Assignment_8_Aldol_and_related_rxns_Answers

Assignment_8_Aldol_and_related_rxns_Answers - CHM 2120...

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CHM 2120 – Assignment 8 -- ANSWERS In this assignment: - The aldol reaction - Haloform reaction - Synthetic applications Note : many questions incorporate earlier material 1. Draw the mechanism for the tautomerization of 1-phenyl-1-butanone (also known as butyrophenone and phenyl propyl ketone) under a. acid catalysis b. base catalysis Ph O Ph OH a. Acid (HA) H A Ph O H H A Ph O H b. Base (B - ) Ph O H B Ph O H B Ph O H
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Assignment 8 – Enols/Enolates and related reactions ANSWERS 2 2. a. What is the key tautomer of the molecule shown below? b. Which would be the major tautomer at equilibrium? Why? c. How could you prove the structure of the major tautomer by IR? O OH more stable tautomer is aromatic IR: keto form: would see C=O stretch (peak) at about 1715 cm -1 -- NOT observed enol form (phenol): would see the broad OH stretch (peak) at about 3200 cm - OBSERVED OH C=C
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Assignment 8 – Enols/Enolates and related reactions ANSWERS 3 3. a. Draw a mechanism for the generation of the compounds shown below. O D 2 O O O D D D NaOD Na OD H O D OD O D H OD O D D OD
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Assignment 8 – Enols/Enolates and related reactions ANSWERS 4 b. (3. b.) Describe how the 1 H NMR of the starting material would differ from the 1 H NMR of the two products. Note: H = 1 H Æ detected by 1 H NMR (peaks are seen) D = 2 H Æ not detected by 1 H NMR (no peaks) C H 2 O CH 3 2H δ =2-3ppm quartet 5H δ =7-8 .2ppm multiplet 3H δ =1-1.6ppm triplet O 3 D H 1H (only 1 H detected by 1 HNMR;Dnotobserved) δ quartet 5H δ multiplet δ =1-1 .6ppm doublet (only "sees" 1 H neighbor; the D is not observed) O 3 D D 0H (no 1 H's to detect by 1 HNMR ;Dno tobserved) 5H δ =7-8.2ppm multiplet 3H δ singlet (D is not observed; no 1 H neighbors present)
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Assignment 8 – Enols/Enolates and related reactions ANSWERS 5 4. A solution of dimedone (5,5-dimethylcyclohexane-1,3-dione) in chloroform contains 33% of an enol tautomer. Draw this tautomer and explain why it is found in much higher proportion than most enols. O O O OH O Enol form is resonance stabilized and consequently exists in a detectable amount in a sample of 5,5-dimethyl-1,3-cyclohexanedione 33% 67% 5. The tautomer of vitamin C shown below is not the most stable one. a. Draw the more stable tautomer b. Why is vitamin C, also called ascorbic acid , so acidic? Note: the 1 st pKa of vitamin C is 4.6 (similar to that of a carboxylic acid) whereas typical alcohols have pKa’s in the range of 16-18. O HO O H O O O OH H O H O O H O H B O O OH H O O O OH H O Conjugate base is resonance-stabilized with another carbonyl group, making it much more acidic than a typical alcohol.
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Assignment 8 – Enols/Enolates and related reactions ANSWERS 6 6.
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This note was uploaded on 01/25/2011 for the course CHM 2120 taught by Professor Dr.flynn during the Spring '10 term at University of Ottawa.

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Assignment_8_Aldol_and_related_rxns_Answers - CHM 2120...

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