ChAPTER 3 CONFORMATIONS OF ALKANES AND CYCLOALKANES SOLUTIONS TO TEXT PROBLEMS 3.1(b) The sawhorse formula contains four carbon atoms in an unbranched chain. The compound is butane, CH CH,CH,CH CH3 (c) Rewrite the structure to show its constitution. The compound is CH,CH,CH(CH,)2; it is 2-methylbutane H3 (d) In this structure, we are sighting down the C-3--C-4 bond of a six-carbon chain. It is CH CH, CH,CHCH, CH3, or 3-methy lhexane CH3 H1CH_ CH, 46 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
CHAPTER 3 CONFORMATIONS OF ALKANES AND CYCLOALKANES SOLUTIONS TO TEXT PROBLEMS 3.1 (b) The sawhorse formula contains four carbon atoms in an unbranched chain. The compound is butane, CH3CH2CH2CH3. (c) Rewrite the structure to show its constitution. The compound is CH3CH2CH(CH3)2; it is 2-methylbutane. (d) In this structure, we are sighting down the C-3GC-4 bond of a six-carbon chain. It is CH3CH2CH2CHCH2CH3, or 3-methylhexane. = CH3 H CH2CH3 H H CH3 CH2CH3 C H H H H3C CH3 CH3 C H H H CH3 CH3 CH3 H H H H CH3 CH3 46 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONFORMATIONS OF ALKANES AND CYCLOALKANES 3.2 Red circles gauche: 60 and 300. Red circles anti: 180. Gauche and anti relationships occur only in staggered conformations; therefore, ignore the eclipsed conformations(0%, 1200, 2400, 3600) 3.3 All the staggered conformations of propane are equivalent to one another, and all its eclipsed con- formations are equivalent to one another. The energy diagram resembles that of ethane in that it is a symmetrical one. H H H D H H, C H H H 240 300 Torsion angle(degrees) The activation energy for bond rotation in propane is expected to be somewhat higher than that in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed nformation. This strain is, however, less than the van der Waals strain between the methyl groups of butane, which makes the activation energy for bond rotation less for propane than for 3.4(b) To be gauche, substituents X and a must be related by a 60 torsion angle. If A is axial as specified in the problem, X must therefore be equatorial. gauche (c) For substituent X at C-1 to be anti to C-3, it must be equatorial (d) When X is axial at C-1, it is gauche to C-3. 3.5(b) According to the numbering scheme given in the problem, a methyl group is axial when it is up"at C-1 but is equatorial when it is up at C-4. Since substituents are more stable when they Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
3.2 Red circles gauche: 60° and 300°. Red circles anti: 180°. Gauche and anti relationships occur only in staggered conformations; therefore, ignore the eclipsed conformations (0°, 120°, 240°, 360°). 3.3 All the staggered conformations of propane are equivalent to one another, and all its eclipsed conformations are equivalent to one another. The energy diagram resembles that of ethane in that it is a symmetrical one. The activation energy for bond rotation in propane is expected to be somewhat higher than that in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed conformation. This strain is, however, less than the van der Waals strain between the methyl groups of butane, which makes the activation energy for bond rotation less for propane than for butane. 3.4 (b) To be gauche, substituents X and A must be related by a 60° torsion angle. If A is axial as specified in the problem, X must therefore be equatorial. (c) For substituent X at C-1 to be anti to C-3, it must be equatorial. (d) When X is axial at C-1, it is gauche to C-3. 3.5 (b) According to the numbering scheme given in the problem, a methyl group is axial when it is “up” at C-1 but is equatorial when it is up at C-4. Since substituents are more stable when they 3 X A 3 X A X and A are gauche. X A Potential energy 0 60 120 180 240 300 360 Torsion angle (degrees) H3C H H H H H H H H H H H3C H CH3 H H H H H3C H H H H H H H H H H H3C H H H CH3 H H H H H H CH3 H CONFORMATIONS OF ALKANES AND CYCLOALKANES 47 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
