Copy of Organic Chemistry Jonh Mc Murry5

Copy of Organic Chemistry Jonh Mc Murry5 - 3 C we

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Unformatted text preview: 3 C we cis-1,2-Dimethylcyclopropane 4.2 Dis—Trans lsomerism in Cycloalkanes 111 Cyclopropane, for example, must be a rigid, planar molecule because three points (the carbon atoms) define a plane. No bond rotation can take place around a cyclopropane carbon—carbon bond without breaking open the ring (Figure 4.1). lbl Figure 4.1 (a) Rotation occurs around the carbon—carbon bond in ethane, but lb) no rotation is possible around the carbon—carbon bonds in cyclopropane Without breaking open the ring. Larger cycloalkanes have increasing rotational freedom, and the very large rings (C25 and up) are so floppy that they are nearly indistinguishable from open-chain alkanes. The common ring sizes (C37C7), however, are severely restricted in their molecular motions. Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a “bottom” face. AS a result, isomerism is possible in substituted cydoalkanes. For example, there are two different 1,2-dimethyl— cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the rnethyls on opposite faces (Figure 4.2). Both isomers are sta— ble compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. H We H trans—1,Z-Dimethylcyclopropane Figure 4.2 There are two different l,2«dimethylcyclopropane isomers, one with the methyl groups on the same face of the ring (cisl and the other with the methyl groups on opposite faces of the ring (trans). The two isomers do not interconvert. Unlike the constitutional isomers butane and isobutane (Section 3.2), which have their atoms connected in a different order, the two 1,2-dlmethyl- cyclopropanes have the same order of connections but differ in the spatial ori- entation of the atoms. Such compounds, which have their atoms connected in the same order but differ in three-din'iensional orientation, are called stereo— chemical isomers, or stereoisoniers. Constitutional lson'lers CH», (different connections I 7 between atoms) CH3*CH*CH3 and CH3—CH2—CH2—Cl l3 Stereoismners lsame connections ‘ a 1d d . 7 but different three- >V< I dimensional geometry) H H H 112 CHAPTER 4 Organic Compounds: Cycloalkanes and Their Srereochemistry weaken-EXAMPLE 4:1 Thomson Click Organic Interactive to learn to write lUPAC names for simple cycloalkanes. Thomson Click Organic Interactive to use an online palette to draw cycloalkane structures from their IUPAC names. Strategy Solution Problem 4.4 Problem 4.5 Problem 4.6 The 1,2-dimethylcyclopropanes are members of a subclass of stereoisomers called cis—trans isomers The prefixes cis- (Latin "on the same side") and trans- (Latin "across") are used to distinguish between them. Cisetrans isomerism is a common occurrence in substituted cycloalkanes. H3C 3 EH? t; r r. 'i /H H A H H 7 tiller-a; cis-1,3-Dimethylcyclobutane ire-ris-1-Bromo-3-ethylcyclopentane Naming Cycloalkanes Name the following substances, including the cis- or trans— prefix: (a) H CH3 [b) H \ 1,43! H3O” \‘H l t‘cr H In these views, the ring is roughly in the plane of the page, a wedged boncl protrudes out of the page, and a dashed bond recedes into the page. Two Stibstituents are cis if they are both out ofor both into the page, and they are trans itone is out ofand one is into. (a) tram- i ,B—Di methylcyclopentane (b) ris—1,2-Dichlorocyclohexane Name the following substances, including the cis- 0r DHHS- prefix: (a) H (b) H3C CHZCHg Draw the structures Of the following molecules: (3) trans-1-Btomo~3vmethylcyclohexane (b) ci5-1,2-Dimethylcyclobutane (c) trnns~1-tert—Butyl-Z-ethylcyclohexane Prostaglandin F3“, a hormone that causes uterine contraction during childbirth, has the following structure. A re the two hydroxyl groups (—OH) on the cyclopentane ring cis or trans to each other? What about the two carbon chains attached to the ting? Prostaglandin F2“ CH3 Problem 4.7 Adolf von Baeyer “835—19?” was born in Berlin, Germany, and received his PhD, atthe Univer- sity of Berlin in l858, working with Robert Bunsen and August Kekulé. Alter holding positions at Berlin and Strasbourg, he was a professor at Munich from 1875 to 1917. He was the first to synthee size the blue dye indigo and was also discoverer of the first barbi~ turare sedative. which he named after his friend Barbara. Baeyer was awarded the Nobel Prize in chemistry in i805. dil' heir-g 'lt'll"-El." Cyclopropane 4.3 Stability onycloalkanes:Ring Strain 113 Name the following substances, including the cis- or tnms- prefix (red-brown = Br): (3) (bl Stability of Cyclualkanes: Ring Strain Chemists in the late 1800s knew that cyclic molecules existed, but the limita- tions on ring size