Ch4 problems - 162 Chapter I CYELOALKANES [email protected] as many structures as you can that have the formula C5H10 and contain one ring Name [email protected]

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Unformatted text preview: 162 Chapter I. CYELOALKANES blems @Write as many structures as you can that have the formula C5H10 and contain one ring. Name them. @me the following molecules according to the IUPAC nomenclature system. 1 HC ,C CH Cl a w so It c. Br Br ('3') CH3 (3) (f) Br Br Draw structural representations of each of the following molecules: (3) trans-1— chloro-2—ethylcyclopropane; (b) cis-l-bromo—2-chlorocyclopentane; (c) 2—chloro—1, 1-diethylcyclopropane; (d) trans-2-bromo-3-chloro—l,l-diethylcyclopropane; (e) cis—1, 3-dichloro—2,2—dimethylcyclobutane; (f) cis~2-chloro—1,1-difluoro—3-methylcyclopentane. . - . .. H [I .. . @The kinetic data for the radical chain chlorination of several cycloalkanes (see the ad- I .P-Er F W?“ joining table) illustrate that the C—H bonds of cyclopropane and, to a lesser extent, cy- ' 1} fl " . ' cIobutane are somewhat abnormal. (a) What do these data tell you about the strength of " the cyclopropane C—H bond and the stability of the cyclopropyl radical? (h) Suggest a reason for the stability characteristics of the cyclopropyl radical. (Hint: Consider bond- angle strain in the radical relative to cyclopropane itself.) Cycloalkane Reactivity Cyclopentane 0.9 Cyclobutane 0.7 Cyclopropane 0.1 Note: Relative to cyclohexane =1.0; at 63°C, hv, CC14 solvent. @Use the data in Tables 3-2 and 4-2 to estimate the DH" value for a C—C bond in (a) cyclopropane; (b) cyclobutane; (c) cyclopentane; and (d) cyclohexane. 22. Draw each of the following substituted cyclobutanes in its two interconverting “puckered” conformations (Figure 4—3). When the two conformations differ in energy, identify the more stable shape and indicate the form(s) of strain that raise the relative energy of the less stable one. (Hint: Puckered cyclobutane has axial and equatorial posi- tions similar to those in chair cyclohexane.) (a) Methylcyclobutane (c) trans- 1 ,2—Dimethy1cyclobutane (e) trans-1,3—Dimethylcyclobutane Which is more stable: cis- or trans-1,2-dimethylcyclobutane; cis- or trans-l 3—dimethylcyclobutane? (b) cis— 1 ,2-Dimethylcyolobutane (d) cis—1,3—Dimethylcyclobutane ’ @For each of the following cyclohexane derivatives, indicate (i) whether the molecule is a cis or trans isomer and (ii) whether it is in its most stable conformation. If your answer to (ii) is no, flip the ring and draw its most stable conformation. ‘ CH3 flows (b) NH2 (0 HO (d) C / CH((:H3)2 o \OCH3 (g) H3&/OCH3 (a) m CH3 CHZCH3 (a) £7 (f) (i) Br NH; O OH \ / C \ F (1!) 0') Cl fl Br CH(CH3)2 24. Usir ' the l of y 25.I)ra then cont than cyclt cych 26. For ‘ stab] 300 1?. Skett show 00011 1h“. Draw 29. What why‘. 3'”. The 1 chair. 31. The t taut-c 1 cis-Ck droinl 32. {in I.'i IS the .33. Save: tlccai ll: ta. L'll'ELES does The c int: I'c U-IHJEL 34. The 11: Forms Ir,- me 1e ring. rergy, lative 'ial posi- :ule is a answer lCHg, I Icing the data in Table 4-3, calculate the AG° for ring flip to the other conformation of the molecules depicted in Problem 23. Make sure that the sign (i.e., positive or negative) of your values is correct. @nrnw the most stable conformation for each of the following substituted cyclohexanes; "I r then. in each case, flip the ring and redraw the molecule in the higher energy chair i conformation: (a) cyclohexanol; (b) trans-S-methylcyclohexanol (see structures in the margin); (c) cis-l-(1-methylethyl)-3-methylcyclohexane; (d) trans-1-ethyl—3-methoxy— nvclohexane (see structure in the margin); (e) trans-1-chloro-4-(1,1—dimethylethyl) cyclohexane. @ur each molecule in Problem 25, estimate the energy difference between the most J’im‘uls and next best conformation. Calculate the approximate ratio of the two at 300 K. ‘ Showing the two possible chair conformations at the left and right ends of the reaction coordinate for conformational interconversion. 33. Draw all the possible all-chair conformers of cyclohexylcyclohexane. @ What is the most stable of the four boat conformations of methylcyclohexane, and why? 39 Ten.- most stable conformation of trans-1,3-bis(1,1-dimethylethyl)cyclohexane is not a chair. What conformation would you predict for this molecule? Explain. :H. The bicyclic hydrocarbon formed by the fusion of a cyclohexane ring with a cyclopen- tane ring is known as hexahydroindane (in margin). Using the drawings of trans- and cis—decalin for reference (Figure 4-13), draw the structures of trans— and cis—hexahy- droindane, showing each ring in its most stable conformation. @n viewing the drawings of cis- and trans-decalin in Figure 4-13, which do you think is the more stable isomer? Estimate the energy difference between the two isomers. 