48 CONFORMATIONS OF ALKANES AND CYCLOALKANES occupy equatorial rather than axial sites, a methyl group that is up at C-1 is less stable than one that is up at C-4 →H2C (c) An alkyl substituent is more stable in the equatorial position. An equatorial substituent at C-3 C 3.6 A tert-butyl group is much larger than a methyl group and has a greater preference for the equator- rial position. The most stable conformation of l-tert-butyl-1-methylcyclohexane has an axial methyl group and an equatorial tert-butyl group CH3 C(CHi) I-ferr-Butyl-1-methylcyclohexane 3.7 Ethylcyclopropane and methylcyclobutane are isomers(both are CsHo) The less stable isomer has the higher heat of combustion. Ethylcyclopropane has more angle strain and is less stable(has higher potential energy)than methylcyclobutane CHCH Less stable More stable Heat of combustion: 3384 kI/m (808.8 kcal/mol) (801.2 kcal/mol) 3. 8 The four constitutional isomers of cis and trans-1, 2-dimethylcyclopropane that do not contain double bond CH,CH3 I, l-Dimethylcyclopropane Ethy cyclopropane CH Methylcyclobutane Cyclopentane 3.9 When comparing two stereoisomeric cyclohexane derivatives, the more stable stereoisomer is the one with the greater number of its substituents in equatorial orientations. Rewrite the structures as chair conformations to see which substituents are axial and which are equatorial. H3C H3 H,C cis-1, 3. 5-Trimethy lcycloher Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
48 CONFORMATIONS OF ALKANES AND CYCLOALKANES occupy equatorial rather than axial sites, a methyl group that is up at C-1 is less stable than one that is up at C-4. (c) An alkyl substituent is more stable in the equatorial position. An equatorial substituent at C-3 is “down.” 3.6 A tert-butyl group is much larger than a methyl group and has a greater preference for the equatorial position. The most stable conformation of 1-tert-butyl-1-methylcyclohexane has an axial methyl group and an equatorial tert-butyl group. 3.7 Ethylcyclopropane and methylcyclobutane are isomers (both are C5H10). The less stable isomer has the higher heat of combustion. Ethylcyclopropane has more angle strain and is less stable (has higher potential energy) than methylcyclobutane. 3.8 The four constitutional isomers of cis and trans-1,2-dimethylcyclopropane that do not contain double bonds are 3.9 When comparing two stereoisomeric cyclohexane derivatives, the more stable stereoisomer is the one with the greater number of its substituents in equatorial orientations. Rewrite the structures as chair conformations to see which substituents are axial and which are equatorial. cis-1,3,5-Trimethylcyclohexane H H H H3C CH3 CH3 CH3 CH3 H H3C H H CH3 CH3 1,1-Dimethylcyclopropane CH3 Methylcyclobutane Ethylcyclopropane CH2CH3 Cyclopentane CH2CH3 CH3 More stable 3352 kJ/mol (801.2 kcal/mol) Less stable 3384 kJ/mol (808.8 kcal/mol) Heat of combustion: 1-tert-Butyl-1-methylcyclohexane CH3 C(CH3)3 H H3C Up Down H CH3 Up Down H H3C Up Down 4 5 6 3 2 1 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONFORMATIONS OF ALKANES AND CYCLOALKANES All methyl groups are equatorial in cis-1, 3,5-trimethylcyclohexane. It is more stable than trans 1, 3, 5-trimethylcyclohexane(shown in the following), which has one axial methyl group in its most H3C CH H CH H,C H CH 3.10 In each of these problems, a tert-butyl group is the larger substituent and will be equatorial in the most stable conformation. Draw a chair conformation of cyclohexane, add an equatorial tert-butyl group, and then add the remaining substituent so as to give the required cis or trans relationship to the tert-butyl group (b) Begin by drawing a chair cyclohexane with an equatorial tert-butyl group. In cis-l-tert-butyl 3-methylcyclohexane the C-3 methyl group is equatorial (c) In trans-l-tert-butyl-4-methylcyclohexane both the tert-butyl and the C-4 methyl group are equatorial H3C H (d) Again the tert-butyl group is equatorial; however, in cis-l-tert-butyl-4-methylcyclohexane the methyl group on C-4 is axial. H 3.11 Isomers are different compounds that have the same molecular formula. Compare the molecular formulas of the compounds given to the molecular formula of spiropentane H=CH, CH Spiropentane(CsHg) CsHs CHIO CsH& Only the two compounds that have the molecular formula Cs Hg are isomers of spiropentane 3. 12 Two bond cleavages convert bicyclobutane to a noncyclic species; therefore, bicyclobutane is bicycl A一一不个 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
