1559T_ch04_55-6910/22/0520:19Pa9e55 ⊕ EQA 4 Cycoalkanes It is convenient to co wer ordinary alkanes and cyclic ones in separate chapters of on nic chemistry textbooks properties or its chemical behavior.What you have leamned eroups.and therefore are relatively unreactive.like acvelic akans.For most of tem.theon actions are radical reactions.The major topics of concern are those dealing with the shapes (conformations) of the types ring system es on the bonc topic that is nota direct of what has gone before:the concept of bond ange strain in compounds containing small rings. Outline of the Chapter 4-1 Nomenclature and Physical Properties Basic material. he most common and most important ring size(six carbons).Its shapes,and their consequences 4-4 Substituted Cyclohexanes More of the same. 45legn9oahane 46 Polycycic Alkanes 4-7 Carbocyclic Natural Products Common ring-containing molecules of biological importance Keys to the Chapter 4-1.Nomenclature and Physical Properties The naming of ring compounds requires two new procedures in addition to those associated with acyclicsys tems.First,because rings have no"ends,"numbering starts at that carbon around the ring giving the lowes 55
4 Cycloalkanes It is convenient to cover ordinary alkanes and cyclic ones in separate chapters of organic chemistry textbooks. In most cases, however, the presence or absence of a ring in a molecule makes little difference to its physical properties or its chemical behavior. What you have learned in Chapters 2 and 3 can be applied virtually without change to the molecules presented in Chapter 4. Cyclic alkanes are nonpolar, lacking in any functional groups, and therefore are relatively unreactive, like acyclic alkanes. For most of them, the only important reactions are radical reactions. The major topics of concern are those dealing with the shapes (conformations) of the types of ring systems, and the effects of these shapes on the bonding and stability of each size ring. Some new points of nomenclature are presented. On the whole, however, the chapter contains only one new topic that is not a direct extrapolation of what has gone before: the concept of bond angle strain in compounds containing small rings. Outline of the Chapter 4-1 Nomenclature and Physical Properties Basic material. 4-2 Ring Strain and Structure The bonding consequences of closing a chain of atoms into a ring of three, four, or five carbons. 4-3 Cyclohexane The most common and most important ring size (six carbons). Its shapes, and their consequences. 4-4 Substituted Cyclohexanes More of the same. 4-5 Larger Cycloalkanes Very brief overview. 4-6 Polycyclic Alkanes Ditto. 4-7 Carbocyclic Natural Products Common ring-containing molecules of biological importance. Keys to the Chapter 4-1. Nomenclature and Physical Properties The naming of ring compounds requires two new procedures in addition to those associated with acyclic systems. First, because rings have no “ends,” numbering starts at that carbon around the ring giving the lowest 55 1559T_ch04_55-69 10/22/05 20:19 Page 55
1559r_ch04.55-6910/22/0520:19Page56 EQA 56.Chapter 4 CYCLOALKANES numbers for using the same crit either be on the same face oron principles of nomenclature follow unchanged. 4-2.Ring Strain and Structure Electron pairs ach othe er and try to be as far apart as possible.Rings with only three r four atoms force ature of compoundsan is the physi cal cause of therin to i the text. To examine th ese n youill find your set of s to b frompstructureduceclpsninribetween niornbon-hydroge bonds. 4-3 and 4-4.Cyclohexanes Before not cap ding the CH bonds pointing straight down (the bonds).Starting from this oint.you should be able to bine oatmeerbr ween the tw possible s of a substituted cyclohexane.This is an example of a tran end of the chapter. 4-5 and 4-6.Larger Rings;Polycyclic Molecules The material in these sections is int nded only to give a very brief introduction to are lature where appropriate. Solutions to Problems 17.Start with the largest ring and systematically go through successively smaller rings: CH CH:CH Cyclopentane Methyleyclobutane 1,1-Dimethylcyclopropan CH,CH:
