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. 45legn9oahanes 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_eh04.55-6910/22/0520:19Page56 EQA 56.Chapter 4 CYCLOALKANES 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 cal cause of therin to i the text. To examine th ese n set of s to b frompstructureduceclpsninribetween niornbon-hydroge bonds. 4-3 and 4-4.Cyclohexanes Before not be too floppy ar asy cap ng the CH bonds pointing straight down (the bonds).Starting from this oint.you should be able to 一a ween the tw possible ha 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 (a) (b) (e) -C d cal.the bon carbon in cyclopropane itself (15645 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_ch04.55-6910/22/0520:19Page5日 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 e (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 @cH人cH,=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) OCH
(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