All methyl groups are equatorial in cis-1,3,5-trimethylcyclohexane. It is more stable than trans- 1,3,5-trimethylcyclohexane (shown in the following), which has one axial methyl group in its most stable conformation. 3.10 In each of these problems, a tert-butyl group is the larger substituent and will be equatorial in the most stable conformation. Draw a chair conformation of cyclohexane, add an equatorial tert-butyl group, and then add the remaining substituent so as to give the required cis or trans relationship to the tert-butyl group. (b) Begin by drawing a chair cyclohexane with an equatorial tert-butyl group. In cis-1-tert-butyl- 3-methylcyclohexane the C-3 methyl group is equatorial. (c) In trans-1-tert-butyl-4-methylcyclohexane both the tert-butyl and the C-4 methyl group are equatorial. (d) Again the tert-butyl group is equatorial; however, in cis-1-tert-butyl-4-methylcyclohexane the methyl group on C-4 is axial. 3.11 Isomers are different compounds that have the same molecular formula. Compare the molecular formulas of the compounds given to the molecular formula of spiropentane. Only the two compounds that have the molecular formula C5H8 are isomers of spiropentane. 3.12 Two bond cleavages convert bicyclobutane to a noncyclic species; therefore, bicyclobutane is bicyclic. Spiropentane (C5H8) CH CH2 C5H8 C6H10 C5H8 C5H10 CH2 CH3 H C(CH3)3 H H3C H C(CH3)3 H H H3C H C(CH3)3 H H H H3C CH3 H3C trans-1,3,5-Trimethylcyclohexane CH3 CH3 H H CH3 H CONFORMATIONS OF ALKANES AND CYCLOALKANES 49 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONFORMATIONS OF ALKANES AND CYCLOALKANES The two bond cleavages shown convert camphene to a noncyclic species; therefore, camphene is bicyclic. (Other pairs of bond cleavages are possible and lead to the same conclusion 3. 13 (b) This bicyclic compound contains nine carbon atoms. The name tells us that there is a five- carbon bridge and a two-carbon bridge. The 0 in the name bicyclo[5.2.0]nonane tells us that the third bridge has no atoms in it-the carbons are common to both rings and are directl Bicyclo[5.2.0]nonane (c) The three bridges in bicyclo[3. 1. I ]heptane contain three carbons, one carbon, and one carbon The structure can be written in a form that shows the actual shape of the molecule or one that Three- carbol One-carbon (d) Bicyclo[3.3. 0]octane has two five-membered rings that share a common side. Three-carbon Three-carbon 3. 14 Since the two conformations are of approximately equal stability when r= H, it is reasonable to expect that the most stable conformation when R= CH3 will have the CH3 group equa R =H: both conformations similar in energy R=CH3: most stable conformation has CH, equ 3.15(a) Recall that a neutral nitrogen atom has three covalent bonds and an unshared electron pair he three bonds are arranged in a trigonal pyramidal manner around each nitrogen in hydrazine(H,NNH,) H H H Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
The two bond cleavages shown convert camphene to a noncyclic species; therefore, camphene is bicyclic. (Other pairs of bond cleavages are possible and lead to the same conclusion.) 3.13 (b) This bicyclic compound contains nine carbon atoms. The name tells us that there is a fivecarbon bridge and a two-carbon bridge. The 0 in the name bicyclo[5.2.0]nonane tells us that the third bridge has no atoms in it—the carbons are common to both rings and are directly attached to each other. (c) The three bridges in bicyclo[3.1.1]heptane contain three carbons, one carbon, and one carbon. The structure can be written in a form that shows the actual shape of the molecule or one that simply emphasizes its constitution. (d) Bicyclo[3.3.0]octane has two five-membered rings that share a common side. 3.14 Since the two conformations are of approximately equal stability when R H, it is reasonable to expect that the most stable conformation when R CH3 will have the CH3 group equatorial. 