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Geometrical Isomerism

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Describes Geometrical isomerism aspects of olefins

Describes Geometrical isomerism aspects of olefins

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    Geometrical Isomerism Geometrical Isomerism Presentation Transcript

    • Presented By: Ms. Harpreet Kaur Pannu Mpharm 1 st semester Roll No: 206 Guided By: Dr Amit. G. Nerkar M.pharm, PhD Department of Pharmaceutical Chemistry
    • Introduction Nature of Geometrical isomers Geometrical isomers due to C ═N and N═N Physical properties of geometrical isomers Stability of geometrical isomers Stereochemistry of addition reactions Conclusion References
    • Isomers : Molecules with same chemical formula but different spatial arrangement of atoms Term - invented by Jons Jakob Berzelius (1830) Constitutional isomers : isomers with a different connectivity Urea CH 4 N 2 O Ammonium cyanate CH 4 N 2 O
    • Stereoisomers - isomeric molecules having same molecular formula & sequence of bonded atoms but which differ only in 3-D orientations of their atoms in space Stereoisomerism is of two types: Enantiomers Diastereomers
    • Many biomolecules exist as stereoisomers that are non-superimposable mirror images of one another, called Enantiomers. Eg: Fig: Enantiomers of lactic acid
    • Stereoisomerism is of following types: Geometrical isomerism Fig: Geometrical isomerism in cis 2-butene & trans 2-butene
    • Diastereomers - stereoisomers not related through a reflection operation Not mirror images of each other Include meso compounds, cis-trans (E-Z) isomers, and non-enantiomeric optical isomers Fig: Meso compound
    • Optical Isomerism- optical isomers are called enantiomers .
    • Enantiomers- Stereoisomers that are non-superimposable mirror images; only properties that differ are direction (+ or -) of optical rotation Drug molecules exist in various optical isomers with only one isomer having the biological activity Fig: (S)-thalidomide (effective drug) Fig : ( R)-thalidomide (dangerous drug)
    • Structures containing double bonds or rings exhibit geometrical isomerism Examples of Geometrical Isomers Fig: Geometrical Isomers
    • The double bond leads to increase in the number of optical isomers Here ‘A’ groups have the same configuration From the structures it is clear that the structures are dissymmetric & mirror images of each other
    • Structures with n different double bonds exist in 2 n geometrically isomeric forms example where n =2 is shown as Geometrical Isomerism in a diene Fig: Geometrical Isomerism in a diene
    • Configuration of Oximes identified by prefixes “syn” & “anti” instead of cis & trans In Aldoxime the syn isomer- in which –OH group of the oxime is on the side of the hydrogen of the aldehyde carbon In Ketoxime - specify the group with respect to which the oxime -OH group is syn
    • Fig: Geometrical isomerism due to C ═N and N═N
    • Following are the Physical Properties of Geometrical Isomerism: Dipole moment Melting Point, Boiling Point, Densities Acid Strength Ultraviolet Spectra Infrared and Raman Spectra NMR Spectra X-ray & Electron Diffraction
    • Dipole Moment is variable for cis and trans isomer, sometimes higher for trans and at times for cis isomer compound of the type abC = Cab, C-a has a strong bond moment but C-b has not, the cis isomer should have a considerable over-all dipole moment
    • Dipole Moments of: cis - 1,2dichloro-ethylene cis - 1,2dibromo-ethylene (1.89) (1.35) whereas the moments of the corresponding trans isomer are zero
    • Trans isomer has greater symmetry than the cis - therefore trans has higher melting point Examples are maleic acid m.p. 130˚, and fumaric acid m.p. 300˚
    • Boiling point, Density and Refractive indices- not related directly to configuration Boiling Point, density, and RI- Inverse function of molecular volume Isomer that has the higher value for any of the above properties has the higher value for the other two also
    • Acid strengths are strongly dependent on configuration Depends upon Resonance Example: pka of cis and trans isomers of crotonic acid Cis- crotonic acid trans-crotonic acid (4.44) (4.70)
    • Cis isomer has absorption maxima at slightly shorter wavelength than the trans isomer Example: trans- Stilbene is planar, cis-Stilbene is coplanar and Resonance between the double extremely crowded and thus bond and phenyl rings is has less resonance energy and at a maximum less stable
    • Differences in infrared spectra of geometrical isomers is in the following regions: 1650cm -1 (C=C) 970-690cm -1 (=C-H out of plane vibration) For trans -1,2-dichloroethylene dipole moment is zero Therefore it shows no double-bond streching in IR but shows strong absorption in the raman at 1577cm -1
    • Comparison of trans & cis 1,2-dichloro ethylene: Shows no IR absorption Shows Raman absorption at 1577cm -1 Shows strong IR absorption at 1590cm -1 Shows no raman absorption
    • Difference exists between the coupling constants of cis and trans protons Trans isomer- strongly coupled Coupling constant for cis : 8 to 11 c.p.s and Coupling constant for trans : 17 to 18 c.p.s Differences in chemical shifts- differentiates cis & trans isomers Example: Dimethyl citraconate Dimethyl mesaconate δ = 190 c.p.s δ = 184 c.p.s
    • X- ray analysis of isomers- indicates which isomer is at hand Example: Stretched rubber - all cis form of polyisoprene Gutta-percha - all trans form of polyisoprene Rubber Gutta-percha
    • Thermochemical stability differences between geometrical isomers measured as difference in heat of combustion Alternatively, Heat of hydrogenation used Difference in the heat of hydrogenation = Difference in the heat of formation Therefore, (∆H cis – ∆H trans )=(H trans – H cis ) The cis isomer has higher heat content
    • Difference in heat of hydrogenation of cis & trans stilbene = 5.7 kcal/mole Cis- stilbene Trans - stilbene Stability of the cis- stilbene is less as compared to the trans isomer
    • Six different types of addition reactions: Hydrogenation Electrophilic addition Molecular addition Free radical addition Addition of carbenes and methylenes Nucleophilic addition
    • Study is concerned to tetra substituted double bonds E.g. Hydrogenation is exclusively cis
    • These involves addition of halogens, hydrogen halides,hypohalous acids, sulfenyl halides & formaldehyde etc. Electrophilic addition reactions are generally trans E.g.
    • Molecular addition reactions proceed in the cis fashion E.g. oxidation of maleic acid= meso tartaric acid oxidation of fumaric acid= (±)-tartaric acid
    • Addition of hydrogen bromide to 1-methylcyclohexene(Br, Cl) under influence of light proceeds in the trans fashion gives cis-1,2- di substituted cyclohexanes E.g.
    • Involves addition of divalent carbon compounds: CR 2 to olefins Carbenes add stereospecifically: cis gives cis & trans gives trans
    • Nucleophilic reagents add to the double bond when activated by electron withdrawing groups Trans product is dominant
    • Geometrical isomers differentiates between various forms of drug molecules Various physical properties aid in determining as which isomer is dominant Determines the dominant product in any chemical reaction The intercoversion between various isomers can be brought about by various reagents
    • C.S. Marvel,Geometric Isomerism, in H. Gilman,ed., “Organic Chemistry,” John Wiley & sons,Inc., New York,1943,pp. 444-477 L. Crombie, Geometrical isomerism about Carbon-Carbon double Bonds, Quart. Revs., 6, 101 C.C. Price, “Mechanism Of Reactions at carbon- carbon Double Bonds,” Interscience Publishers, Inc., New York,1946 W. E Truce, nucleophilic Reactions of Thiols with Acetylenes and Chloroethylenes, in N. Kharasch, ed., “Organic Sulfur Compounds” vol 1, Pergamon press,ltd., London, 1961