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) a)Saturated: (Contain only single bonds) Alkanes (C n H 2N + 2 ) Cycloalkanes (C n H 2N ) b) Unsaturated: contain Alkenes: double bonds (,,,C n H 2N ) Alkynes: triple bonds ((C n H 2N - 2 ) Aromatic: benzene like compounds

Facts about double and triple bonds

Bonding in acetylene (ethyne)

Nomenclature of alkenes and Alkynes 1)The ending ene is used for alkenes and yne for alkynes 2)Select the longest chain that includes both carbons of the multiple bond 3)Number the chain from the end nearest to the multiple bond 4)Indicate the position of the multiple. Examples

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.

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 vi nilyc halides since substitution of alkenes is difficult

2) Increasing the size of a terminal alkyne

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

A Lindlar catalyst is a heterogeneous catalyst 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.heterogeneouscatalystpalladiumcalcium carbonateleaddeactivatelead acetatelead oxide hydrogenationalkynesalkenes 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][1][2]palladium chloridequinolinealkanes phenylacetylenestyrene[1] Alkyne reduction is stereoselective, occurring via syn addition to give the cis-alkene.[3]stereoselectivesyn additioncis-alkene[3]

Lindlar’s Catalyst