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Stereochemistry Structural Or Constitutional Isomers

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Stereochemistry Structural Or Constitutional Isomers Stereochemistry Structural or constitutional isomers... have the same molecular formula but different connectivity (skeletal, positional, functional) Stereoisomers... have the same connectivity but a different arrangement of atoms in space Recall: 2-butene H H H CH3 C C C C H3C CH3 H3C H cis-2-butene trans-2-butene . Trans and Cis can also be used for disubstituted cycloalkanes (no bond rotation): 1,2-dimethylcyclopropane H3C CH3 H3C H H H H CH3 cis trans . What if there are less than 2 H atoms? . What if more than one bonded to the alkene is the same? H CH2F H CH2Cl C C C C H3C CH2Cl H3C CH2F The Cahn-Ingold-Prelog Rules for determining substituent priority: Substituents are evaluated by comparing atoms that are equidistant f rom the point of interest. 1) The higher the atomic number, Z, the higher the priority (higher atomic weight). 2) For atoms with the same Z, the heavier (isotope) has higher priority (13C vs. 12C 2H (D) vs. H). 3) If the atoms attached are the same, then move out to the next shell of atoms one bond f urther always comparing atoms that are equidistant. 4) If a substituent contains -bonds, interpret them as multiple -bonds between the same atoms (e.g. an aldehyde has a carbon which is bonded to 2 oxygen atoms and one hydrogen atom). CH2CH3 C CH2OH CH2F C CH2CH2Br H OCH3 C OCH3 C C OCH H 3 H CF3 C CH2SH H3C CF3 C C H CH2SH Another level of complexity with multiple bonds: N000 C000 C N C N C N000 000 O O C000 C C O H OH O000 CH3 C C H vs. C CH3 CH3 How do you know when molecules are the same? Molecules are identical if, in equivalent conformations, they are superimposable. H H H3C H C C C C H3C CH2CH3 H CH2CH3 H3HC H C C H3HC CH2CH3 . If the connectivity of two molecules differs, then they are constitutional isomers. If the connectivity is the same but the molecules are not superimposable, then they are stereoisomers. How about these two? Y axis F HO X axis F Z axis OH Rotate about y-axis to place most atoms in the plane of the page. Rotate ~160º F counterclockwise HO about z-axis F OH Stereochemistry at tetrahedral centres: H H C C CH3 H C Cl 3 Cl CH2CH3 H3CH2C Are these molecules the same? H H Rotate 180° C around CH bond C CH CH CH Cl 3 Cl 2 3 CH2CH3 CH3 Configuration: the actual arrangement in space of the substituents around a given asymmetric atom that can be designated R or S. Also applies to E/Z. Chiral or stereogenic centre: Any tetrahedral atom, frequently carbon, which has 4 different substituents. Also called stereocentre There are only 2 possible configuration for tetrahedral centres: H C CH Cl 3 CH2CH3 These two configurations cannot be interchanged without breaking bonds! H If you switch any two substituents, then the configuration changes: C CH CH H Cl Cl 2 3 H CH3 C C C CH2CH3 CH3 Cl Cl H H3C CH3 CH2CH3 CH2CH3 Make 2 models, convince yourself this is true… Solutions that rotate plane-polarized light are said to be "optically active". H H C C CH3 H C HO 3 OH CH2CH3 H3CH2C (-)-2-Butanol (+)-2-Butanol Dextrotatory - rotates ppl in a clockwise direction. Levorotatory - rotates ppl in counterclockwise direction How do we distinguish between different absolute configurations? How do we label them? Historically…(bit still used today for certain classes of molecules) The D/L Convention of Fischer Absolute configuration: The actual three-dimensional arrangement of groups around an asymmetric center. H All sugars with the same absolute configuration next to the CH2OH group are "D". Those with opposite absolute L HO CH2OH configuration are " " CHO D-Glyceraldehyde Note that this designation now has nothing to do with the direction the molecule rotates ppl. How do we name compounds such that their absolute configuration is indicated? 1. We assign priorities, 1 to 4 (using the rules established by Cahn, Ingold and Prelog), to the substituents of an asymmetric atom 2. Reorient the centre such that the lowest priority, 4, is positioned into the plane of the board. 