17_all_ - 17.1 For all practical purposes, the compound

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17.1 For all practical purposes, the compound 2,4-cyclohexadien-1-one exists totally in its enol form. Write the structure of 2,4-cyclohxandien-1-one and of its enol form. What special factor account for the stability of the enol form? Answer: OH O Because phenol is a kind of aromatic compounds. It has aromatic ring, so it is much more stable. 17.2 Would you expect optically active ketones such as the following to undergo acid-or base-catalyzed racemization? Explain your answer. C 3 H 7 C C 2 H 5 H 3 C CC 6 H 5 O C 3 H 7 C H 3 C H CH 2 CC 6 H 5 O (a) (b) Answer: (a) It doesn’t have any α -H, therefore, it will not tautomerize to any enol form, no racemization. (b) It has the α -H, but the sterocenter is the β -carbon and thus enol formation doesn’t affect it, no racemization. 17.3 When sec -bultyl phenyl ketone is treated with either OD¯ or D 3 O + in the presence of D 2 O, the ketone undergoes hydrogen-deuterium exchange and produces this compound: C 2 H 5 C D CH 3 COC 6 H 5 Write mechanisms that account for this behabior. Answer: OD - Catalyzed: CC C 2 H 5 CH 3 H O C 6 H 5 C 2 H 5 C C CH 3 O - C 6 H 5 DO D C 2 H 5 C C CH 3 OD C 6 H 5 C 2 H 5 C D D CH 3 C O C 6 H 5 - D 3 O + Catalyzed:
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CC C 2 H 5 CH 3 H O C 6 H 5 DO D D C 2 H 5 CH 3 H O OD D C 6 H 5 D C 2 H 5 C C CH 3 OD C 6 H 5 C 2 H 5 C D D CH 3 C O C 6 H 5 17.4 Write a mechanism using sodium ethoxide in ethoxide in ethanol for the epimerization of cis -decalone to trans -decalone. Draw chair conformational structures that show why trans -decalone is more stable than cis -decalone. Answer: Mechanism: H H O -O C 2 H 5 H O - HO C 2 H 5 H H O or H H O chair conformations: trans form: O H H cis form: O H H 17.5 Why do we say that the halogenation of ketones in base is “base promoted” rather than “base catalyzed”? Answer: The reaction is said to be “base promoted” because base is consumed as the reaction takes place. A catalyst is, by definition, not consumed. 17.6 Addition evidence for the halogenation mechanisms that we just presented comes from the following facts: (a) Optically active sec-butyl phenyl ketone undergoes acid-catalyzed racemization at a rate exactly equivalent to the rate at which it undergoes acid-catalyzed halogenation. (b) sec-Butyl phenyl ketone undergoes acid-catalyzed iodination at the same rate
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that it undergoes acid-catalyzed bromination. (c) sec-Butyl phenyl ketone undergoes base-catalyzed hydron-deuterium exchange at the same rate that it undergoes base-promoted halogention. Explain how each of these observations supports the mechanisms that we have presented. Answer: (a) From this, we know that the slow step in acid-catalyzed racemization is the same as the that in acid-catalyzed halogenation. (b) According to he mechanism given, the slow step for acid-catalyzed iodination is the same as that for acid-catalyzed bromination. Thus, we would expect both reactions to occur at the same rate.
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17_all_ - 17.1 For all practical purposes, the compound

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