Chapter 8 - Alkynes: An Introduction to Organic Synthesis Draw structures corresponding to each of the following names. 1. ethynylcyclopropane Answer: CCH 2. 3,10-dimethyl-6-sec-butylcyclodecyne Answer: 3. 4-bromo-3,3-dimethyl-1-hexen-5-yne CH3 Br Answer: H 2C CH C CH C C H CH3 4. acetylene Answer: H CCH Provide names for each compound below. CH3 5. CH3C CCHCH2CH2CH3 Answer: 4-methyl-2-heptyne CH 3 6. CCH Answer: 1-ethynyl-2-methylcyclopentane Test Items for McMurry’s Organic Chemistry, Seventh Edition 59 The compound below has been isolated from the safflower plant. Consider its structure to answer the following questions. H H CCCCCCCC H H3C C C C H H C H H 7. What is the molecular formula for this natural product? Answer: C13H10 8. What is the degree of unsaturation for this compound? Answer: We can arrive at the degree of unsaturation for a structure in two ways. Since we know that the degree of unsaturation is the number of rings and/or multiple bonds in a compound, we can simply count them. There are three double bonds (3 degrees) and three triple bonds (six degrees), so the degree of unsaturation is 9. We can verify this by using the molecular formula, C13H10, to calculate a degree of unsaturation. The saturated 13-carbon compound should have the base formula C13H28, so (28 - 10) ÷ 2 = 18 ÷ 2 = 9. 9. Assign E or Z configuration to each of the double bonds in the compound. Answer: H H E CCCCCCCCE H H3C C C C H H C H H 10. Provide the name for this unusual natural product. Answer: (3E, 11E)-1,3,11-tridecatrien-5,7,9-triyne or (3E,11E)-trideca-1,3,11-trien-5,7,9-triyne To answer the following questions, consider the information below: The heat of hydrogenation of 1,4-pentadiene is 254.4 kJ/mol. The heat of hydrogenation of 1-pentyne is approximately 292.9 kJ/mol. 11. Which structure is more stable, 1,4-pentadiene or 1-pentyne? Answer: 1,4-pentadiene 60 Chapter 8: Alkynes: An Introduction to Organic Synthesis 12. Show the energy differences between the hydrogenation of 1,4-pentadiene and 1-pentyne on a reaction energy diagram. Answer: A = 1-pentyne B = 1,4-pendatiene C = 1-pentene A D = pentane Energy B C 292.9 kJ/mol 254.4 kJ/mol D Reaction Progress Consider an unknown with the molecular formula CH46. 13. How many degrees of unsaturation are present? Answer: A saturated four carbon compound has the formula CH410. Therefore this compound has ()10− 6÷= 2 2 degrees of unsaturation. 14. Draw six of the nine possible structures with this formula. Answer: H C CCH2CH3 CH3CCCH3 H2CCHCHCH2 H2C C CHCH3 CH3 CH3 CH2 Predict the product of each reaction below. Be sure to indicate stereochemistry when appropriate. Br 15. 2 KOH ethanol Br Test Items for McMurry’s Organic Chemistry, Seventh Edition 61 Answer: Br 2 KOH ethanol Br D 16. CH CH CCCH 2 3 2 3 Lindlar catalyst Answer: CH3CH2 CH3 D CH CH CCCH 2 CC 3 2 3 Lindlar catalyst DD 17. 1. BH , THF CCH 3 2. H2O2, NaOH, H2O Answer: O 1. BH3, THF CCH CH2 C H 2. H2O2, NaOH, H2O 18. 1. NaNH , NH CCH 2 3 2. CH3CH2I Answer: 1. NaNH2, NH3 CCH CCCH2CH3 2. CH3CH2I 19. KMnO4 + H3O Answer: COOH KMnO4 + H3O COOH 62 Chapter 8: Alkynes: An Introduction to Organic Synthesis HgSO , H SO CH CH CCH 4 2 4 20. 