3.3. Several tricyclic compounds exist in nature with a cyclopropane ring fused to a cis- decalin structure, as shown in the molecule tricyclo[5.4.01’3.01’7}undecane (margin). In various countries, some of these substances have a history of use as folk medi- cines for purposes such as contraception. Make a model of this compound. How does the cyclopropane ring affect the conformations of the two cyclohexane rings? The cyclohexane rings in cis-decalin itself are capable of (simultaneous) chair—chair interconversion (recall Exercise 4-10). 15 the same true in tricyclo[5.4.01'3.01'7] t ,1 '3'. Sketch a potential-energy diagram (similar to that in Figure 4-9) for methylcyciohexane undecane? GZDThe naturally occurring sugar glucose (Chapter 24) exists in the two isomeric cyclic * forms shown below. These are called a and ,8, respectively. and they are in equilibrium by means of chemical processes that are introduced in Chapter 17. CHZOH CH20H | HO 0 Ho 0 HO H HO OH ' OH HO OH H i a form of glucose ,3 form of glucose (8.) Which of the two forms is more stable? (1)) At equilibrium the two forms are present in a ratio of approximately 64: 36. Calcu- late the free energy difference that corresponds to this equilibrium ratio. How closely does the value you obtained correlate with the data in Table 4-3? Problems 1 63 5 Cyclohexanol OH i “CH3 trans-3-Methylcyclohexanol CHZCH3 : “0cm trans-1~Ethyl—3-methoxycyclohexane OD Hexahydroindane do Tricyclo[s.4.01'3.0"fiundecane 164 HZN thapter l, CYElflAlKANES 35. Identify each of the following molecules as a monoterpene, a sesquiterpene, or a diterpene (all names are common). CH3 H30 (1)) CH3 (a) —— ‘ (c) H3C CH2 0 CH2 CH3 — OH 0 CH3 CHZOH CH2 CH3 Geraniol Eremanthin Eudesmol CH3 COZCH3 CH3 0 CH3 (d) (e) \ (f) HOCH2 O CH O / \ 3 / 0 HOCHE OH 0 / Ipomeamarone Genipin Castoramine O O H3C CH3 CH3 CH3 CH H (g) CH3 0 a" \ \ \ \ 20 H3C 0 CH3 Cantharidjn Vitamin A Circle and identify by name each functional group in the structures pictured in Problem 35. 37. Find the 2-methyl—1,3-butadiene (isoprene) units in each of the naturally occurring organic molecules pictured in Problem 35. 38. Circle and identify by name all the functional groups in any three of the steroids illus- trated in Section 4-7. Label any polarized bonds with partial positive and negative charges (5+ and 6—). 39. Several additional examples of naturally occurring molecules with strained ring struc- tures are shown here. + O 0 CH3 H3C HN NH carboxylic acid (Present in plants, this molecule plays a role in the ripening of fruits and the dropping of autumn leaves) (Present in cedar-wood oil) H3C\ A * COOH ‘ 0 N N O L H3C CH3 ——H Hocn20 H H O CHZOH cia bi l-Aminocyclopropane- 3 CH3 ,1 \ CH3 0 CH3 HO OH Thymidine dimer (A component of DNA that has been exposed to ultraviolet light) 0! "Pinefle Africanone (Also a plant-leaf oil) Identify the terpenes (if any) in the preceding group of structures. Find the 2-methyl-L3' butadiene units in each structure and classify the latter as a mono-, sesquih, or diterpene- If cyclobutane were flat, it would have exactly 90° C—CAC bond angles and could conceivably use pure 1) orbitals in its C~C bonds. What would be a possible hbe to b are t 4]. Con decz near 42. Fusi‘ a but SUPP syntl (a) l. (b) It {E} It (Li) J“. 'Sleret th'nrla 43. The e rcneti Custer (a, is) done: Elf if“: D" ['Mt'ul l-f-t' tl |-=~|IIE:I ,._V W Problems 1 65 hybridization f0r the carbon atoms of the molecule that would allow all the C—H bonds [0 be equivalent? Exactly where would the hydrogens on each carbon be located? What are the real structural features of the cyciobutane molecule that contradict this hypothesis? .tl . Compare the structure of cyclodecane in an all-chair conformation with that of trans- decalin. Explain why all-chair cyclodecane is highly strained, and yet trans-decalin is nearly strain free. Make models. Q27 m CH3 All-chair cyclodecane trans-Decalin 0] .13. Fusidic acid is a steroidlike microbial product that is an extremely potent antibiotic with CH3 a broad spectrum of biological activity. Its molecular shape is most unusual and had supplied important clues to researchers investigating the methods by which steroids are synthesized in nature. 