numbers for substituent groups, using the same criteria for “lowest numbers” presented earlier. Second, rings have a “top” and a “bottom” face, relatively speaking. Therefore, substituents on different ring carbons may either be on the same face or on opposite faces, necessitating the cis or trans denotation in the name. All other principles of nomenclature follow unchanged. 4-2. Ring Strain and Structure Electron pairs repel each other and try to be as far apart as possible. Rings with only three or four atoms force the electron pairs of the COC bonds to be closer together than is normal for carbon atoms in molecules. The repulsion that results is the major cause of the high-energy nature of small ring compounds and is the physical cause of the ring strain referred to in the text. To examine the structural aspects of these molecules, you will find your set of models to be indispensable. Cyclopropane is the only flat cycloalkane ring. All larger cycloalkanes are nonplanar. Ring distortion away from a planar structure reduces eclipsing interactions between neighboring carbon–hydrogen bonds. 4-3 and 4-4. Cyclohexanes Before you do anything else, make a model of cyclohexane. Be sure to use the correct atoms and bonds from your kit. The completed model should not be too floppy and should be easily capable of holding the shape shown in Figure 4-5(B). This is the chair conformation, with three COH bonds pointing straight up and three COH bonds pointing straight down (the axial COH bonds). Starting from this point, you should be able to construct the other important cyclohexane conformations by moving an “end” carbon through the plane of the “middle” four carbons of the ring; that is, Learn to recognize axial and equatorial positions and their cis/trans interrelationships around the ring. Again, use your model in conjunction with the chapter text and illustrations. Note the congestion associated with large groups in axial positions, a result of 1,3-diaxial interactions, the main effect that causes differences in energy between the two possible chair conformations of a substituted cyclohexane. This is an example of a transannular (literally, “across the ring”) interaction, arising in this case from the ring structure forcing groups to adopt gauche conformational relationships. Be sure to use your models when trying to do the problems at the end of the chapter. 4-5 and 4-6. Larger Rings; Polycyclic Molecules The material in these sections is intended only to give a very brief introduction to areas of organic chemistry that are important in current research but are generally beyond the scope of a course at this level. Only a small number of selected molecules are mentioned with relevant points of structure and nomenclature presented where appropriate. Solutions to Problems 17. Start with the largest ring and systematically go through successively smaller rings: Cyclopentane cis-1, 2-Dimethylcyclopropane CH3 Methylcyclobutane 1, 1-Dimethylcyclopropane CH3 CH2CH3 CH3 CH3 CH3 trans-1, 2-Dimethylcyclopropane Ethylcyclopropane (Did you forget this one? Lots of students miss it.) CH3 CH3 Boat and boatlike conformations (rather floppy, too) Move up 56 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 56
