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PHARMACEUTICAL ORGANIC CHEMISTRY-I

PHARMACEUTICAL ORGANIC CHEMISTRY-I. ALKANES AND ITS REACTIONS SATHEESH KUMAR G. Alkanes: Fuels from Petroleum. Branched-chain hydrocarbons tend to perform better in internal combustion engines. Reactivity of Alkanes. Alkanes have only strong, nonpolar  bonds

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PHARMACEUTICAL ORGANIC CHEMISTRY-I

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  1. PHARMACEUTICAL ORGANIC CHEMISTRY-I ALKANES AND ITS REACTIONS SATHEESH KUMAR G

  2. Alkanes: Fuels from Petroleum • Branched-chain hydrocarbons tend to perform better in internal combustion engines

  3. Reactivity of Alkanes • Alkanes have only strong, nonpolar  bonds • No reaction with nucleophiles or electrophiles • Not much reactivity - paraffins (little affinity)

  4. Chlorination and Bromination of Alkanes

  5. Chlorination and Bromination of Alkanes • Initiation: Homolytic cleavage Note that when an arrowhead with a single barb is used, it denotes movement of a single electron radicals

  6. Chlorination and Bromination of Alkanes

  7. Relative Stabilities of Alkyl Radicals

  8. Reactivity–Selectivity Principle • The very reactive chlorine atom will have lower selectivity and attack pretty much any hydrogen available on an alkane • The less reactive bromine atom will be more selective and tends to react preferentially with the easy targets, i.e. tertiary hydrogens

  9. Radical Substitution of Benzylic and Allylic Hydrogens

  10. Stereochemistry of Radical Substitution • If a chirality center already exists, it may affect the distribution of products • A pair of diastereomers will be formed, but in unequal proportions

  11. Stereochemistry of Radical Substitution

  12. Cyclopropane undergoes electrophilic addition much like an alkene Reactions of Cyclic Compounds

  13. Reactions of Cyclic Compounds • The bond angles in cyclopropane are 60o, which is considerably smaller than the ideal of 109o • The sp3hybrid orbital cannot overlap head-to-head - the bonds are weaker than normal • Consequently three-membered rings undergo ring opening with electrophilic reagents

  14. Radical Reactions in Biological Systems • Alkanes (toxic) are converted to alcohols (nontoxic) in the liver via a radical mechanism • An iron-containing enzyme, cytochrome P450, catalyzes the reaction

  15. Radicals and Stratospheric Ozone • Chlorofluorocarbons (CFCs) are exceptionally stable, but under the intense ultraviolet radiation present in the stratosphere, they undergo a radical dissociation

  16. Radicals and Stratospheric Ozone • The chlorine radicals are ozone-removing reagents • It has been estimated that each chlorine radical destroys 100,000 ozone molecules in a radical chain reaction Overall

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