were unclear. Although numerous compounds containing five- and six-membered rings were known, smaller and larger ring sizes had not been prepared, despite many efforts. A theoretical interpretation of this observation was proposed in 1885 by Adolf Von Baeyer, who suggested that small and large rings might be unstable due to angle strainithe strain induced in a molecule when bond angles are forced to deviate from the ideal 109° tetrahedral value. Baeyer based his sugges- tion on the simple geometric notion that a three-mem bered ring (cyclopropane) should be an equilateral triangle with bond angles of 60° rather than 109°, a tour-membered ring (cyclobutane) should be a square with bond angles of 90°, a five-membered ring should be a regular pentagon with bond angles of 108°, and so on. Continuing this argument, large rings should be strained by having bond angles that are much greater than 109°. Cyclobutane Cyclopentane Cyclohexane What are the facts? To meaSure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest way to do this for a cycloalkane is to measure its heat ofcombustion, the amount of heat released when the compound burns completely with oxygen. The more energy (strain) the compound contains, the more energy (heat) is released on combustion. 4) (CHZJH + 311/2 02 i1 C02 -l* n HZO + Heat 114 CHAPTER 4 Organic Compounds: Cycloalkanes and Their Stereochemistry Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CHZ and acyclic alkane heat of combus- tion per CH2, and multiplying by the number ofCl-lz units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free. Problem 4.8 Problem 4.9 Because the heat of combustion of a cycloalkane depends on size, we need to look at heats of combustion per CH2 unit. Subtracting a reference value derived from a strain-free acyclic alltane and then multiplying by the number of CH2 units in the ring gives the overall strain energy. Figure 4.3 shows the results. 120-l ’7; 100 -'~23.9 E 2 80-- 49.1 A ; E E“ 60 143 5 m (U C 8 d) a. : E 40 9.6 ‘43 O O 5 20— : - 4.8 O l "—__—“—l-“—Iu I i i l O 3 4 S 6 7 8 9 10 11 12 13 14 Ring size The data in Figure 4.3 show that Baeyer’s theory is only partially correct. Cyclopropane and cyclobutane are indeed strained, just as predicted, but cyclopentane is more strained than predicted, and cyclohexane is strain-free. Cycloalkanes of intermediate size have only modest strain, and rings of 14 car- bons or more are strain-tree. Why is Baeyer’s theory wrong? Baeyer's theory is wrong for the simple reason that he assumed all cyclo- alkanes to be flat. In fact, as we’ll see shortly, most cycloalkanes are not flat; they adopt puckered three-dimensional conformations that allow bond angles to be nearly tetrahedral. As a result, angle strain occurs only in three- and four- membered rings that have little flexibility. For most ring sizes, particularly the medium-ring (C7—C11)cycloalkanes, torsional strain caused by tie—9H eclipsing interactions on adjacent carbons (Section 3.6) and steric strain caused by the repulsion between nonbonded atoms that approach too closely (Section 3.7) are the most important factors. Thus, three kinds otstrain contribute to the Overall energy of a cycloalkane. - Jingle strainkthe strain due to expansion or compression of bond angles ‘n—the strain due to eclipsing of bonds on neighboring atoms O .‘sit‘r‘icg strain—the strain due to repulsive interactions when atoms approach each other too closely Each H <—> H eclipsing interaction in ethane costs about 4.0 kJ/mol. How many such interactions are present in cyclopropane? What traction ot‘ the overall 115 kjr‘nioi (27.5 kcal/niol) strain energy of cyclopropane is due to torsional strain? cis-l,Z-Dimethylcyclopropane has more strain than trans-l ,Z-dimethylcyclo- propane. How can you account for this difference? Which of the two compounds is more stable? 4.4 Figure 4.4 The structure of cyclopropane, showing the eclipsing of neighboring CiH bonds that gives rise to torsional strain. Part (bl is a Newman prof jection along a C~C bond. 4.4 Conformations of Cycloalkanes 115 Conlormations of Cyclualkanes Cyclopropane Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60° (.Z—C—C bond angles, In addition, cyclopropane also has considerable torsional strain because the Cell bonds on neighboring carb0n atoms are eclipsed (Figure 4.4). (a) lb) H H E .I- l ‘ ‘1 \ H LL. |lt.:..