Chapter 12 Review (Lecture 2)

Chapter 12 Review (Lecture 2) - Chapter 12 Alcohols from...

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Chapter 12 Alcohols from Carbonyl Compounds. Oxidation-Reduction and Organometallic Compounds. The carbonyl group is found in several functional groups: C=O : R H C=O R R' C=O R HO C=O R R'O aldehyde ketone carboxylic acid ester The carbonyl group is polar : C=O C O : - + C O δ + δ− (a resonance theory description of the polarity) Nu: - + C O δ + δ− = sp 2 C Nu O : - sp 3
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Hydride Ion and Carbanions as Nucleophiles Two important nucleophiles in reactions with carbonyl functions are: H: - R: - hydride ion carbanions Hydride ions are readily available from the metal hydride reagents : Na + H-B-H H H - sodium borohydride (sodium tetrahydridoborate) Li + H-Al-H H H - lithium aluminum hydride (lithium tetrahydridoaluminate) Carbanions are available from organometallic reagents such as: R-Li R-MgX organolithium organomagnesium halide The addition of hydride or carbanion to a carbonyl is formally a reduction of that function.
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Alcohols by Reduction of Carbonyl Compounds Primary and secondary alcohols may be prepared by the reduction of several carbonyl functional groups: R-C-H O = aldehyde [H] R-C-H H OH primary alcohol R-C-R' O [H] ketone R-C-R' H OH secondary alcohol R-C-OH O [H] carboxylic acid R-C-H H OH primary alcohol R-C-OR' O [H] ester R-C-H H OH primary alcohol (+ R'OH)
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Lithium Aluminum Hydride Al H H H H - Li + a near white non-volatile crystalline solid soluble in various ethers LAH is more properly called lithium tetrahydridoaluminate, which more accurately describes the structure. Aldehydes and Ketones R-C-R' O = : : + Li + H-Al-H H H - hydride addition ether solvent The alkoxide ion produced from attack by hydride adds to the electron-deficient AlH 3 producing an alkoxyaluminate species. R-C-H R' O AlH 3 - Li + H 2 O (workup) R-C-H R' O H + Al(OH) 3 + LiOH Aldehydes give primary alcohols and ketones give secondary alcohols.
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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 . Na + B H H H H - a white crystalline solid, stable in dry air, insoluble in diethyl ether, but soluble in water and THF The hydridoborate anion is a source of hydride (H: - ) in reduction reactions: B H - C O = C :O: : : : - H Because of the greater electronegativity of boron (compared with Al), the
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Chapter 12 Review (Lecture 2) - Chapter 12 Alcohols from...

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