Ch_322b_12.03

Ch_322b_12.03 - Overall Stoichiometry in Water 4 RCR' O = +...

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Reductions with Sodium Borohydride Sodium borohydride , also called sodium tetrahydridoborate, is an ionic compound with a tetrahedral geometry for the borate ion . The hydridoborate anion is a source of hydride (H: - ) in reduction reactions: Because of the greater electronegativity of boron (compared with Al), the tetrahydridoborate ion (BH 4 - ) is more stable and less reactive than AlH 4 - .
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• These reactions may be carried out safely in protic solvents such as water and alcohols because H 2 formation is slow at room temperature and below. • NaBH 4 is a selective reducing agent . It reduces aldehydes and ketones, but not esters or carboxylic acids. • As with LAH, all four H are active as hydrides. Some Key Observations about Sodium Borohydride Reductions:
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Unformatted text preview: Overall Stoichiometry in Water 4 RCR' O = + NaBH 4 4 RCHR' OH primary or secondary alcohol + 3 H 2 O + NaH 2 BO 3 A Possible Mechanism B H-+ C=O R R' hydride transfer B + H-C-O R' R-reduction H-C-O R' R-+ H-O-H proton transfer H-C-OH R' R-+ HO alcohol product B-+ HO B-OH-B HO HO O-Na + sodium dihydrogenborate active hydride source Some Examples of Reduction by Sodium Borohydride CH 3 CH 2 CH O = propanal NaBH 4 H 2 O O NaBH 4 H 2 O cyclopentanone CH 3 C-O COCH 3 O NaBH 4 H 2 O/CH 3 OH selective reduction Quiz 12.03 Provide the organic products of the following metal hydride reductions. O = COOCH 3 (i) excess LiAlH 4 /ether (ii) H 2 O O COOCH 3 NaBH 4 H 2 O, CH 3 OH OH CH 2 OH OH COOCH 3...
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_12.03 - Overall Stoichiometry in Water 4 RCR' O = +...

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