| e — / in. ' H H J Eclipsetl How can the hybrid-orbital model of bonding account for the large distor- tion of bond angles from the normal 109° tetrahedral value to 60° in cyclo- propane.7 The answer is that cyclopropane has bent bonds. In an unstrained alkane, maximum bonding is achieved when two atoms have their overlapping orbitals pointingdirectly toward each other. in cyclopropane. though, the orbitals can't point directly toward each other; rather, they overlap at an angle. The result is that cyclopropane bonds are weaker and more reactive than typical alkarie bonds—255 kJ/mol (61 kcal/mol) tor a CfC bond in cyclopropane versus 355 kJ/mol (85 kcal/moll fora CeC bond in open-chain propane. x... 7 we ' /"' C | | l | | | Typical alkane C—C bonds Typical bent cyclopropane C—C bonds Cycluhutane Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. As a result/ the total strain for the two compounds is nearly the same—110 kj/mol (26.4 kcal/mol) for cyclobutane versus 115 til/moi (27.5 kcal/mol) for cyclopropane. Experiments shOW that cyclobutane is not quite flat but is slightly bent so that one carbon atom lies about 25” above the plane otthe other three (Figure 4.5). The effect of 115 CHAPTER 4 Organic Compounds: Cycloalkanes and Their Stereochemistry la} (a) Problem 4.10 this slight bend is to increase angle strain but to decrease torsional strain, until a minimum-energy balance between the two opposing effects is achieved. lb) lcl Hot (ltrrii-t izizlipserr / 'H H \ i W .l-l L //\H 33 \ H -i Not ti trite R E-ICiilJ'riii'ii Figure 4.5 The conformation of oyclobutane. Part la) is a Newman projection along the C1—C2 bond. showing that neighboring C—H bonds are not quite eclipsed. Cyclopentane Cyclopentane was predicted by Baeyer to be nearly strain-free but in fact has a total strain energy of 26 kJ/mol (6.2 kcal/mol). Although planar cyclopentane has practically no angle strain, it has a large amount of torsional strain. Cyclopentane therefore twists to adopt a puckered, nonplanar conformation that strikes a balance between increased angle strain and decreased torsional strain. Four of the cyclopentane carbon atoms are in approximately the same plane, with the fifth carbon atom bent out of the plane. Most of the hydrogcns are nearly staggered with respect to their neighbors (Figure 4.6). lb) Observer Figure 4.6 The conformation of cyclopentane. Carbons 1. 2, 3, and 4 are nearly planar, but carbon 5 is out of the plane. Part la) is a Newman projection along the C1702 bond. showing that neighboring C—H bonds are nearly staggered, How many H H H eclipsing interactions would be present itcyclopentane were pia- nar? Assuming an energy cost of 4.0 kJ/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have? Since the measured total strain of cyclopentane is 26 kj/mol, how much of the torsional strain is relieved by puckerin g7 4.5 Con‘formationsoleclohexane 11‘] Problem 4.11 iwo conformations of (is-lI3-dimethylcyclobutane are shown. What is the differ- ence between them, and which do you think is likely to be more stable? la) I . (bi 4.5 Coniormations of Cyclnhexane Substituted cyciohexanes are the most common cycloalkanes and occur wider in nature. A large number of compounds, including steroids and many phar- maceutical agents, have cyclohexane rings. The flavoring agent menthol, for instance, has three substituents on a six—membered ring. D 7 x, , e5 H ,i I HEC/Ctl H R CH3 “I; _ " Menthol Cyclohexane adopts a strain-tree, three-dimensional shape, called a chair confermation because of its similarity to a lounge chair/ with a back, a seat, and a footrest (Figure 4.7). Chair cyclohexane has neither angle strain nor torsional strain—all CvCeC bond angles are near 109°, and all neighboring C—H bonds are staggered. (ai " (bi H r H i i Observer Figure 4.7 The strainefree chair conformation of cyclohexane. All CiC—C bond angles are 111.5“, close to the ideal 1095“ tetrahedral angle, and all neighboring CfiH bonds are staggered, 118 CHAPTER 4 Organic Compounds: Cycloalkanes and Their Stereochemistry Step 1 Step 2 Step 3 The easiest way to visualize chair cyclohexane is to build a molecular model. (In fact, do it now.) Two—dimensional drawings like that in Figure 4.7 are useful, but there’s no substitute for holding. twisting, and turning a three-dimensional model in your own hands. The chair con formation of cyclohexane can be drawn in three steps. 0‘... a“ O o... “9 set from each other. This means that four of the eyclohexane carbons lie in a plane. a\. 7 g '7 Place the topmost carbon atom above and to the right of the plane of the other four, and COnnect the bonds. Draw two parallel lines, slanted downward and slightly off- i r’.\./. / Place the bottommost carbon atom below and to the left of .( .\. the plane of the middle four, and connect the bonds. Note in that the bonds to the bottommost carbon atom are parallel to the bonds to the topmost carbon. /" a o‘ When viewing cyclohexan e, it’s helpful to remember that the lower bond is in front and the upper bond is in back. if this convention is not defined, an opti- cal illusion can make it appear that the reverse is true. For clarity, all cyclo- hexane rings drawn in this book will have the front (lower) bond heavily shaded to indicate nearness to the viewer. “ This brand is m liiiizls. llIIF l .