1559T_ch04_55-6910/22/0520:19Pa9e57 ⊕ EQA 5 ouions o Problems·57 18.(a)lodocyclopropane (b)trans-1-Methyl-3-(1-methylethylcyclopentane (c)cis-1.2-Dichlorocyclobutane (d)cis-1-Cyclohexyl-5-methylcyclodecane (e)To tell whether this is cis or trans,draw in the hydrogens on the substituted carbons: Groups on bottom of ring One Br on top.one on bottom,.frans-1,3-dibromocyclohexane (f)Similarly. On top一⑧ ⊕ .cis-1.2-dibromocyclohexane (b) (e) -C d cal.the bon carbon in cyclopropane itself (156495 bond angle comp ssion).Forming the radical value to begi h is the DH for the C-C bond between CHa (2)groups the t
18. (a) Iodocyclopropane (b) trans-1-Methyl-3-(1-methylethyl)cyclopentane (c) cis-1,2-Dichlorocyclobutane (d) cis-1-Cyclohexyl-5-methylcyclodecane (e) To tell whether this is cis or trans, draw in the hydrogens on the substituted carbons: One Br on top, one on bottom, trans-1,3-dibromocyclohexane. (f ) Similarly, cis-1,2-dibromocyclohexane 19. (a) (b) (c) (d) (e) (f ) 20. (a) The very low relative radical chlorination reactivity of cyclopropane implies abnormally strong COH bonds and an abnormally unstable cyclopropyl radical. (b) Radicals prefer sp2 hybridization, with 120° bond angles. So in the cyclopropyl radical, the bond angle strain at the radical carbon is greater (120° 60° 60° bond angle compression) than at a carbon in cyclopropane itself (109.5° 60° 49.5° bond angle compression). Forming the radical therefore increases ring strain and is more difficult in cyclopropane than in a molecule lacking bond angle distortion to begin with. 21. In all cases the reference value to begin with is the DH° for the COC bond between CH2 (2°) groups, i.e., DH° for CH3CH2OCH2CH3, 88 kcal mol1 (Table 3-2). (a) Cleavage of a COC bond in cyclopropane requires a smaller net energy input because ring strain is relieved in the process. Breaking a “normal” COC bond would require 88 kcal mol1 input, but because 28 kcal mol1 is recovered as a result of strain relief in opening the three-membered CH3 Cl F F CH3 CH3 Cl CH3CH2 CH2CH3 Cl Br Cl CH3CH2 CH2CH3 Cl Br Cl CH2CH3 Cl Br H On top On bottom H Br Br Br H H Groups on top of ring Groups on bottom of ring Solutions to Problems • 57 1559T_ch04_55-69 10/22/05 20:19 Page 57
1559r_eh04.55-6910/22/0520:19Page58 58.Chapter 4 CYCLOALKANES gopening (Section 4-2 CH,CH>+CH,CHs orle CH.CH2-CH.CH2 cn CH (b)For cyclobutane.our estimated DH=88-26=62 kcal mol- (e)D 88-7=81 kcal mol-1 (d)DH=88-0=88 kcal mol-1 22.Here is a drawing of cyclobutane,with axial (a)and equatorial (e)positions labeled. axial and equatorial positions CH H (1.3-diaxial)interactio b)CH CH: This (the t ns-1 2 c 4e (c) ime.In the cis-1.2(b)abovel
ring, the DH° actually required is 88 28 60 kcal mol1 . Note that this is consistent with the Ea of 65 kcal mol1 for ring opening (Section 4-2). (b) For cyclobutane, our estimated DH° 88 26 62 kcal mol1 . (c) DH° 88 7 81 kcal mol1 (d) DH° 88 0 88 kcal mol1 Thus, the unusual ring-opening reactions of cyclopropane and cyclobutane (relative to other alkanes and cycloalkanes) are thermodynamically reasonable. 22. Here is a drawing of cyclobutane, with axial (a) and equatorial (e) positions labeled. All the carbons are equivalent, and flipping the puckered form exchanges axial and equatorial positions, exactly as does flipping chair conformations in cyclohexane. (a) (b) (c) H H H H CH3 CH3 CH3 CH3 Both CH3’s equatorial; more stable This (the trans-1,2 compound) is more stable because both CH3’s can be equatorial at the same time. In the cis-1,2 [(b) above] there is always one axial group in either conformation. H H H H CH3 CH3 CH3 CH3 H H H CH3 CH3 Equatorial; more stable Transannular (1,3-diaxial) interaction a a a a e e e e CH3CH2· ·CH2CH3 CH2CH2 ·CH2CH2CH2· CH3CH2 CH2 CH2 CH2 (88 input) vs. 88 kcal mol1 required to break a “normal” COC bond 28 kcal mol1 recovered as result of relief of ring strain 60 kcal mol1 net input actually required 58 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 58
