% % Important Concepts. Problems. Problems C h a p t e r

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1 Problems C h a p t e r Important Concepts 1. The rules for naming alkynes are essentially the same as those formulated for alkenes. Molecules with both double and triple bonds are called alkenynes, the double bond receiving the lower number if both are at equivalent positions. ydroxy groups are given precedence in numbering alkynyl alcohols (alkynols). 2. The electronic structure of the triple bond reveals two p bonds, perpendicular to each other, and a s bond, formed by two overlapping sp hybrid orbitals. The strength of the triple bond is about 229 kcal mol 21 ; that of the alkynyl C bond is 131 kcal mol 21. Triple bonds form linear structures with respect to other attached atoms, with short C C (1.20 Å) and C (1.06 Å) bonds. 3. The high s character at C1 of a terminal alkyne makes the bound hydrogen relatively acidic (pk a < 25). 4. The chemical shift of the alkynyl hydrogen is low (d ppm) compared with that of alkenyl hydrogens because of the shielding effect of an induced electron current around the molecular axis caused by the external magnetic field. The triple bond allows for long-range coupling. IR spectroscopy indicates the presence of the CqC and qc bonds in terminal alkynes through bands at cm 21 and cm 21, respectively. 5. The elimination reaction with vicinal dihaloalkanes proceeds regioselectively and stereospecifically to give alkenyl halides. 6. Selective syn dihydrogenation of alkynes is possible with Lindlar catalyst, the surface of which is less active than palladium on carbon and therefore not capable of hydrogenating alkenes. Selective anti hydrogenation is possible with sodium metal dissolved in liquid ammonia because simple alkenes cannot be reduced by one-electron transfer. The stereochemistry is set by the greater stability of a trans disubstituted alkenyl radical intermediate. 7. lkynes generally undergo the same addition reactions as alkenes; these reactions may take place twice in succession. ydration of alkynes is unusual. It requires an g(ii) catalyst, and the initial product, an enol, rearranges to a ketone by tautomerism. 8. To stop the hydroboration of terminal alkynes at the alkenylboron intermediate stage, modified dialkylboranes particularly dicyclohexylborane are used. xidation of the resulting alkenylboranes produces enols that tautomerize to aldehydes. 9. The eck reaction links alkenes to alkenyl halides in a metal-catalyzed process. Problems 27. Draw the structures of the molecules with the following names. 1-Chloro-1-butyne (Z )-4-romo-3-methyl-3-penten-1-yne (c) 4-exyn-1-ol 28. Name each of the compounds below, using the IUPC system of nomenclature. (c) (d) (e) (f) 29. Compare C bond strengths in ethane, ethene, and ethyne. Reconcile these data with hybridization, bond polarity, and acidity of the hydrogen. 30. Compare the C2 C3 bonds in propane, propene, and propyne. Should they be any different with respect to either bond length or bond strength? If so, how should they vary? 31. Predict the order of acid strengths in the following series of cationic species: C 2 N 3 1, C N 2 1, C q N 1. [int: Look for an analogy among hydrocarbons (Section 13-2).]

2 600 C h a p t e r 1 3 lkynes 32. The heats of combustion for three compounds with the molecular formula C 5 8 are as follows: cyclopentene, D comb kcal mol 21 ; 1,4-pentadiene, D comb kcal mol 21 ; and 1-pentyne, D comb kcal mol 21. Explain in terms of relative stability and bond strengths. 33. Rank in order of decreasing stability. 1-eptyne and 3-heptyne CqC, C 2 CqC, and (int: Make a model of the third structure. Is there anything unusual about its triple bond?) 34. Deduce structures for each of the following. Molecular formula C 6 10 ; NMR spectrum ; no strong IR bands between 2100 and 2300 or 3250 and 3350 cm 21. Molecular formula C 7 12 ; NMR spectrum ; IR bands at about 2120 and 3330 cm 21. (c) The percentage composition is carbon and 9.59 hydrogen (the remainder is ), and the exact molecular mass is ; NMR and IR spectra C (next page). The inset in NMR spectrum C provides better resolution of the signals between 1.6 and 2.4 ppm. 1 NMR 6 4 ( ) 4 Si Mz 1 NMR spectrum ppm (δ) δ 1 NMR ( ) 4 Si Mz 1 NMR spectrum ppm (δ) δ