3. If the remaining substituents are arrayed 1-2-3 in clockwise order, then the center has R configuration: 1 1 4 2 C C 2 3 4 3 4. If the order is counterclockwise, 3 2 4 the centre is S: 1 C 2 1 4 3 Try to assign these (a model kit is exceedingly helpful!): CH3 CO2H CH2OH Cl C C H3C H H3C H Cl Cl O H H Br H3C Br C C H HO CH3 O Enantiomers: are molecules that are non-superimposable mirror-images of one another. H H C C CH3 H C Cl 3 Cl CH2CH3 H3CH2C R S . Enantiomers have identical physical properties in all respects (thus difficult to separate from each other), except they rotate plane polarized light in opposite directions (unrelated to absolute configuration). Such molecules are said to be optically active. Thus, a 50/50 mixture of two enantiomers (a racemic mixture) will not rotate plane polarized light. A mixture of enantiomers that is optically active (i.e. not a 50/50 mxture) is said to be scalemic. Why would only one enantiomer cause a certain problem/react with a given substrate? H H Cl Cl F F Br Br H H Br Cl F F A - F Cl Br B - Br C - Cl B B A A C C Enzyme Enzyme . An object is chiral if it is nonsuperimposable on its mirror image. Achiral : an object (molecule) which is superimposable upon its mirror image. If a molecule has a plane of symmetry it is not chiral. Can a molecule without any chiral centres be chiral? Br Br Br Br C C C C C C H H H H HO OH OH HO BINAP Can a molecule with centres be achiral? H3C H H CH Br Br 3 C C C C H H H3C Br Br CH3 . Only if there is an even number of chiral centres. Meso compounds have chiral centres, but are achiral (they have an internal plane of symmetry) H H H Invert H OH ring spin 180° HO OH H H OH OH OH H H HO OH If there exists any (accessible) conformation in which a molecule is achiral, then it is achiral, period. H H H HO H HO HO OH Atoms other than carbon? Inversion of Configuration Ph Ph H CH C CH N H CH C N 3 2 3 3 2 N H3CH2C CH3 CH Ph 3 CH3CH2Br Ph H3CH2C CH3 N Br- CH2CH3 P S H3CH2C CH3 H3CH2C O Ph Ph Molecules with two chiral centers... H H H3C Br Br CH3 C C C C H H HO CH3 H3C OH 1 2 Br Br H3C H H CH3 C C C C H H HO CH3 H3C OH 3 4 Diastereomers: non-superimposable, non-mirror images . Diastereomers have different physical properties and can easily be separated from one another. A molecule can have a maximum of 2n possible stereoisomers (n = number of chiral centres). Draw all possible stereoisomers and indicate which are enantiomers and which are diastereomers: H I Br Br H Cl CH3 F H3C Cl Fischer Projections: a shorthand notation for showing absolute configurations. In no way do these diagrams attempt to show the actual shape of molecules. 90° CHO CHO H = H C OH H OH HOH2C CHO CH2OH CH2OH OH = = 180° H HOH2C CHO CH2OH CH2OH OH HO C H = HO H CHO CHO A 90° or 270° rotation of a Fischer projection generates… A 180° rotation of a Fischer projection generates… Use models to convince yourself this is true! Fischer projections were developed to aid in drawing molecules (sugars) with many chiral centers. It is easy to spot enantiomers and superimposable molecules when drawn as these projections. CHO CHO CH2OH CH2OH HO H H OH HO H H OH HO H H OH HO H HO H CH2OH CH2OH CHO CHO Amino acids are chiral and have the same absolute configuration. Amino acids H2NCH2CO2H H2NCHCH3CO2H Glycine Alanine CO2H CO2H CHO D-Glyceraldehyde H2N H H2N H H OH CH3 CH3 CH2OH L-Alanine CO2H CO2H CO2H H2N H H2N H H2N H H2C L-tryptophan CH2SH CH2OH L-cysteine L-Serine NH CO2H H2N H H2C NH These are called L-amino acids not because they all rotate L-Histidine plane-polarized light in a levorotatory fashion (they don't) N but because of their structural relationship to D- glyceraldehyde. H CN C + H CN CH2 CN H CH2(CH2)16CH3 CH (CH ) CH HO O 2 2 16 3 C C OH H C 2 Hydrolase H2C O O Enzyme HC O CH2(CH2)16CH3 H C O CH (CH ) CH C C 2 2 16 3 H2O H C 2 O H2C O O O C CH (CH ) CH 2 2 16 3 C CH2(CH2)16CH3 O O Tristearyl glycerol, a triglyceride A diglyceride. You cannot make one enantiomer of a product preferentially starting only with achiral molecules. In other words, you must use optically active material (catalyst, reactant, solvent etc.) in order to prepare something that is optically active. The chiral pool... CO2H H OH HO H OHO HO H OH HO H H H CO2H H OH R-+-Pinene O D-Tartaric acid R-+-Camphor D-Mannose Ph CO2H Asymmetric Ph O P Catalysis O Ru O P H ,cat.
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