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Biological Chemistry First Year Organic Chemistry Lecture Nine : Stereochemistry Convener : Dr. Fawaz Aldabbagh Isomers are different compounds that have the same molecular formula Constitutional isomers are isomers that differ because their atoms are connected in a different order C2H10 Butane Isobutane Stereoisomers differ only in the arrangement of their atoms in space Geometric Isomers Cl H Cl H C2H2Cl2 Cl H cis-1,2-Dichloroethene H Cl trans-1,2-Dichloroethene Enantiomers are stereoisomers whose molecules are nonsuperimposable mirror images of one another Objects that are superimposable on their mirror images are said to be achiral Involves a tetrahedral sp3 atom H CH3 C CH2 CH3 OH Chiral Centre CH3 HO H C 2-Butanol CH3 Interchanging any two groups at a chiral H OH C centre (stereocentre) that bears four CH2 CH2 different groups converts one enantiomer CH3 CH3 into another One structure can be superimposed on another CH3 H C OH CH3 CH3 HO C H CH3 If any of the attached groups onto the tetrahedral atom are the same, the centre is achiral. 2-Propanol The ultimate way to test for molecular chirality is to construct models of the molecule and its mirror image and then determine whether they are superimposable Screwdriver is achiral Socks are achiral Golf club is chiral Gloves are chiral A molecule will not be chiral if it possess a centre or plane of Symmetry Properties of Enantiomers Enantiomers have identical melting points and boiling points Enantiomers have identical solubilities in solvents Enantiomers have identical spectra and refractive index Enantiomers interact, and react with achiral molecules in the same manner Enantiomers interact and react with other chiral molecules at different rates Enantiomers rotate plane-polarised light by equal amounts but in opposite directions Chiral molecules are optically active Plane-polarised light Oscillation of electrical field of ordinary light occurs in all possible directions Polarimeter is a devise used to measure the effect of planepolarised light on an optically active compound Nomenclature of Enantiomers (R-S) System Enantiomers have opposite configuration at the chiral or stereocentre Priority 1. Highest atomic number first; 2. If the first atom attached to the chiral centre has the same mass, then next set of atoms in the unassigned groups are examined; 3. We now rotate the formula (or model) so that the group with lowest priority is away from us; 4. Unsaturated substituents have greater priority than saturated substituents The Absolute Configuration c CH3 HO H a C d b CH2 CH3 c CH3 H d C OH a b CH2 CH3 H Clockwise R-2-butanol H anticlockwise S-2-butanol Assign (R) or (S) H2 C H CH3 Cl C C H2C H H OH C C C(CH3)3 CH2CH3 Which structures represent enantiomers or two molecules of the same compound in different orientations? H Br H F C Br Cl C and Cl F CH3 H Cl C Cl Br Br and C H CH3 No Correlation between the direction of rotation of plane polarised light and the absolute configuration of a molecule Clockwise Rotation (+) – dextrorotatory Anti-Clockwise Rotation (-) – levorotatory Same Configuration CH3 H2 C C H HO CH2CH3 (R)-(+)-2-Methyl-1-butanol CH3 H2 C C H Cl CH2CH3 (R)-(-)-1-Chloro-2-methylbutanol An equimolar mixture of two enantiomers is called a Racemic Mixture It is Optically Inactive O O H S-(+)-Carvone Principle component of Caraway seed oil and responsible for the characteristic odour H R-(-)-Carvone Principle component of Spearmint oil and responsible for the characteristic odour Receptor Sites in the Nose are Chiral Biochemical effects arise when molecules interact with biomolecules such as enzymes or receptors. These are constructed of chiral building blocks called amino acids. Therefore, the biomolecule only accepts molecules of a certain configuration or shape. Only the S- is pharamacologically active COOH H3 C H Ibuprofen - anti-inflammatory drug COOH H3 C Ph H COOH H3 C H R- H O * N O CH3 CH3 O N * O H Ph S- Thus, only one enantiomer will fit into the receptor or enzyme active site thalidomide S – induces a teratogenic response R – induces a hypnotic or seductive response Draw the R- and S- forms of thaildomide Nobel Prize 2001 Professor William Knowles Professor Ryoji Noyori Professor K. Barry Sharpless For synthesis of optically active compounds – asymmetric synthesis SN2 H3C HO H C Br C6H13 R-(-)-2-Bromooctane CH3 CH3 HO C Br H C6H13 HO H C6H13 S-(+)-2-Octanol Transition State Inversion of Stereochemistry