‘r‘u'l is iiiji ' In addition to the chair conformation of cyclohexane, a second arrange- ment called the twisbboat conformation is also nearly free of angle strain. it does, however, have both steric strain and torsional strain and is about 23 kJ/mol (5.5 kcal/mol) higher in energy than the chair conformation. As a result, molecules adopt the twist-boat geometry only under special circumstances. Steric Strain l-l Hii H : Torsional strain Twist-boat cyclohexane " ** H i (23 kJ/mol strain) 4.6 Cyclohexane (chair conformation) Figure 4.8 Axial (red) and equatorial (bluel positions in Chair cyclohexane. The six axial lwdrogens are parallei to the ring axis, and the six equatorial lwclrogens are in a band around the ring equator. 4.6 Axial and Equatorial Bonds in Cyclohexane 119 Axial and Equatorial Bonds in Cyclohexane 'l'he chair conformation of cyclohexane has many consequences. We’ll see in Section 1i.9, for instance, that the chemical behavior of many substituted cyclohexanes is inuenced by their conformation. In addition, we’ll see in Sec- tion 25.5 that simple carbohydrates such as glucose adopt a conformation based on the cyclohexane Chair and that their chemistry is directly affected as a result. L ,.@=~. Glucose (chair conformation] Another consequence of the chair conformation is that there are two kinds of positions for substituents on the cyclohexane ring: axial positions and aqua— mrial positions (l’igure 4.8). The six axial positions are perpendicular to the ring, parallel to the ring axis, and the six equatorial positions are in the rough plane of the ring, around the ring equator. Fling axis As shown in Figure 4.8, each carbon atom in cyclohexane has one axial and one equatorial hydrogen. Furthermore, each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement. For example, if the top face of the ring has axial hydrogens on carbons 'l, 3, and S, then it has equatorial hydrogens on carbons Z, 4,. and 6. Exactly the reverse is true for the bottom face: carbons 1, 3, and 5 have equatorial hydrogens, but carbons 2, 4. and 6 have axial hydrogens (Figure 4.9}. Note that we. haven't used the words (i5 and trans in this discussion of cyclo hexane contm'mation. Two hydrogens on the same face of the ring are always cis, regardless of whether they're axial or equatorial and regardless of whether they’re adjacent. Similarly, two hydrogens on opposite faces of the ring are always trans. 120 CHAPTEFM Figure 4.9 Alternating axial and equatorial positions in chair cyclohexane. as shown in a view looking directly down the ring axis. Each carbon atom has one axial and one equatorial position, and each face has alternating axial and equatorial positions. Axial bonds: Equatorial bonds; bonds, one on each carbon, come in three sets of two parallel lines. Each set is also parallel to two ring bonds. Equatorial The six axial bonds, one on each carbon, are parallel and alternate upvdown, Organic Compounds: Cyclaalkanes and Their Stereochemistry l'fl!i:lifll'l7£l_ Axial Axial and equatorial bonds can be drawn following the procedure in Figure 410. Look at a molecular model as you practice. The six equatorial bonds alternate between sides around the ring. Completed cyclohexane Figure 4.10 A procedure for drawing axial and equatorial bonds in chair cyclohexane. Because chair cyclohexane has two kinds of positions, axial and equatorial, we might expect to find two isomeric forms of a monOSLibstituted cyclohexane. In fact, we don’t. There is only one methylcyclohexane, one bromocyclohexane, one cyclohexanoi (hydroxycyclohexane), and so on, because cyclohexane rings are cor/for;nationally mobile at room temperature. Different chair conformations readily interconvert, exchanging axial and equatorial positions. This intercon— version, usually called a ring-flip, is shown in Figure 4.11. As shown in Figure 4.11, a chair cycloliexane can be ring-flipped by keep- ing the middle four carbon atoms in place while folding the two end carbons in opposite directions. In so doing, an axial substituent in one chair form becomes an equatorial substituent in the ring-flipped chair form and viceversa. For example, axial bromocyclohexane becomes equatorial bromocyclohexane after ring-flip. Since the energy barrier to chair—chair interconversion is only ...
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Copy of Organic Chemistry Jonh Mc Murry5 - 3 C we

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