0, "‘-. IE Fusidic acid (5:) Locate all the rings in fusidic acid and describe their conformations. (b) Identify all ring fusions in the molecule as having either cis or trans geometry. (c) Identify all groups attached to the rings as being either a- or lB-substituents. (11) Describe in detail how this molecule differs from the typical steroid in structure and E stereochemistry. (As an aid to answering these questions, the carbon atoms of the frame- )blem 3-"- work of the molecule have been numbered.) “g 4.5. The enzymatic oxidation of alkanes to produce alcohols is a simplified version of the reactions that produce the adrenocortical steroid hormones. In the biosynthesis of corti~ s illus- Costerone from progesterone (Section 4-7), two such oxidations take place successively ve (a, b). It is thought that the monooxygenase enzymes act as complex oxygen-atom donors in these reactions. A suggested mechanism, as applied to cyclohexane, consists Struc_ of the two steps shown below the biosynthesis. HOCE; I-I HO Steroid CH3 :0 monooxygenases, CH20H .0: _____ __+ j 0 \OH Progesterone Corticosterone OH been ‘ + 0 (atom) + ~0H —+ :thyl—1,3- Itemene’ Calculate AH° for each step and for the overall oxidation reaction of cyclohexane. .ngles Use the following DH“ values: cyclohexane C—H bond, 98.5 kcal mol"; bond in 0—H )SSible radical, 102.5 kcal mol“; cyclohexanol C—O bond, 96 kcal mol". 166 Chapter; CYEIDAIKANES (am Iodobenzene dichloride, formed by the reaction of iodobenzene and chic» ine, is a reagent for the chlorination of alkaue C—H bonds. Chlorinations in which iodobenzene dichloride is used are initiated by light. Q. . a C1 (a) Propose a radical chain mechanism for the chlorination of a typical alkane RH by iodobenzene dichloride. To get you started, the overall equation for the reaction is given / Cl below, as is the initiation step. Iodobenzene dichloride t RH + Olen —~—> RC] + HCl + Q1 .. . /C1 m . Initlatlon: I ~—> I—Cl + Cl- \c1 (b) Radical chlorination of typical steroids by iodobenzene dichloride gives, predomi- nantly, three isomeric monochlorination products. On the basis of both reactivity (terti- ary, secondary, primary) considerations and steric effects (which might hinder the ap- proach of a reagent toward a 0—H bond that might otherwise be reactive), predict the three major sites of chlorination in the steroid molecule. Either make a model or care- fully analyze the drawings of the steroid nucleus in Section 4-7. I: . . IClg —v> 3 major monochlorosterords 45. As Problem 43 indicates, the enzymatic reactions that introduce functional groups into the steroid nucleus in nature are highly selective, unlike the laboratory chlorination described in Problem 44. However, by means of a clever adaptation of this reaction, it is possible to partly mimic nature’s selectivity in the laboratory. Two such examples are illustrated below. CH3 (a) I I I (2/0 H \\0 (b) Propose reasonable explanations for the results of these two reactions. Make a model of the product of the addition of C12 to each iodocompound (compare Problem 44) to help in analyzing each system. 'Irsm Pr 4.6. ("onsir Comte] compc (a) M: rial or hybrid that 1e (b) M: analys compa projec l’l'epmfe 4?. which (a) Cy '48. The fc has (a carbor and to W. In this (a) the (c) the Sit. Which (a) ’CH3 Problems 167 II‘ ' “1i? Problem El Chm- it; nsider the following compounds: tch ‘ O O H by is given Hz, catalyst —w—) A B Conformational analysis reveals that, though compound A exists in a chair conformation, compound B does not. r. (a) Make a model of A. Draw chair conformations and label the substituents as equato- 101111- rial or axial. Circle the most stable conformation. (Note that the carbonyl carbon is sp2 (terti- hybridized and therefore the attached oxygen is neither equatorial nor axial. Do not let 5 aP‘ that lead you astray.) :t the (b) Make a model of B. Consider both transannular and gauche interactions in your Care- analysis of its two chair forms. Discuss the steric problems of these conformations in comparison with those of A. Illustrate the key points of your discussion with Newman projections. Suggest a less statically encumbered conformation for B. Prepi'oi'essinnal Problems ids 41‘. Which of the following cycloalkanes has the greatest ring strain? (a) Cyclopropane (b) Cyclobutane (c) Cyclohexane (d) Cycloheptane 45. The following molecule H ictional Cl 31 of this H’ZCICH such H CH 3 has (a) one axial chlorine and one sp2 carbon, (b) one axial chlorine and two sp2 r", carbons, (c) one equatorial chlorine and one sp2 carbon, or (d) one equatorial chlorine I and two Sp2 carbons. H 45". In this compound H D Cl Br CH3 H3C :13 H CH3 (:31 (a) the D is equatorial (b) the methyl-s are both equatorial Lt} the Cl is axial (d) the deuterium is axial 51!. Wh fish of the following isomers has the smallest heat of combustion? H CH3 H CH3 CH3 H odel of CH3 H H CH3 1 :0 help (b) H (6) CH3 (6) H tat CH3 ...
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This note was uploaded on 09/30/2009 for the course CHEM 3A taught by Professor Fretchet during the Spring '08 term at University of California, Berkeley.

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Ch4 problems - 162 Chapter I CYELOALKANES [email protected] as many structures as you can that have the formula C5H10 and contain one ring Name [email protected]

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