3.15 (a) Recall that a neutral nitrogen atom has three covalent bonds and an unshared electron pair. The three bonds are arranged in a trigonal pyramidal manner around each nitrogen in hydrazine (H2NNH2). H N N H H H H H H H H H H H N H N H H H R H: both conformations similar in energy R CH3: most stable conformation has CH3 equatorial R N N R Three-carbon bridge Three-carbon bridge One-carbon bridge One-carbon bridge Three-carbon bridge Bicyclo[5.2.0]nonane CH2 CH3 CH3 CH2 CH3 CH3 CH2 CH3 CH3 50 CONFORMATIONS OF ALKANES AND CYCLOALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONFORMATIONS OF ALKANES AND CYCLOALKANES 51 (b) The o-H proton may be anti to one N-H proton and gauche to the other(left)or it may gauche to both(right) H H 3.16 Conformation (a) most stable; all its bonds are staggered. Conformation(c)is the least stable all its bonds are eclipsed 3. 17 (a) First write out the structural formula of 2, 2-dimethylbutane in order to identify the substituent groups attached to C-2 and C-3. As shown at left, C-2 bears three methyl groups, and C-3 bears two hydrogens and a methyl group. The most stable conformation is the staggered one shown at right. All other staggered conformations are equivalent to this one Sight along this bond CH/H CH H 3C CH H (b) The constitution of 2-methylbutane and its two most stable conformations are shown. Sight along this bond. C-C -CH H H CH Both conformations are staggered. In one(left), the methyl group at C-3 is gauche to both of the C-2 methyls. In the other (right), the methyl group at C-3 is gauche to one of the C-2 methyls and anti to the other. (c) The hydrogens at C-2 and C-3 may be gauche to one another(left), or they may be anti (right). CH H3 HC H HC-C-C--CH3 CH Sight along this bond 3.18 The 2-methylbutane conformation with one gauche CH3... CH3 and one anti CH3.CH3 rela- tionship is more stable than the one with two gauche CH3..CH3 relationships. The more stable conformation has less van der waals strain CH H3C. HH C H More stable Less stable Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) The OGH proton may be anti to one NGH proton and gauche to the other (left) or it may be gauche to both (right). 3.16 Conformation (a) is the most stable; all its bonds are staggered. Conformation (c) is the least stable; all its bonds are eclipsed. 3.17 (a) First write out the structural formula of 2,2-dimethylbutane in order to identify the substituent groups attached to C-2 and C-3. As shown at left, C-2 bears three methyl groups, and C-3 bears two hydrogens and a methyl group. The most stable conformation is the staggered one shown at right. All other staggered conformations are equivalent to this one. (b) The constitution of 2-methylbutane and its two most stable conformations are shown. Both conformations are staggered. In one (left), the methyl group at C-3 is gauche to both of the C-2 methyls. In the other (right), the methyl group at C-3 is gauche to one of the C-2 methyls and anti to the other. (c) The hydrogens at C-2 and C-3 may be gauche to one another (left), or they may be anti (right). 3.18 The 2-methylbutane conformation with one gauche CH3 . . . CH3 and one anti CH3 . . . CH3 relationship is more stable than the one with two gauche CH3 . . . CH3 relationships. The more stable conformation has less van der Waals strain. CH3 H3C CH3 H H H CH3 H3C CH3 H H H More stable Less stable CH3 H3C CH3 CH3 H H CH3 H CH3 CH3 H3C H H3C C C CH3 CH3 CH3 H H Sight along this bond. CH3 CH3 H H3C H H CH3 H CH3 H3C H H H3C CH C C 3 H CH3 H H Sight along this bond. CH3 CH3 CH3 H3C H H H3C CH C C 3 CH3 CH3 H H Sight along this bond. N H O H H N H H O H CONFORMATIONS OF ALKANES AND CYCLOALKANES 51 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
52 CONFORMATIONS OF ALKANES AND CYCLOALKANES 3. 19 All the staggered conformations about the C-2--C-3 bond of 2, 2-dimethylpropane are equivalent to one another and of equal energy; they represent potential energy minima. All the eclipsed confor mations are equivalent and represent potential energy maxima. CH3 HC个CH H CH H H一H CH CH H3C TCH H,CCH, H,CTCH3 H H H Torsion angle(degrees) The shape of the potential energy profile for internal rotation in 2, 2-dimethylpropan ne more closely resembles that of ethane than that of butane. 3.20 The potential energy diagram of 2-methylbutane more closely resembles that of butane than that of propane in that the three staggered forms are not all of the same energy. Similarly, not all of the eclipsed forms are of equal energy H HC一CH3H3CH HCH H-h H-TH H H C-H H H3 H3 120180240300360 Torsion angle(degrees) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
