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SCAN0011 - \g" VII Alkyne Reactions(read Ch.9.8 —...

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Unformatted text preview: \g" VII. Alkyne Reactions: (read Ch.9.8 — 9.14) Like alkenes, alkynes undergo addition reactions. ,A. a. Reduction _ I 1. Catalytic hydrogenation (complete and partial) . _ , a) complete reduction of alkynes in the presence of finely divided Pt, Pd, N1 or Rh With 2 b equivalents of hydrogen yields an alkane. Alkene is an intermediate. /~_ /# «X cat. ’ , . R_ EG_RI + 2H2 —> R—CHchZ—R I work SO T€CLC£IWJ 6/ all/KW C” catalyst = Pt, Pd, Ni, or Rh Heats of hydrogenation: CH3CH2CECH CH3CHECHCH3 229 kJ/mol 275 kJ/mol Alkyl groups stabilize triple bonds in the same way that they stabilize double bonds. *Internal triple bonds are more Stable than terminal ones. b) partial reduction Alkynes could he used to prepare alkenes if a catalyst were available that is active enough to catalyze the hydrogenation of alkynes, but not active enough for the hydrogenation of alkenes. H2 H2 R-CEC-R' cat R CH=CH R' X > R CHZCHZ R' ' cat. There is a catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not that of alkenes to alkanes. It is called the Lindlar catalyst and consists of palladium supported on [d CaCO3, which has been poisoned with lead acetate and quinoline. syn-Hydrogenation occurs; cis alkenes free of their trans isomer are formed. ‘ fl,“ 1* g x a) [email protected]—R' 2 CZC V6 l M ~— CA Lindlar Pd R’ ‘R Lass readh® 5 2. Dissolving metal reduction (partial reduction to trans-alkene) Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. tra’ . ! .r n s are formed. R-CEC- +> R—CHZCHz—R' KM) jLL l K 2 ATM% Metal (Li, Na, K) is reducing agent; H2 is not involved Mechanism: four steps Overall reaction: H\ ,R' R—cEc—R' + 2 M + 2 NH3 pzq + 2 MNH2 R H /3 trans-alkene ...
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