addition of bromine to ethylene similarly gives BrCH2CH2Br irrespective of the face of approach. The two faces of the C=O bond of acetone and of the C=C bond of ethylene are, thus homotopic. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Faces: Addition Criterion • Ethanol is formed by the addition of MeMgI to either face of the C=O bond of formaldehyde. Thus, the two faces of the C=O bond of formaldehyde are homotopic. • Consequently, two faces of the C=O of any symmetrically substituted carbonyl compounds, such as ketones of the type R2CO, e.g, acetone, 3-pentanone, benzophenone, etc., are homotopic. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Symmetry Criterion • Ligands are homotopic (by internal comparison) if they can interchange places through operation of a Cn symmetry axis. Thus the bromine atoms in methylene bromide (A; symmetry point group C2v) are homotopic since they exchange places through a 1 180° turn around the C2 axis (C 2). • Similarly, the methine hydrogen atoms of (+)-tartaric acid (B) are interchanged by operation of the C2 axis (the molecule belongs to point group C2). Homotopic atoms in methylene bromide, and active-tartaric acid are shown in Figure 8. • The three methyl hydrogen atoms of CH3CO2H are homotopic when rotation is fast. Rotation around the H3C-CO2H axis is rapid on the time scale of most experiments. Under this condition the three methyl H’s will exchange their places under the operation of C3 symmetry axis. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Symmetry Criterion Homotopic Ligands: Symmetry Criterion • The presence of a symmetry axis in a molecule does not guarantee that homomorphic ligands will be homotopic. It is necessary that operation of the symmetry axis make the nuclei in question interchange places. Thus in 1,3-dioxolane (Figure 9), in its average planar conformation, the hydrogen atoms (HE and HF) at C-2 are homotopic, since they are interchanged by operation of the C2 axis (the symmetry point group of the molecule is C2v). • On the other hand, the geminal hydrogen atoms at C-4, or C-5, are not interconverted by the C2 symmetry operation and are therefore heterotopic (HA with respect to HB and HC with respect to HD). However, HA and HD are homotopic (as are HB and HC), being interchanged once again by the C2 axis. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Symmetry Criterion • The two hydrogen atoms in each of the three dichloroethylenes (1,l-, cis-1,2-, trans-1,2-) and the four hydrogen atoms in methane (CH4), ethylene (H2C=CH2), and allene (H2C=C=CH2), are homotopic. It might be noted that, in a rigid molecule, the number of homotopic ligands in a set cannot be greater than the symmetry number of the molecule in question. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Ligands: Symmetry Criterion • In allene, the geminal hydrogens are interchangeable in pairs by rotation around the molecular axis (C2 axis) while the non-geminal hydrogens are interchangeable through rotation around the two C2 axes perpendicular to the former. All the four hydrogen atoms are thus homotopic. • This proves that if ligands A and B are found homotopic through rotation around one Cn axis and ligands B and C through rotation around another Cn axis, all three (A, B, and C) form a set of homotopic ligands. • The two methine hydrogens (as also two OH and the two CO2H groups) of (+)-tartaric acid are interchangeable through rotation around a C2 axis either in the eclipsed conformation or in an anti conformation. These pairs of ligands are, therefore, homotopic. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Faces: Symmetry Criterion • Faces of double bonds are homotopic when they can be interchanged by operation of a symmetry axis. Since there are only two such faces, the pertinent axis must, of necessity, be of even multiplicity so as to contain C2. • Thus, the two faces of acetone are interchanged by the operation of the C2 axis (the molecule is of symmetry C2v); the two faces of ethylene (D2h) are interchanged by operation of two of the three C2 axes (either the one containing the C=C segment or the axis at right angles to the first one and in the plane of the double bond). • The faces in acetone, ethylene, 1,1-dichloroethylene, cis-1,2- dichloroethylene, and allene (H2C=C=CH2) are homotopic. This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata Homotopic Faces: Symmetry Criteria • The faces in acetone, ethylene, 1,1-dichloroethylene, cis-1,2- dichloroethylene, and allene are exchangeable by C2 axis.
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