3.19 All the staggered conformations about the C-2GC-3 bond of 2,2-dimethylpropane are equivalent to one another and of equal energy; they represent potential energy minima. All the eclipsed conformations are equivalent and represent potential energy maxima. The shape of the potential energy profile for internal rotation in 2,2-dimethylpropane more closely resembles that of ethane than that of butane. 3.20 The potential energy diagram of 2-methylbutane more closely resembles that of butane than that of propane in that the three staggered forms are not all of the same energy. Similarly, not all of the eclipsed forms are of equal energy. Potential energy 0 60 120 180 240 300 360 Torsion angle (degrees) H3C CH3 H H H CH3 H3C H H H H3C CH3 H CH3 H H H3C CH3 CH3 H H H3C H CH3 CH3 H H H3C CH3 H CH3 H H H CH3 CH3 Potential energy 0 60 120 180 240 300 360 Torsion angle (degrees) H H H3C CH3 H CH3 H H H3C CH3 CH3 H H H H3C CH3 CH3 H H H CH3 H3C CH3 H CH3 H3C CH3 H H H CH3 H3C CH3 H H H CH3 H3C CH3 H H H 52 CONFORMATIONS OF ALKANES AND CYCLOALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONFORMATIONS OF ALKANES AND CYCLOALKANES 3.21 Van der Waals strain between the tert-butyl groups in 2, 2, 4, 4-tetramethylpentane causes the C-2--C-3--C-4 angle to open to 125-128 This angle is CH3 CH H, C-C--CH2-C--CH, is equivalent to CH3 3.22 The structure shown in the text is not the most stable conformation because the bonds of the group are eclipsed with those of the ring carbon to which it is attached. The most stable con tion has the bonds of the methyl group and its attached carbon in a staggered relationship H Bonds of methyl group eclipsed Bonds of methyl group staggered with those of attached carbon with those of attached carbon 3.23 Structure A has the hydrogens of its methyl group eclipsed with the ring bonds and is less stable than B. The methyl group in structure B has its bonds and those of its attached ring carbon in a stag gered relationship H心H H Furthermore, two of the hydrogens of the methyl group of A are uncomfortably close to two axial hydrogens of the ring 3.24 Conformation B is more stable than A. The methyl groups are rather close together in A, resulting in van der Waals strain between them. In B, the methyl groups are farther apart Van der Waals strain between cis methyl groups. Methyl groups remain cis, but are far apart H3 H H 3.25 (a) By rewriting the structures in a form that shows the order of their atomic connections, it is apparent that the two structures are constitutional isomers H H is equivalent to CH_ CCH H C (2, 2-Dimethylpropane CH is equivalent to CH CH, CHCH3 H3 2-Methylbutane) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
3.21 Van der Waals strain between the tert-butyl groups in 2,2,4,4-tetramethylpentane causes the C-2GC-3GC-4 angle to open to 125–128°. 3.22 The structure shown in the text is not the most stable conformation, because the bonds of the methyl group are eclipsed with those of the ring carbon to which it is attached. The most stable conformation has the bonds of the methyl group and its attached carbon in a staggered relationship. 3.23 Structure A has the hydrogens of its methyl group eclipsed with the ring bonds and is less stable than B. The methyl group in structure B has its bonds and those of its attached ring carbon in a staggered relationship. Furthermore, two of the hydrogens of the methyl group of A are uncomfortably close to two axial hydrogens of the ring. 3.24 Conformation B is more stable than A. The methyl groups are rather close together in A, resulting in van der Waals strain between them. In B, the methyl groups are farther apart. 3.25 (a) By rewriting the structures in a form that shows the order of their atomic connections, it is apparent that the two structures are constitutional isomers. CH3 H3C CH3 H H H is equivalent to CH3CCH3 CH3 CH3 (2,2-Dimethylpropane) CH3 CH3 H3C H H H is equivalent to CH3CH2CHCH3 CH3 (2-Methylbutane) H H CH3 CH3 A Van der Waals strain between cis methyl groups. H H H3C CH3 B Methyl groups remain cis, but are far apart. A (less stable) B (more stable) H H H H H H H H H H Bonds of methyl group eclipsed with those of attached carbon Bonds of methyl group staggered with those of attached carbon H H H H H H H H CH is equivalent to H3C C C 2 CH3 CH3 CH3 CH3 CH3 This angle is enlarged. CONFORMATIONS OF ALKANES AND CYCLOALKANES 53 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