3 Problems C h a p t e r NMR ( ) 4 Si Mz 1 NMR spectrum ppm (δ) δ 100 Transmittance () C IR cm 1 Wavenumber 35. The IR spectrum of 1,8-nonadiyne displays a strong, sharp band at 3300 cm 21. What is the origin of this absorption? Treatment of 1,8-nonadiyne with NaN 2, then with D 2, leads to the incorporation of two deuterium atoms, leaving the molecule unchanged otherwise. The IR spectrum reveals that the peak at 3300 cm 21 has disappeared, but a new one is present at 2580 cm 21. What is the product of this reaction? What new bond is responsible for the IR absorption at 2580 cm 21? (c) Using ooke s law, calculate the approximate expected position of this new band from the structure of the original molecule and its IR spectrum. ssume that k and f have not changed. 36. Write the expected product(s) of each of the following reactions. C 2 CCC 2 3 NaN 2, liquid N3 C 2 C 2 C 2 CC r r 2 NaN 2, liquid N 3 Na (c) meso- CC 2 CCC 2 C (1 equivalent), C3 (d) (4R,5R)- CC 2 CCC 2 C Na (1 equivalent), C3

4 D G 602 C h a p t e r 1 3 lkynes 37. Write the expected product of the reaction of 3-octyne with Na in liquid N 3. When the same reaction is carried out with cyclooctyne (Problem 33b), the product is cis-cyclooctene, not trans-cyclooctene. Explain, mechanistically. 38. Write the expected major product of reaction of 1-propynyllithium, C q C 2 Li 1, with each of the following molecules in TF. C 2 r (c) Cyclohexanone (d) C (e) C C 2 f i (f) KK 39. Write the mechanism and final product for the reaction of 1-propynyllithium with trans-2,3- dimethyloxacyclopropane. 40. Which of the following methods is best suited as a high-yield synthesis of 2-methyl- 3-hexyne,? 2, Lindlar's catalyst NaN 2, liquid N 3 (c) 1. 2, C4 2. NaN 2, liquid N 3 (d) Li r (e) Li r 41. Propose reasonable syntheses of each of the following alkynes, using the principles of retrosynthetic analysis. Each alkyne functional group in your synthetic target should come from a separate molecule, which may be any two-carbon compound (e.g., ethyne, ethene, ethanal). (c) (d) ( ) 3 CC C [e careful! What is wrong with ( ) 3 C 1 2 :C C?] 42. Draw the structure of (R)-4-deuterio-2-hexyne. Propose a suitable retro-s N 2 precursor of this compound. 43. Reaction review. Without consulting the Reaction Road Map on p. 598, suggest reagents to convert a general alkyne RCqC into each of the following types of compounds. R G CPC D r r R r r CC r r (c) RCC (Markovnikov product)

5 D G Problems C h a p t e r (d) R I CC I (e) RCqCð M (f) RCqCCR R (g) RCqCR (h) RCqCC 2 C 2 (i) RCC (j) R G CPC D (k) R CC (anti-markovnikov product) 44. Give the expected major product of the reaction of propyne with each of the following reagents. D 2, Pd CaC 3, Pb( 2 C ) 2, quinoline; Na, ND 3 ; (c) 1 equivalent I; (d) 2 equivalents I; (e) 1 equivalent r 2 ; (f) 1 equivalent I; (g) 2 equivalents I; (h) 2, gs 4, 2 S 4 ; (i) dicyclohexylborane, then Na, What are the products of the reactions of dicyclohexylethyne with the reagents in Problem 44? 46. Write the structures of the initially formed enol tautomers in the reactions of propyne and dicyclohexylethyne with dicyclohexylborane followed by Na and 2 2 (Problems 44, part i, and 45, part i). 47. Give the products of the reactions of your first two answers to Problem 45 with each of the following reagents. 2, Pd C, C 2 ; r 2, C 4 ; (c) 3, TF, then Na, 2 2 ; (d) MCP, C 2 2 ; (e) s 4, then 2 S. 48. Propose several syntheses of cis-3-heptene, beginning with each of the following molecules. Note in each case whether your proposed route gives the desired compound as a major or minor final product. 3-Chloroheptane; 4-chloroheptane; (c) 3,4-dichloroheptane; (d) 3-heptanol; (e) 4-heptanol; (f) trans-3-heptene; (g) 3-heptyne. 49. Propose reasonable syntheses of each of the following molecules, using an alkyne at least once in each synthesis. r I I (c) meso-2,3-dibromobutane (d) Racemic mixture of (2R,3R)- and (2S,3S)-2,3-dibromobutane (e) r (f) (g) (h) (i)