1559T_ch04_55-6910/22/0520:19Pa9e59 EQA Solufions o Problems5 @cHcH,=H Lat the Both CH conformation. 23.Refer to answers to Problems 18(e)and 18(f)for guidelines. (a)Trans.Not most stable form.Ring flip gives diequatorial conformation: CiCH (b)Trans!(Surprise)Note positions of hydrogens H The two hydrogens are trans.so clearly the NH and OCH groups must be trans.too.The NH is = H e)Cis. HO CH(CHs) is not the most stable conformation because the ring can flip to the form on the right,in which CH(CH3)is equatorial and OH axia 1)Tran Most stable conformation(CH equatorial)
(d) (e) 23. Refer to answers to Problems 18(e) and 18(f) for guidelines. (a) Trans. Not most stable form. Ring flip gives diequatorial conformation: (b) Trans! (Surprise!) Note positions of hydrogens. The two hydrogens are trans, so clearly the NH2 and OCH3 groups must be trans, too. The NH2 is cis to the top H, and the OCH3 is cis to the bottom H. Both groups are equatorial, so this is the most stable conformation. (c) Cis. From Table 4-3, we see that CH(CH3)2 prefers an equatorial position more (2.2 kcal mol1 ) than does OH (0.94 kcal mol1 ). In the structure drawn, CH(CH3)2 is axial and OH is equatorial. This is not the most stable conformation because the ring can flip to the form on the right, in which CH(CH3)2 is equatorial and OH axial. (d) Trans. H H Most stable conformation (CH3 equatorial). CH3 O OCH3 C H H HO H HO H CH(CH3)2 CH(CH3)2 H H NH2 OCH3 Trans Trans Cl CH3 H H H H CH3 CH3 CH3 CH3 Equal in energy: one methyl axial and one equatorial in each conformation. H H H CH3 CH3 H CH3 CH3 Both CH3’s equatorial; more stable Now it is the cis-1,3 compound that can have both CH3’s equatorial at the same time. It is more stable than the trans (below). Solutions to Problems • 59 1559T_ch04_55-69 10/22/05 20:19 Page 59
1559r_ch04.55-6910/22/0520:19Page60 EQA 60.Chapter 4 CYCLOALKANES (e)Cis. Most stable form (CH CHcqutoria). H (f)Trans Most stable form (both groupsqur). (g Cis. -OCH; Most stable form (oth group) Not most stable form.Ring flip makes it diequatorial ()Cis. Not most stable form.Ring flip makes HO-Cgroupqurial which is preferable (Table 4-3). (j)Trans.Most stable form [compare (b),above]. i the poblem is the le one and (a）-(1.70+0.52)=-2.22 kcal mol- b)14+0.75=2.2 kcal mol- (c)-(2.20-0.94)=-1.26 kcal mol (d1.70-1.29=0.41 kcal mol- (e)1.75-0.46=1.29 kcal mol- f)1.4+0.55=2.0 kcal mol (g1.70+0.75=2.45 kcal mol-1 h)-0.94+0.25)=-1.19 kcal mol- ①-(1.29-0.55)=-0.74 kcal mol-1 25 (j)2.20+0.52=2.72 kcal mol- ror (ao OH w尺a HO CH (CHCH(CH)
(e) Cis. (f ) Trans. (g) Cis. (h) Cis. (i) Cis. (j) Trans. Most stable form [compare (b), above]. 24. The sign for G° will be negative if the conformation shown in the problem is the less stable one and will be positive if the conformation shown is the more stable one. (a) (1.70 0.52) 2.22 kcal mol1 (b) 1.4 0.75 2.2 kcal mol1 (c) (2.20 0.94) 1.26 kcal mol1 (d) 1.70 1.29 0.41 kcal mol1 (e) 1.75 0.46 1.29 kcal mol1 (f ) 1.4 0.55 2.0 kcal mol1 (g) 1.70 0.75 2.45 kcal mol1 (h) (0.94 0.25) 1.19 kcal mol1 (i) (1.29 0.55) 0.74 kcal mol1 (j) 2.20 0.52 2.72 kcal mol1 25. Most stable Least stable conformation conformation (a) (b) (c) CH(CH3)2 CH3 CH3 CH(CH3)2 HO CH3 CH3 OH OH OH COOH Br H H Not most stable form. Ring flip makes HO C group equatorial, which is preferable (Table 4-3). O F OH Not most stable form. Ring flip makes it diequatorial. H H CH3 OCH3 H Most stable form (both groups equatorial). H H Br H NH2 Most stable form (both groups equatorial). I H H Most stable form (CH3CH2 equatorial). CH2CH3 60 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 60