54 CONFORMATIONS OF ALKANES AND CYCLOALKANES (b) Both models represent alkanes of molecular formula C6Hig In each one the carbon chain is unbranched. The two models are different conformations of the same compound CH CH, CH,CH,CH, CH,(hexane (c) The two compounds have the same constitution; both are( CH3)2CHCH(CH3)2. The Newman ojections represent different staggered conformations of the same molecule: in one the hydrogens are anti to each other, whereas in the other they are gauche. H3C. CH H3C H are different conformations of H3C H 2, 3-dimethylbutane Hydrogens at C-2 and and C-3 are anti (d) The compounds differ in the order in which the atoms are connected. They are constitutional isomers. Although the compounds have different stereochemistry(one is cis, the other trans), they are not stereoisomers. Stereoisomers must have the same constitution CH cis-1, 2-Dimethylcyclopentane trans-1, 3-Dimethylcyclopentane (e) Both structures are cis-l-ethyl-4-methylcyclohexane(the methyl and ethyl groups are both up"). In the structure on the left, the methyl is axial and the ethyl equatorial. The orientations are opposite to these in the structure on the right. The two structures are ring-flipped forms of each other--different conformations of the same compound (f) The methyl and the ethyl groups are cis in the first structure but trans in the second. The two compounds are stereoisomers; they have the same constitution but differ in the arrangement of their atoms in space CH CHa CH2 CHCH cis-l-Ethyl-4-methylcyclohexane trans-l-Ethyl-4-methylcyclohexane both alkyl groups are up) (ethyl group is down; methyl group is up) Do not be deceived because the six-membered rings look like ring-flipped forms Remember, chair-chair interconversion converts all the equatorial bonds to axial and vice versa. Here the ethyl group is equatorial in both structures () The two structures have the same constitution but differ in the arrangement of their atoms in space; they are stereoisomers. They are not different conformations of the same compound, because they are not related by rotation about C-C bonds. In the first structure as shown ere the methyl group is trans to the darkened bonds, whereas in the second it is cis to these H Methyl is trans to Methyl is cis to Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) Both models represent alkanes of molecular formula C6H14. In each one the carbon chain is unbranched. The two models are different conformations of the same compound, CH3CH2CH2CH2CH2CH3 (hexane). (c) The two compounds have the same constitution; both are (CH3)2CHCH(CH3)2. The Newman projections represent different staggered conformations of the same molecule: in one the hydrogens are anti to each other, whereas in the other they are gauche. (d) The compounds differ in the order in which the atoms are connected. They are constitutional isomers. Although the compounds have different stereochemistry (one is cis, the other trans), they are not stereoisomers. Stereoisomers must have the same constitution. (e) Both structures are cis-1-ethyl-4-methylcyclohexane (the methyl and ethyl groups are both “up”). In the structure on the left, the methyl is axial and the ethyl equatorial. The orientations are opposite to these in the structure on the right. The two structures are ring-flipped forms of each other—different conformations of the same compound. ( f ) The methyl and the ethyl groups are cis in the first structure but trans in the second. The two compounds are stereoisomers; they have the same constitution but differ in the arrangement of their atoms in space. Do not be