streochemistry

streochemistry

• 1.
  Presented by Dr. Gopalkrushna H.Murhekar
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   Introduction  History Chiraland Achiral molecules Optical Activity Racemic Mixtures Baeyer Strain Theory  Conformations References
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  Stereochemistry, a subdisciplineof chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. An important branch of stereochemistry is the study of chiral molecules. Stereochemistry is also known as 3D chemistry because the prefix "stereo-" means "three-dimensionality". The study of stereochemistry focuses on streoisomer and spans the entire spectrum of organic, inorganic, physical, boilogical and especially supramolecular chemistry.
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  History Louis Pasteur couldrightly be described as the first stereochemist, having observed in 1849 that salts of tartaric acid collected from wine production vessels could rotate plane polarized light , but that salts from other sources did not. This property, the only physical property in which the two types of tartrate salts differed, is due to optical isomerism. In 1874, Jacobus Henricus van 't Hoff and Joseph LeBel explained optical activity in terms of the tetrahedral arrangement of the atoms bound to carbon.
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  Stereoisomers – compoundswith the same connectivity, different arrangement in space. Enantiomers – stereoisomers that are non-superimposible mirror images; only properties that differ are direction (+ or -) of optical rotation. Diastereomers – stereoisomers that are not mirror images; different compounds with different physical properties.
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  • Although everythinghas a mirror image, mirror images may or may not be superimposable. • Some molecules are like hands. Left and right hands are mirror images, but they are not identical, or superimposable. Chiral and Achiral Molecules
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  7 • Other moleculesare like socks. Two socks from a pair are mirror images that are superimposable. A sock and its mirror image are identical. • A molecule or object that is superimposable on its mirror image is said to be achiral. • A molecule or object that is not superimposable on its mirror image is said to be chiral.
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  Optical Activity • Enantiomersrotate the plane of polarized light in opposite directions, but same number of degrees.
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  Polarimeter
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  Racemic Mixtures If opticallyactive reagents combine to form a chiral molecule, a racemic mixture is formed. It is optically inactive.
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  Johann Friedrich Wilhelm Adolfvon Baeyer • First graduate student of Kekule’ • Developed theory that cycloalkanes possess different amounts of strain or stability, depending on the size of the ring. • Awarded Nobel Prize in 1905 for work with organic dyes. "I have never set up an experiment to see whether I was right, but to see how the materials behave". Baeyer Strain Theory
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  The bond angelis of 109˚ 28' (or 109.5˚) for carbon atom in tetrahedral geometry (methane molecule). Baeyer observed different bond angles for different cycloalkanes and also observed some different properties and stability. On this basis, he proposed angle strain theory. The theory explains reactivity and stability of cycloalkanes. Baeyer proposed that the optimum overlap of atomic orbitals is achieved for bond angel of 109.5o . In short, it is ideal bond angle for alkane compounds. Effective and optimum overlap of atomic orbitals produces maximum bond strength and stable molecule. If bond angles deviate from the ideal then ring produce strain. Higher the strain higher the instability. Higher strain produce increased reactivity and increases heat of combustion. Baeyer proposed “any deviation of bond angle from ideal bond angle value (109.5o ) will produce a strain in molecule. Higher the deviation lesser the instability”
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  4-13
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  Conformations different spatial arrangementsof atoms that result from rotations about single (σ) bonds Conformer: a specific conformation of a molecule Conformational Analysis of Ethane Sawhorse C C H H H H H H HH H H H H
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  Staggered EclipsedNewman projection Thereare two conformations of ethane: Dihedral (torsion) angle: angle between an atom (group) on the front atom of a Newman Projection and an atom (group) on the back atom H H H H H H Front carbon Back carbon
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  16 Energy vs. dihedralangle for ethane
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  Conformation of Cyclohexane
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  Energy Diagram ofCyclohexane
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  References