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instructor_supplement_22 - Instructor Supplemental...

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Instructor Supplemental Solutions to Problems © 2010 Roberts and Company Publishers Chapter 22 The Chemistry of Enolate Ions, Enols, and a , b -Unsaturated Carbonyl Compounds Solutions to In-Text Problems 22.1 (b) The acidic hydrogens are the a -hydrogens on the carbons between the two carbonyl groups. (—OEt = ethoxy group = —OCH 2 CH 3 .) The reason that these hydrogens are particularly acidic is that the conjugate-base enolate ion is stabilized by the polar effects and resonance effects of two carbonyl groups, whereas the conjugate-base enolate ion of an ordinary ester is stabilized by the corresponding effects of only one carbonyl group. The resonance structures of the conjugate-base enolate ion of ethyl acetoacetate are as follows: 22.3 A mechanism for replacement of one hydrogen in the reaction of Eq. 22.6, text p. 1051, is shown in the following equation. (The mechanisms for replacement of the others are identical.) Only the a -hydrogens are replaced because the enolate ion is the only carbanion stable enough to be formed. The carbanion intermediates required in a similar mechanism for the replacement of the hydrogens other than the a -hydrogens are not resonance-stabilized. 22.6 (b) All a -hydrogens are exchanged for deuterium. (The methyl hydrogens of the tert -butyl group are not a -hydrogens.)
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INSTRUCTOR SUPPLEMENTAL SOLUTIONS TO PROBLEMS • CHAPTER 22 2 22.7 (b) (c) Benzaldehyde, PhCH A O, has no enol forms because it has no a -hydrogens. 22.9 (b) The “enol” form of an amide that has a carbon–nitrogen double bond is called an imidic acid; see Eq. 21.20b, text p. 1009. Notice that this particular amide cannot enolize toward the a -carbon because it has no a - hydrogens on that carbon. 22.11 (b) The enol is formed by the mechanism shown in Eq. 22.17b on text p. 1056, except that D 3 O + is the acid. This results in the “washout” of one a -hydrogen into the large excess of deuterated solvent. Protonation of the double bond by D 3 O + gives a deuterium at the a -position. Replacement of one a -hydrogen by deuterium is shown; the mechanism for replacement of the other a -hydrogens is identical. 22.13 (b) Because the rate of ketone halogenation is independent of the halogen concentration, the rates of halogenation of the same ketone with two different halogens are also independent of halogen concentration and therefore independent of the identity of the halogen itself. In fact, the rate in both cases is the rate of enolization, as in part (a). The two processes are compared at the same acid concentration because the enolization process is acid-catalyzed (Eq. 22.24, text p. 1058). 22.14 In part (b), benzophenone does not halogenate because it cannot form an enolate ion (it has no a -hydrogens), and the aromatic rings are deactivated by the carbonyl group toward electrophilic halogenation.
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