deceived because the six-membered rings look like ring-flipped forms. Remember, chair–chair interconversion converts all the equatorial bonds to axial and vice versa. Here the ethyl group is equatorial in both structures. (g) The two structures have the same constitution but differ in the arrangement of their atoms in space; they are stereoisomers. They are not different conformations of the same compound, because they are not related by rotation about CGC bonds. In the first structure as shown here the methyl group is trans to the darkened bonds, whereas in the second it is cis to these bonds. Methyl is trans to these bonds. CH3 H Methyl is cis to these bonds. H CH3 cis-1-Ethyl-4-methylcyclohexane (both alkyl groups are up) trans-1-Ethyl-4-methylcyclohexane (ethyl group is down; methyl group is up) CH3 CH3CH2 CH3 CH3CH2 CH3 CH3 CH3 CH3 cis-1,2-Dimethylcyclopentane trans-1,3-Dimethylcyclopentane and are different conformations of 2,3-dimethylbutane Hydrogens at C-2 and C-3 are gauche. Hydrogens at C-2 and C-3 are anti. CH3 CH3 H H H3C H3C CH3 CH3 H H3C H H3C 54 CONFORMATIONS OF ALKANES AND CYCLOALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CONFORMATIONS OF ALKANES AND CYCLOALKANES 55 3.26(a) Three isomers of Cs H contain two rings and have no alkyl substituents: (b) Five isomers of C6Hlo contain two rings and have no alkyl substituents Spirohexane Bicyclo[2.2. 0Jhexane Bicyclo[3. 1.0]hexane Bicyclo[2. 1. 1]hexane Cyclopropylcyclopropane 3.27 (a) The heat of combustion is highest for the hydrocarbon with the greatest number of carbons Thus, cyclopropane, even though it is more strained than cyclobutane or cyclopentane, has the lowest heat of combustion Cyclopentane Heat of combustion 3291 kJ/mol Cyclobutane Heat of combustion 2721 kJ/mol (650.3 kcal/mol) △ Cycloprop Heat of combustion 2091 kJ/mol (499.8 kcal/mol) A comparison of heats of combustion can only be used to assess relative stability when the ompounds are isomers. (b) All these compounds have the molecular formula CHi. They are isomers, and so the one with the most strain will have the highest heat of combustion 1, 1, 2, 2-Tetramethylcyclopropane Heat of combustion (high in angle strain; bonds are 4635 kJ/mol eclipsed; van der Waals strain (1107.9 kcal/mol) between cis methyl groups) cis-1, 2-Dimethylcyclopentane Cow angle strain; some torsional 4590 kJ/mol strain van der waals strain (1097. 1 kcal/mol) CH between cis methyl groups) Methylcyclohexane Heat of combustion CHs(minimal angle, torsional, and 4565 kJ/mol van der Waals strain) (1091. 1 kcal/mol) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
3.26 (a) Three isomers of C5H8 contain two rings and have no alkyl substituents: (b) Five isomers of C6H10 contain two rings and have no alkyl substituents: 3.27 (a) The heat of combustion is highest for the hydrocarbon with the greatest number of carbons. Thus, cyclopropane, even though it is more strained than cyclobutane or cyclopentane, has the lowest heat of combustion. A comparison of heats of combustion can only be used to assess relative stability when the compounds are isomers. (b) All these compounds have the molecular formula C7H14. They are isomers, and so the one with the most strain will have the highest heat of combustion. H3C CH3 H3C CH3 1,1,2,2-Tetramethylcyclopropane (high in angle strain; bonds are eclipsed; van der Waals strain between cis methyl groups) Heat of combustion 4635 kJ/mol (1107.9 kcal/mol) H3C CH3 H H cis-1,2-Dimethylcyclopentane (low angle strain; some torsional strain; van der Waals strain between cis methyl groups) Heat of combustion 4590 kJ/mol (1097.1 kcal/mol) Methylcyclohexane (minimal angle, torsional, and van der Waals strain) Heat of combustion 4565 kJ/mol (1091.1 kcal/mol) CH3 Cyclopentane Heat of combustion 3291 kJ/mol (786.6 kcal/mol) Cyclobutane Heat of combustion 2721 kJ/mol (650.3 kcal/mol) Cyclopropane Heat of combustion 2091 kJ/mol (499.8 kcal/mol) Spirohexane Bicyclo[2.2.0]hexane Bicyclo[2.1.1]hexane Cyclopropylcyclopropane Bicyclo[3.1.0]hexane Spiropentane Bicyclo[2.1.0]pentane Bicyclo[1.1.1]pentane CONFORMATIONS OF ALKANES AND CYCLOALKANES 55 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
