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216 W11-Expt 4-2 - Chem 216 W11 Topic_Experiment 4 page 1...

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Chem 216 W11 Topic : _Experiment 4__ page 1 of 3. Experiment 4: NaBH 4 Reduction of an Unknown Aromatic Ketone [See: Ege’s book, Section 14.4, pp 545-549] Two commonly used hydride-based reducing agents: Na + B - H H H H - mild reducing agent - relatively stable reagent (against moisture, air) electronegativity values: H 2.1; B 2.0; Al 1.5 e.n.| for B-H: 0.1 | Δ e.n.| for Al-H: 0.6 1. Sodium borohydride (NaBH 4 ) Li + Al - H H H H - powerful reducing agent - reacts violently with water, ROH to produce H 2 gas - Reactions with LiAhH are usually carried out in a polar aprotic solvent such as anhydrous tatrahydrofuran (THF) and anhydrous (diethyl) ether (CH 3 CH 2 OCH 2 CH 3 ) 2. Lithium aluminum hydride (LiAlH 4 ) more polarized, more on the H a stronger H donor O O THF (diethyl) ether In addition, the size differences between B and H and Al and H should make the dissociation of H- more effective for the Al-H bond. In addition, the difference in the coordination power of Na + and Li + (stronger) on the carbonyl oxygen further contributes to make the reactivity of LiAlH 4 stronger.
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