1559T_ch04_55-6910/22/0520:19Pa9e61 EQA Solfions to Problems61 d CH.CH ac、C(CH) C(CH3) 26.From Table 4-3 Ratios,using△G°--RT In Keg K=484=083: ..837mi (b)1.7-0.94=0.8 kcal mol (g2.2+1.7=3.9 kcal mol Ka=10:99.9/0.1 ratic (d1.75-0.75=1.00 keal mol- Ke=5.3:84/16 ratio (e)5+0.52=5.5 kcal mol- Kea≈10；>99.90.1 ratio e e即油rot ahe information toestimate the of the twist-boat and boat conformations in the middle of the diagram except to assume that they will probably be equal to orresponding conformations of
(d) (e) 26. From Table 4-3 Ratios, using G° RT ln Keq (a) 0.94 kcal mol1 (less stable conformation Keq 4.8; 0.83; is higher in energy) 83/17 ratio (in favor of more stable conformation) (b) 1.7 0.94 0.8 kcal mol1 Keq 3.8; 0.79; 79/21 ratio (c) 2.2 1.7 3.9 kcal mol1 Keq 103 ; 99.9/0.1 ratio (d) 1.75 0.75 1.00 kcal mol1 Keq 5.3; 84/16 ratio (e) 5 0.52 5.5 kcal mol1 Keq 104 ; 99.9/0.1 ratio In each case the more stable conformation is the one in which the group with the largest G° value from Table 4-3 is equatorial. 27. The basic idea is that the two extremes of the diagram, the two chair conformations, will no longer be equal in energy: One has the methyl group equatorial, but the other has it axial. You do not have sufficient information to estimate the energies of the twist-boat and boat conformations in the middle of the diagram, except to assume that they will probably be equal to or (more likely) higher in energy than the corresponding conformations of cyclohexane itself, relative to the more stable chair conformations. E Reaction coordinate to conformational interconversion CH3 CH3 CH3 Chair Chair Boat 1.7 kcal mol–1 3.8 3.8 1 4.8 4.8 1 Cl C(CH3)3 C(CH3)3 Cl CH3O CH2CH3 CH2CH3 CH3O Solutions to Problems • 61 1559T_ch04_55-69 10/22/05 20:19 Page 61
1559r_ch0455-6910/22/0520:19Page62 EQA 62.Chapter 4 CYCLOALKANES 28.Both rings can flip.so there are four possible combinations.two of which are identical: Most stable:each ri 2.e ( rhospieleepineahdiearetpeodpoionandeuamineeaeh Methyl is pseu CH CH,LH Methyl is pseudoaxial cnergy.One a 30.Only a boat-related conformation permits both bulky groups to avoid axial positions.This molecule will adopt a shape in which both groups are"pseudoequatorialIt will be based on the twist-boat of
28. Both rings can flip, so there are four possible combinations, two of which are identical: 29. Notice how some positions around a boat conformation of cyclohexane are axial-like (“pseudoaxial”) and some are equatorial-like (“pseudoequatorial”): If you draw conformations placing the methyl group in each different type of position and examine each conformation for strain, you will see the following: Of the two with the methyl in an equatorial-like position, the one on the left has the bond to the CH3 staggered with respect to neighboring COH bonds. The other possibility is higher in energy as a result of the eclipsing interaction shown. The two conformations with pseudoaxial CH3 are both quite high in energy. One actually has three diaxial interactions involving the methyl group (only one is shown; make a model to see the rest!), and the worst of them all has a serious transannular interaction due to the close approach between the CH3 and the H shown. 30. Only a boat-related conformation permits both bulky groups to avoid axial positions. This molecule will adopt a shape in which both groups are “pseudoequatorial.” It will be based on the twist-boat of Diaxial interactions Worst conformation: transannular interaction CH3 H CH3 H Methyl is pseudoaxial CH3 CH3 H Best conformation Methyl is pseudoequatorial Eclipsed a a a a e e e e a pseudoaxial e pseudoequatorial H H Most stable: each ring is attached by an equatorial bond to the other Least stable: each ring is attached by an axial bond to the other These two are identical H H H H H H 62 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 62
