2444-111611

2444-111611 - Reduction: Hydrogenation 1 3 H 2 , Ni(R) 1...

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Unformatted text preview: Reduction: Hydrogenation 1 3 H 2 , Ni(R) 1 eq. H 2 , Ni(R) 3 Addition of one molar equivalent of hydrogen (H-H) will generate a diene and addition of two molar equivalents of H-H) will give an alkene. Complete reduction of benzene to cyclohexane (addition of three molar equivalents of H- H) typically requires the use of an excess of hydrogen. The aromatic character of the benzene ring makes the reduction more difFcult, so a hydrogenation reaction requires more vigorous conditions when compared to hydrogenation of an alkene. This means that higher temperatures and/or higher pressures of hydrogen gas are necessary. When benzene reacts with three molar equivalents of hydrogen gas in the presence of a Raney nickel catalyst, abbreviated Ni(R), reduction to cyclohexane occurs if the reaction is heated. Hydrogenation of benzene is also possible using a palladium catalyst or a rhodium (Rh) catalyst. Reduction: Birch Reduction 2 When benzene is treated with sodium and ethanol in liquid ammonia, the product is the non-conjugated 1,4- cyclohexadiene, 89 . EtO H Na H H 86A H H 86B EtO H Na H H H H 86C 87 88 89 Reduction: Birch Reduction 3 Although there are three resonance contributors to 86 , the lowest energy resonance contributor will arise by separation of the two electrons of the carbanion and the single electron of the radical in order to diminish electronic repulsion. This means that 86B represents the lowest energy electronic distribution in the intermediate, and it will lead to the Fnal product. EtO H Na H H 86A H H 86B EtO H Na H H H H 86C 87 88 89 Reduction: Birch Reduction 4 OMe Na OMe OMe OMe !" !" or 21 90 91 92 N O O N O O N O O Na NO 2 or 40 93 94 95 In 91 , the negative charge on the ipso carbon leads to electron repulsion that greatly...
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This note was uploaded on 02/25/2012 for the course CHEM 244 taught by Professor Jardin,j during the Fall '08 term at UConn.

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2444-111611 - Reduction: Hydrogenation 1 3 H 2 , Ni(R) 1...

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