tb22 - Chapter 22 Carbonvl Alpha-Substitution Reactions...

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Chapter 22 - Carbonvl Alpha-Substitution Reactions Refer to the compounds below to answer the following questions: aDO ~OB 0 II rYC,- LJH 0 II ~N- CH2- c- CH3 I II III IV 1. Indicate all the acidic hydrogens in Compounds I through IV. Answer: Acidic hydrogens are underlined. i; H C~Et 0 H H H H ~OB \.J H /\H 0 II rYC,- yH 0 II ~N- Cfu- C- Clli H H I II III IV 2. Indicate which hydrogens in Compound II are the most acidic. Explain your answer. Answer: The methylene hydrogens are the most acidic because they are flanked by two electron- withdrawing groups, -N02 and C=O. The enolate generated by their removal is highly resonance stabilized; it has three resonance forms. o~ :~ ~0 :O:~ .. x/ £- II . ...y 11-. .) ~N rn- C- CH3 +N \" CHr C- CH3 - / - / \.J- :0: :0: .. .. .. O' :0: .. ~ I +N- CH==C- CH3 - / :0: .. In contrast, the enolate generated by the abstraction of a methyl proton is stabilized by only one electron-withdrawing group and has two resonance forms: .. 8. .".:0: O' :0:- .9~ \ II{'\;; .. ~ 1- +N- CH2- C- CH2 + N- CH2- C- CH2 - / - / :0: :0: .. .. 3. Choose the most acidic compound from Compounds I - IV. Explainyour choice. Answer: Compound III is the most acidic compound because the methine hydrogen is flanked by three carbonyl groups so the enolate ion generated by its removal has four resonance forms. .o~ .0-' -.:0. .o~ .6 ~ ." COEt .0. _.:a t] ." COEt .0. t) ° COEt .0. ~ ." COEt .0:-- U ~ . .:Y .. \j. ~. - - - Test Items for McMurry's Organic Chemistry, Seventh Edition 241
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4. Draw the structures for all enols of Compound I. OH I a~H Answer: Consider the structures below to answer the following questions. 0 d'°H 0 II aCH2C-H II 0 0 II II c C H3C/ 'CH;-- , CH3 I III 5. Indicate the most acidic hydrogens in each of the molecules. Answer: 0 d'°H I 0 II uQ!,C-H II 0 0 II II C C H3C""""'~ 'CH3 III The most acidic hydrogens are underlined. 6. Rank the molecules above in order of increasing acidity (least acidic to most acidic). a. III, II, I b. II, III, I c. I, II, III d. II, I, III Answer: b 7. Nitroethane [CH3CH2N02, pKa = 8.6] is a much stronger acid than ethane [CH3CH3, pKa ~ 60]. Explain. Answer:
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tb22 - Chapter 22 Carbonvl Alpha-Substitution Reactions...

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