1559T_ch04_55-6910/22/0520:19Pa9e63 EQA Solutions to Problems.63 mize eclipsing interactions of the true boa conformation(Section 4-3). C(CHa (CH3)2C- 31.Models may be helpful here.You should be able to construct structures similar to those pictured below 本B rans-Hexahydmoindane cis-Hexahydroindan roindane and cis to each other envelopes.around the bond away from the give iseto the cyclopentane half-chair conformation,which is similar in energy but harder to draw. 32.In trans-decalin cach of the carbon- al p as A and B,and numbered the four relevant bonds 1-4: ring fusior respect to both rings. In cis-decalin the situation is different.Look at the picture: )Bond with respeet to ring A (rotate the pase this n r bo nds ar to the ring-rusion hydrogens are now equatorial and ax to the we would conclude that the cis isomer is 3.5 keal mol higher in energy dess stable)than the urans.This turns out to be an ove re d has three more than does the trans:
cyclohexane in order to minimize eclipsing interactions of the true boat conformation (Section 4-3). (Make a model!) 31. Models may be helpful here. You should be able to construct structures similar to those pictured below. Notice how the ring-fusion hydrogens are trans to each other in trans-hexahydroindane and cis to each other in cis-hexahydroindane. In the drawings, the cyclohexane rings are chairs and the cyclopentane rings are envelopes. A slight twist around the cyclopentane bond away from the envelope “flap” would give rise to the cyclopentane half-chair conformation, which is similar in energy but harder to draw. 32. In trans-decalin each of the carbon–carbon bonds attached to the ring fusion occupy an equatorial position with respect to the ring on which they are attached. In the illustration below we have labeled the rings as A and B, and numbered the four relevant bonds 1–4: Bonds 1 and 2 (which are part of ring B) are equatorial substituents with respect to ring A. Bonds 3 and 4 (which are part of ring A) are equatorial substituents with respect to ring B. Both hydrogen atoms on the ring fusion carbons are axial with respect to both rings. In cis-decalin the situation is different. Look at the picture: Bond 1 (in ring B) is now axial with respect to ring A (rotate the page clockwise by 60° to see this more clearly). Bond 2 is still equatorial. Also, bond 3 (in ring A) is now axial with respect to ring B (bond 4 is still equatorial). So two of these four bonds are axial with respect to the ring on which they are substituents and, as a result, give rise to 1,3-diaxial interactions that raise the enthalpy of the compound. Notice that the ring-fusion hydrogens are now equatorial with respect to one ring and axial with respect to the other. If we were to assign an energy of about 1.75 kcal mol1 to each axial bond to a carbon in cis-decalin, we would conclude that the cis isomer is 3.5 kcal mol1 higher in energy (less stable) than the trans. This turns out to be an overestimate, in part because carbon atoms in rings cannot rotate freely and therefore do not generate as much steric interference as do simple alkyl groups, which can rotate a full 360°. An alternative way to estimate the energy difference is to search for butane structural fragments that possess gauche conformations. The cis isomer has three more than does the trans; assuming each gauche butane raises the energy content by about 0.9 kcal mol1 , one arrives at an energy difference of 2.7 kcal mol1 . H B A 2 3 4 1 H H A B 3 2 4 1 H H H H H trans-Hexahydroindane cis-Hexahydroindane H H C(CH3)3 (CH3)3C Solutions to Problems • 63 1559T_ch04_55-69 10/22/05 20:19 Page 63
