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12_ - P1 OXT/SRB JWDD052-12 P2 xxx JWDD052-Solomons-v2 12...

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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-12 JWDD052-Solomons-v2 May 1, 2007 14:8 12 ALCOHOLS FROM CARBONYL COMPOUNDS: OXIDATION-REDUCTION AND ORGANOMETALLIC COMPOUNDS SOLUTIONS TO PROBLEMS 12.1 (a) H C H H O H = oxidation state of C 3 H = - 3 1 O = + 1 Total = - 2 H C O O H = oxidation state of C 1 H = - 1 3 O = + 3 Total = + 2 H C O H = oxidation state of C 2 H = - 2 2 O = + 2 Total = 0 (b) - 4 CH 4 + 4 CO 2 - 2 + 2 0 CH 3 OH H C O O H H C O H (c) A change from 2 to 0 (d) An oxidation, since the oxidation state increases (e) A reduction from + 6 to + 3 12.2 (a) H C H H C OH H H 3 H = 3 1 C = 0 Total = 3 2 H = 2 1 C = 0 1 O = + 1 Total = 1 H C H H C H O 3 H = 3 1 C = 0 Total = 3 1 H = 1 1 C = 0 2 O = + 2 Total = + 1 208
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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-12 JWDD052-Solomons-v2 May 1, 2007 14:8 ALCOHOLS FROM CARBONYL COMPOUNDS 209 (b) Only the carbon atom of the CH 2 OH group of ethanol undergoes a change in oxida- tion state. The oxidation state of the carbon atom in the CH 3 group remains unchanged. (c) H C H H C OH O 3 H = 3 1 C = 0 Total = 3 1 C = 0 3 O = + 3 Total = + 3 The oxygen-bearing carbon atom increases its oxidation state from + 1 (in acetaldehyde) to + 3 (in acetic acid). 12.3 (a) If we consider the hydrogenation of ethene as an example, we find that the oxidation state of carbon decreases. Thus, because the reaction involves the addition of hydrogen , it is both an addition reaction and a reduction. Ni H C H H C H H H H 2 + H C H C H H 2 H = 2 2 C = 0 Total = 2 3 H = 3 1 C = 0 Total = 3 (b) The hydrogenation of acetaldehyde is not only an addition reaction, but it is also a reduction because the carbon atom of the C = O group goes from a + 1 to a 1 oxidation state. The reverse reaction (the dehydrogenation of ethanol) is not only an elimination reaction, but also an oxidation. Ion-Electron Half-Reaction Method for Balancing Organic Oxidation-Reduction Equations Only two simple rules are needed: Rule 1 Electrons ( e ) together with protons (H + ) are arbitrarily considered the reducing agents in the half-reaction for the reduction of the oxidizing agent. Ion charges are balanced by adding electrons to the left-hand side. (If the reaction is run in neutral or basic solution, add an equal number of OH ions to both sides of the balanced half-reaction to neutralize the H + , and show the resulting H + + OH as H 2 O.) Rule 2 Water (H 2 O) is arbitrarily taken as the formal source of oxygen for the oxidation of the organic compound, producing product, protons , and electrons on the right-hand side. (Again, use OH to neutralize H + in the balanced half-reaction in neutral or basic media.)
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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-12 JWDD052-Solomons-v2 May 1, 2007 14:8 210 ALCOHOLS FROM CARBONYL COMPOUNDS EXAMPLE 1 Write a balanced equation for the oxidation of RCH 2 OH to RCO 2 H by Cr 2 O 7 2 in acid solution.
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