6 604 C h a p t e r 1 3 lkynes 50. Show how the eck reaction might be employed to synthesize each of the following molecules. C 51. Propose a reasonable structure for calcium carbide, CaC 2, on the basis of its chemical reactivity (Section 13-10). What might be a more systematic name for it? 52. Propose two different syntheses of linalool, a terpene found in cinnamon, sassafras, and orange flower oils. Start with the eight-carbon ketone shown here and use ethyne as your source of the necessary additional two carbons in both syntheses.? Linalool 53. The synthesis of chamaecynone, the essential oil of the enihi tree, requires the conversion of a chloroalcohol into an alkynyl ketone. Propose a synthetic strategy to accomplish this task.? Eventually K C q C K Chamaecynone C q C 54. Synthesis of the sesquiterpene bergamotene proceeds from the alcohol shown here. Suggest a sequence to complete the synthesis.? 2 C 0 0 ergamotene 55. n unknown molecule displays 1 NMR and IR spectra D (next page). Reaction with 2 in the presence of the Lindlar catalyst gives a compound that, after ozonolysis and treatment with Zn in aqueous acid, gives rise to one equivalent of CC and two of C. What was the structure of the original molecule? 56. Formulate a plausible mechanism for the hydration of ethyne in the presence of mercuric chloride. (int: Review the hydration of alkenes catalyzed by mercuric ion, Section 12-7.)

7 Problems C h a p t e r NMR ( ) 4 Si Mz 1 NMR spectrum ppm ( ) δ 100 Transmittance () D IR Wavenumber 600 cm synthesis of the sesquiterpene farnesol requires the conversion of a dichloro compound into an alkynol, as shown below. Suggest a way of achieving this transformation. (int: Devise a conversion of the starting compound into a terminal alkyne.)? CqCC 2 Eventually Farnesol

8 606 C h a p t e r 1 3 lkynes Team Problem 58. Your team is studying the problem of an intramolecular ring closure of enediyne systems important in the total synthesis of dynemicin, which exhibits potent antitumor activity. N Dynemicin C ne research group tried the following approaches to effect this process. Unfortunately, all were unsuccessful. Divide the schemes among yourselves and assign structures to compounds through D. (Note: R9 and R0 are protecting groups.) 1. N š? C3S2, (C3C 2) 3N LiNR2 N š? 2. R N š? R R N š? R 1. (C3C2C2C2)4N F, TF (removes R ) 2. S 2, (C 2) 3N C LiNR 2 R N š? 3. R N } LiNR2 r Mild base D R N

9 Problems C h a p t e r successful model study (shown here) provided an alternative strategy toward the completion of the total synthesis. C LD Discuss the advantages of this approach and apply it to the appropriate compound in approaches 1 through 3. Preprofessional Problems 59. The compound whose structure is C q C(C 2 ) 3 is best named (IUPC) 4-chloro-1-pentyne; 5-chloropent-1-yne; (c) 4-pentyne-1-chloroyne; (d) 1-chloropent-4-yne. 60. nucleophile made by deprotonation of propyne is 2 :C 2 ; 2 :C C 2 ; (c) 2 :C q C; (d) 2 :C q C ; (e) 2 :C C. 61. When cyclooctyne is treated with dilute, aqueous sulfuric acid and gs 4, a new compound results. It is best represented as (c) (d) (e) 62. From the choices shown below, pick the one that best describes the structure of compound. 3P 4, p 2, 270 C, 100 atm C 2 C(C 2 ) 2 C 2 CC 2 (c) Cq CCC 2 (d) CqCC 2 CC 2

10 608 C h a p t e r 1 3 lkynes 63. From the choices shown below, pick the one that best describes the structure of compound. r r r2 (2 equivalents) CCC 2 ( primary alcohol; r r C 68.6, 8.6, and 22.9) 2 (2 equivalents), Raney Ni 1-butanol C 2 P CC 2 C 2 C 2 (c) CqCC 2 (d) CPCCP C (e)