1559T_ch04.55-6910/22/0520:19Page6d 64.Chapter 4 CYCLOALKANES left yclohexane in a chair conformation at all?Actually.it is not.The geometry of the th-mmrd ring forces the c cyclohexane ring bonds asso ated with it and four of the of a chair conformation,with the appearance shown below. modoattempt a chair-chair flip in ring B youwlk shape.This model se 34.(a)The (DK=6436 17.the on( xane is -0.94 kcal mol- s largely from the fact thatt ring in glucose is no carbon,and introduction of two polar C-O bonds whose dipoles can give rise to either attractive or repulsive interactions with the CO bonds of nearby substituted ring carbon 合2弘dite pere in the mo.ee0 he molecle onee道5.qp (a)10 carbons,monoterpene (b).(c).and (d)15 carbons,sesquiterpene (e)11 carbons.but only 10 in the contiguous molecular"skeleton":monoterpen (f)15.sesquiterpene (g)10.monoterpene (h)20.diterpene 36. CH. H;C (a) (b) nes (e CH. CH
33. You really do need to go to models to address this question. Start by examining a simpler compound, one in which a cyclopropane ring is fused to a single cyclohexane ring (below, left). Is the cyclohexane in a chair conformation at all? Actually, it is not. The geometry of the three-membered ring forces the cyclohexane ring bonds associated with it and four of the cyclohexane ring carbons to all be coplanar. Only the two cyclohexane ring carbons farthest from the ring fusion are movable. In tricyclo[5.4.01,3.01,7]undecane (above, right), the ring labeled ‘A’ is similarly constrained. Only two of its carbons (the one at the bottom and the one at the lower left) are capable of significant motion. This isn’t much more than a wiggling motion, one up and one down, relative to the plane of the remaining four carbons in that six-membered ring. As for ring ‘B’, it is capable of holding one reasonable approximation of a chair conformation, with the appearance shown below. However, if you attempt a chair-chair flip in ring ‘B’, you will encounter much more resistance from the model set. The flip can be forced, but the conformation that results is a distorted twist-boatlike shape. This is because the two carbons at the ring fusion cannot undergo the corresponding rotation that would finish the chair-chair interconversion. The rigidity of ring ‘A’ prevents that. 34. (a) The form is more stable because all of its substituent groups are equatorial. One of the OOH groups (the one on the ring carbon at the right of the picture) in the form is axial. (b) Keq 64/36 1.78. Using the equation G° 1.36 log10 Keq (Section 2-1) for an equilibrium constant at room temperature (25°C), we get G° 0.81 kcal mol1 . In Table 4-3 the axial/ equatorial free energy difference for an OOH substituent on cyclohexane is 0.94 kcal mol1 . The difference is small but real, and it derives largely from the fact that the ring in glucose is not a cyclohexane but an oxacyclohexane—a cyclic ether. Replacing a ring CH2 group with an oxygen has several effects, including removal of the steric interactions associated with the hydrogens on the carbon, and introduction of two polar COO bonds whose dipoles can give rise to either attractive or repulsive interactions with the COO bonds of nearby substituted ring carbons. 35. Count carbons in the molecule. If there are 10, the molecule is a monoterpene; if 15, a sesquiterpene; if 20, a diterpene. (a) 10 carbons, monoterpene (b), (c), and (d) 15 carbons, sesquiterpene (e) 11 carbons, but only 10 in the contiguous molecular “skeleton’’; monoterpene (f ) 15, sesquiterpene (g) 10, monoterpene (h) 20, diterpene 36. O O CH2 CH2 CH2 H3C H3C CH3 CH3 CH2OH CH3 OH CH3 CH3 Alkenes Alkenes Alcohol Ester (a) (b) (c) Alcohol H B A H A B H H 64 • Chapter 4 CYCLOALKANES 1559T_ch04_55-69 10/22/05 20:19 Page 64
