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2444-102411 - 1 Reactions 1 NaBH4 EtOH 2 aq NH4Cl O OH 1...

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Reactions 1 CHO O CO 2 H CO 2 Me 1. NaBH 4 , EtOH 2. aq. NH 4 Cl 1. LiAlH 4 , THF 2. H 2 O 1. LiAlH 4 , THF 2. H 2 O 1. LiAlH 4 , THF 2. H 2 O OH CH 2 OH CH 2 OH CH 2 OH
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Catalytic Hydrogenation 2 • If an alkene such as 1-heptene ( 27 ) is mixed with hydrogen gas in the solvent methanol, however, there is no reaction. • A comparison of diatomic bromine and diatomic hydrogen shows that diatomic hydrogen (H-H) is not polarizable. Therefore, it does not react directly with the alkene. • Something must added to first break the H-H bond before there is a reaction with the alkene ! -bond. Based on many experiments, addition of a transition metal such as nickel promotes reaction with H-H. • Indeed, addition of a small amount of nickel to a mixture of hydrogen gas and 1- heptene ( 27 ) gives an excellent yield of heptane. • The type of nickel used is called Raney nickel [abbreviated as Ni(R)], which is a finely divided form of nickel. H 2 , Raney Ni 27 MeOH 74%
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Catalytic Hydrogenation 3 • Transition metals react with diatomic hydrogen to produce hydrogen atoms (essentially H) that are bound to the surface of the metal. These hydrogen atoms are very reactive and easily react with an alkene. • The actual mechanism of this reaction is somewhat complex since the surface of the metal contains many reactive sites. • A cartoon version of the mechanism shows the a metal as a surface (see 28 ), and the metal reacts with H-H to give a “hydrogen-loaded” metal, 29 . This means that hydrogen atoms are adsorbed on the surface of the metal. • The transition metal also reacts with the ! -bond of an alkene to form a coordination complex known as an " 2 -complex. 29 reacts with 1-heptene ( 27 ) to form complex 30 , in which the metal is coordinated to the ! - bond as well as the hydrogen atoms. • At the surface of the metal, a hydrogen atom is transferred to the alkene ! -bond to form a # - covalent C–H bond in 31 . • In 31 , hydrogen atoms remain on the surface of the metal, and transfer of a second atom of hydrogen from the metal to the coordinated carbon “releases” heptane, and regenerates a “clean” surface of the catalyst ( 28 ) that can react with additional hydrogen gas. • The net result of this process adds two hydrogen atoms to the C=C unit of an alkene. • This process is called catalytic hydrogenation .
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Catalytic Hydrogenation 4 H-H H H C 5 H 11 H H H H C 5 H 11 H H 1-heptene H-transfer H-transfer 29 30 31 28 metal catalyst The metal catalyst can now react with more H 2 28 C 5 H 11 +
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Catalytic Hydrogenation 5 • Other transition metals catalyze the reaction of hydrogen gas with an alkene, including palladium (Pd), platinum (Pt) as well as nickel (Ni). Rhodium (Rh), ruthenium (Ru) or iridium (Ir), and compounds derived from these metals are also commonly used. This reaction occurs on the surface of the metal, the larger the surface area, the faster the proces s.
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