9.7 Catalytic Hydrogenation The addition of H 2 across a C=C double bond If a chirality center is formed, syn addition is observed Draw the stereoisomers that are produced Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-1 Klein, Organic Chemistry 2e

9.7 Catalytic Hydrogenation Analyze the energy diagram below Why is a catalyst necessary? Does the catalyst affect the spontaneity of the process? Typical catalysts include Pt, Pd, and Ni Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-2 Klein, Organic Chemistry 2e

9.7 Catalytic Hydrogenation The metal catalyst is believed to both adsorb the H atoms and coordinate the alkene The H atoms add to the same side of the alkene pi system Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-3 Klein, Organic Chemistry 2e

9.7 Catalytic Hydrogenation Draw product(s) for the reaction below. Pay close attention to stereochemistry How many chirality centers are there in the alkene reactant above? How does the term, mesocompound, describe the product(s) of the reaction? Practice with SkillBuilder 9.5 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-4 Klein, Organic Chemistry 2e

9.7 Catalytic Hydrogenation If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is heterogeneous. WHY? Homogeneous catalysts also exist What advantage might a homogeneous catalyst have? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-5 Klein, Organic Chemistry 2e

9.7 Asymmetric Hydrogenation In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand A chiral catalyst can produce one desired enantiomer over another. HOW? Why would someone want to synthesize one enantiomer rather than a racemic mixture? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-6 Klein, Organic Chemistry 2e

9.7 Asymmetric Hydrogenation A chiral catalyst allows one enantiomer to be formed more frequently in the reaction mixture Some chiral catalysts give better enantioselectivity than others. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-7 Klein, Organic Chemistry 2e

9.7 Asymmetric Hydrogenation BINAP is a chiral ligand that gives pronounced enantioselectivity For any reaction, stereoselectivity can only be achieved if at least one reagent (reactant or catalyst) is chiral Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-8 Klein, Organic Chemistry 2e

9.7 Asymmetric Hydrogenation Predict the major product(s) for the reactions below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. 9-9 Klein, Organic Chemistry 2e

9.8 Halogenation Halogenation involves adding two halogen atoms across a C=C double bond Halogenation is a key step in the production of PVC Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halogenation Halogenation with Cl 2 and Br 2 is generally effective, but halogenation with I 2 is too slow and halogenation with F 2 is too violent Halogenation occurs with anti addition Given the stereospecificity, is it likely to be a concerted or a multi-step process? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halogenation Let’s look at the reactivity of Br 2. Cl 2 is similar It is nonpolar, but it is polarizable. WHY? What type of attraction exists between the Nuc: 1- and Br 2 ? Does the Br 2 molecule have a good leaving group attached to it? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

We know alkenes can act as nucleophiles Imagine an alkene attacking Br 2. You might imagine the formation of a carbocation 9.8 Halogenation However, this mechanism DOES NOT match the stereospecificity of the reaction. HOW? WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halogenation Mechanism continued on next slide Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halogenation Only anti addition is observed. WHY? Prove to yourself that the products are enantiomers rather than identical Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halogenation Only anti addition is observed Can you design a synthesis for ? Practice with conceptual checkpoint 9.26 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halogenation Predict the major product(s) for the reactions below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halohydrin Formation Halohydrins are formed when halogens (Cl 2 or Br 2 ) are added to an alkene with WATER as the solvent The bromonium ion forms from Br 2 + alkene, and then it is attacked by water Why is the bromonium attacked by water rather than a Br 1- ion? Is water a better nucleophile? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halohydrin Formation A proton transfer completes the mechanism producing a neutral halohydrin product The net reaction is the addition of –X and –OH across a C=C double bond Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halohydrin Regioselectivity The –OH group adds to the more substituted carbon The key step that determines regioselectivity is the attack of water on the bromonium ion Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e

9.8 Halohydrin Regioselectivity When water attacks the bromonium ion, it will attack the side that goes through the lower energy transition state Water is a small molecule that can easily access the more sterically hindered site Practice with SkillBuilder 9.6 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e Transition state

9.8 Halohydrin Regioselectivity Predict the major product(s) for the reactions below Copyright © 2015 John Wiley & Sons, Inc. All rights reserved Klein, Organic Chemistry 2e