Chem357_Alkyne_Chem__FINAL_Student_

Chem357_Alkyne_Chem__FINAL_Student_ - Reactions of Alkynes...

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1 Reactions of Alkynes Reactions of Alkynes Paula Y. Paula Y. Bruice Bruice , , Chapter 6 Chapter 6 2 Some chemistry of Alkynes is similar to that explored already for Alkenes but there are distinct differences. Reactions of Alkynes Reactions of Alkynes e.g. H 2 Pt o r Pd/C H H H 2 (2 equiv.) Pt o r Pd/C H H H H Br 2 Br Br Br 2 Br Br Br Br (2 equiv.) HYDROGENATION HALOGENATION
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3 Reactions of Alkynes Reactions of Alkynes But compare the following…. C C H C OH C H H H C C H C O C H H H a ketone an alcohol H H H 2 O H H 2 O 4 Reactions of Alkynes - Hydrogenation Reactions of Alkynes - Hydrogenation H 2 (2 equiv.) Pd/C H H H H H H excess H 2 H 2 H 2 (1 equiv.) Pd/C H H H H What happens when ONLY one equivalent of H 2 is used? Pd/C Pd/C + unreacted alkyne H H + ONLY small amount Why? H H Recall the surface delivery of hydrogen to just one face giving SYN ADDITION MIXTURE
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5 Reactions of Alkynes - Hydrogenation Reactions of Alkynes - Hydrogenation H 2 Pd/C H H H H H H H 2 Pd/C Alkenes are significantly more reactive in addition reactions than Alkynes. SLOW FAST The alkene reacts as soon as it is formed. Here, the H 2 would be a limiting reagent. sp hybridized carbons hold their electrons more tightly than an sp 2 carbon 6 Reactions of Alkynes - Hydrogenation Reactions of Alkynes - Hydrogenation Consider the following: H H 2 (1 equiv.) Pd/C H 2 (1 equiv.) Pd/C ? ? What is the MAJOR product?
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7 Reactions of Alkynes - Hydrogenation Reactions of Alkynes - Hydrogenation CH 3 CH 2 CH 3 CH 3 CH 3 CH 2 H H Consider the utility in the following reactions... H CH 3 CH 2 H CH 3 cis-alkene trans-alkene ? ? Br 2 Br 2 CH 3 H Br Br H (+ enantiomer) (2 S ,3 R )-2,3-dibromopentane H H Br Br CH 3 (+ enantiomer) (2 R ,3 R )-2,3-dibromopentane 8 Reactions of Alkynes - Hydrogenation Reactions of Alkynes - Hydrogenation CH 3 CH 2 CH 3 CH 3 CH 3 CH 2 H H cis-alkene Lindlar's Catalyst H 2 Lindlar's catalyst is a "POISONED" or "DEACTIVATED" hydrogenation catalyst. hydrogens are still added CIS Lindlar's catalyst is Palladium precipitated on CaCO 3 and further treated with Pb(OAc) 2 and Quinoline N Quinoline heterogeneous reaction (SOLID)
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9 Reactions of Alkynes Reactions of Alkynes CH 3 CH 2 CH 3 What about a method to obtain the trans-alkene? H CH 3 CH 2 H CH 3 trans-alkene ? Since these hydrogens must be delivered in a trans fashion, a heterogeneous catalyst will not work here. 10 CH 3 CH 2 CH 3 H CH 3 CH 2 H CH 3 trans-alkene Na o r Li NH 3 (liquid) -78 o C A slurry of acetone and dry ice has a temp of -78 o C Birch Reduction R C C R Mechanism: Na R C C R H NH 2 R C C R H Na R C C R H H H 2 N H R H R Stereochemistry is determined at this last step Reactions of Alkynes Reactions of Alkynes
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11 Reactions of Alkynes - Hydrogenation
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