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Organic chemistry B Chapter 12 Alkynes By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

Organic chemistry B Chapter 12 Alkynes By Prof. Dr. Adel M. Awadallah Islamic University of Gaza. Alkenes and Alkynes Hydrocarbons (contain only carbon and hydrogen) Saturated: (Contain only single bonds) Alkanes (C n H 2N + 2 ) Cycloalkanes (C n H 2N ) b) Unsaturated: contain

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Organic chemistry B Chapter 12 Alkynes By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

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  1. Organic chemistry B Chapter 12 Alkynes By Prof. Dr. Adel M. Awadallah Islamic University of Gaza

  2. Alkenes and Alkynes Hydrocarbons(contain only carbon and hydrogen) • Saturated: (Contain only single bonds) Alkanes (CnH2N + 2 ) Cycloalkanes (CnH2N ) b)Unsaturated: contain Alkenes: double bonds (,,,CnH2N) Alkynes: triple bonds ((CnH2N - 2) Aromatic: benzene like compounds

  3. Facts about double and triple bonds

  4. Bonding in acetylene (ethyne)

  5. Nomenclature of alkenes and Alkynes • The ending ene is used for alkenes and yne for alkynes • Select the longest chain that includes both carbons of the multiple bond • Number the chain from the end nearest to the multiple bond • Indicate the position of the multiple. Examples

  6. Assigning Priority • Alkenes and alkynes are considered to have equal priority • In a molecule with both a double and a triple bond, whichever is closer to the end of the chain determines the direction of numbering. • In the case where each would have the same position number, the double bond takes the lower number. • In the name, “ene” comes before “yne” because of alphabetization.

  7. Preparation of Alkynes • 1) Generation of a triple bond • Note1) We need a very strong base for the second dehydrohalogenation, since vinylic halides are very unreactive • Note2): This is a method for preparation of vinilyc halides since substitution of alkenes is difficult

  8. 2) Increasing the size of a terminal alkyne

  9. Reduction of alkynes With hydrogen in presence of ordinary nickel, platinum or palladium gives alkanesHydrogentaion of alkynes with Lindlar’s catalyst gives a cis alkene

  10. A Lindlar catalyst is a heterogeneouscatalyst that consists of palladium deposited on calcium carbonate and treated with various forms of lead. The lead additive serves to deactivate the palladium sites. A variety of "catalyst poisons" have been used including lead acetate and lead oxide. The palladium content of the catalyst is usually 5% by weight. The catalyst is used for the hydrogenation of alkynes to alkenes. • As described by its inventor,[1][2] the catalyst is prepared by reduction of palladium chloride in a slurry of calcium carbonate followed by adding lead acetate. By this approach, one obtains a catalyst with a large surface area. Further deactivation of the catalyst with quinoline enhances its selectivity, preventing formation of alkanes. An example of alkyne reduction is the reduction of phenylacetylene to styrene.[1] • Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene.[3]

  11. Lindlar’s Catalyst

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