3 2 H O 2 Answer: O HgSO4, H2SO4 CH3CH2CCH CH3CH2CCH3 H2O CH CH C C H excess H2 21. Pd/C Answer: CH CH C C H CH2CH2CH2CH3 excess H2 Pd/C To answer the following questions consider this reaction: Br HBr CH3CH2CH2CH2 CCH CH3CH2CH2CH2 CCH2 CH3COOH 22. On the structures provided, draw arrows showing the electron flow for the reaction mechanism for the electrophilic addition of hydrogen bromide to 1-hexyne + _ CH3CH2CH2CH2 CCH HBr CH3CH2CH2CH2 CCH2 + Br Br CH3CH2CH2CH2 CCH2 Answer: + _ CH3CH2CH2CH2 CCH HBr CH3CH2CH2CH2 CCH2 + Br Br CH3CH2CH2CH2 CCH2 Test Items for McMurry’s Organic Chemistry, Seventh Edition 63 23. The key intermediate in the electrophilic addition reaction shown above is called: a. an acetylenic carbocation b. a vinylic bromide c. a secondary carbocation d. a vinylic carbocation Answer: d Select the best reagent or sequence of reagents from the list provided which would best accomplish each transformation below. Place the letter corresponding to the reagent(s) in the blank to the left of each reaction. Each transformation requires only one answer. a. H2SO4, H2O, HgSO4 e. 1. Br2, CCl4 b. H2, Lindlar 2. 2 NaNH2, NH3 c. 1. BH3, THF f. 1. HCl 2. H2O2 2. NaNH2, NH3 d. 1. NaNH2, NH3 g. Li/NH3 2. CH3CH2I h. 1. NaNH2, NH3 2. CH3CH2CH2I H C H 3 24. CH3C CCH3 CC H CH3 Answer: g O CH CCH CH2 CCH 2 3 25. Answer: a 26. HCCH CH3CH2CH2CCH Answer: h 27. CH CH CC Answer: e 64 Chapter 8: Alkynes: An Introduction to Organic Synthesis Show all reagents and intermediates necessary to carry out the following conversions. H CH2CH3 28. (CH3)2CHCH2CCH CC (CH ) CHCH H 3 2 2 Answer: (CH3)2CHCH2CCH 1. NaNH2, NH3 2. CH3CH2Br H CH2CH3 Li/NH3 (CH3)2CHCH2C CCH2CH3 CC (CH3)2CHCH2 H OH 29. CH3CH2CCH CH3CH2CHCH2CH2CH3 Answer: 1. NaNH , NH H , Pd/C CH CH CCH 2 3 2 3 2 CH3CH2CCCH2CH3 CH3CH2CH CHCH2CH3 2. CH3CH2Br or Li/NH3 1. Hg(OAc)2, H2O,/THF 1. BH3, THF 2. NaBH 4 or 2. H2O2, NaOH, H2O OH CH3CH2CHCH2CH2CH3 Test Items for McMurry’s Organic Chemistry, Seventh Edition 65 Using acetylene and any alkyl halides as starting materials, synthesize the following compounds. More than one step may be required. Show all reagents and all intermediate compounds in your synthetic scheme. O 30. CH3CCH2CH2CH2CH2CH3 2-heptanone (responsible for the odor of cheddar cheese) Answer: 1. NaNH2, NH3 HC CH HC CCH2CH2CH2CH2CH3 2. CH3CH2CH2CH2CH2I HgSO4, H2SO4 H2O O CH3CCH2CH2CH2CH2CH3 31. Answer: 1. NaNH2, NH3 1. NaNH2, NH3 HC CH CH2CCH 2. C6H5CH2I 2. C6H5CH2I CH2C CCH2 Li/NH3 Cl 32. 1,2-dichloropropane Cl (a degreasing solvent used in dry cleaning) Answer: 1. NaNH2, NH3 H2/Lindlar HC CH HC CCH3 H2CCHCH3 2. CH3I or Li/NH3 Cl2 CCl4 Cl Cl 66 Chapter 8: Alkynes: An Introduction to Organic Synthesis O acetic acid 33. (raw material for the vinyl acetate H CCOH 3 polymer used in paintsand adhesives) Answer: 1. NaNH2, NH3 1. NaNH2, NH3 HC CH CH3CCH CH3C CCH3 2. CH3I 2. CH3I KMnO4 + H3O O 2 H3CCOH Br 34. CH CHCH CH CH 3 2 2 3 Answer: 1. NaNH2, NH3 H2 HC CH HC CCH2CH2CH3 H2CCHCH2CH2CH3 Lindlar catalyst 2. CH3CH2CH2I HBr ether Br CH3CHCH2CH2CH3 Test Items for McMurry’s Organic Chemistry, Seventh Edition 67 .