8Chapter 03

8Chapter 03 - Chapter 3 Conformations of Conformations...

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Unformatted text preview: Chapter 3 Conformations of Conformations Alkanes and Cycloalkanes and 3-1 3.1 Conformational Analysis of Ethane Conformations are different spatial Conformations are different spatial Conformations Conformations arrangements of a molecule that are arrangements of a molecule that are generated by rotation about single bonds. generated by rotation about single bonds. generated generated 3-2 Ethane Eclipsed conformation Eclipsed conformation 3-3 Ethane Eclipsed conformation Eclipsed conformation 3-4 Ethane Staggered conformation Staggered conformation 3-5 Ethane Staggered conformation Staggered conformation 3-6 Projection formulas of the staggered Projection formulas of the staggered conformation of ethane conformation of ethane H H H H H H H Newman H H H H H Sawhorse 3-7 Anti relationships Anti relationships H H H 180° 180° H H H H H H H H H Two bonds are anti when the angle between them is 180°. Two 3-8 Gauche relationships Gauche relationships H H H 60° 60° H H H H H H H H H Two bonds are gauche when the angle between them is 60°. Two 3-9 An important point: An important point: The terms anti and gauche apply The terms anti and gauche apply oonlyto bonds (or groups) on adjacent nly only to bonds (or groups) onaadjacent djacent adjacent only carbons, ,aandonly to staggered nd taggered carbons and only tosstaggered and staggered conformations. conformations. 3-10 12 kJ/mol 0° 60° 120° 180° 240° 300° 360° 3-11 Torsional strain Torsional strain •• The eclipsed conformation of ethane is 12 kJ/mol The eclipsed conformation of ethane is 12 kJ/mol less stable (higher energy) than the staggered. less stable (higher energy) than the staggered. •• The eclipsed conformation is destabilized by The eclipsed conformation is destabilized by torsional strain. torsional strain. •• Torsional strain is the destabilization that results Torsional strain is the destabilization that results from eclipsed bonds. from eclipsed bonds. 3-12 3.2 Conformational Analysis of Butane 3-13 Conformational Analysis of Butane: C2-C3 Rotation 3-14 14 kJ/mol 3 kJ/mol 0° 60° 120° 180° 240° 300° 360° 3-15 van der Waals strain van der Waals strain gauche anti •• The gauche cconformationof butane is 33kJ/mol The ggauche onformation of butane is kJ/mol The gauche The auche lless stable than the anti. . ess stable than theaanti nti lless ess anti •• The gauche cconformationis destabilized by The ggauche onformation is destabilized by The gauche The auche vvander Waals strain (also called steric strain) an van der Waals strain (also called steric strain) van which results from atoms being too close together. which results from atoms being too close together. which which 3-16 van der Waals strain van der Waals strain eclipsed •• The conformation of butane in which the two The conformation of butane in which the two methyl groups are eclipsed with each other is methyl groups are eclipsed with each other is is the least stable of all the conformations. is the least stable of all the conformations. •• ItItis destabilized by both torsional strain is destabilized by both torsional strain (eclipsed bonds) and van der Waals strain. (eclipsed bonds) and van der Waals strain. 3-17 3.3 Conformational Analysis of Conformational Higher Alkanes Higher 3-18 The most stable conformation of unbranched The most stable conformation of unbranched alkanes has anti relationships between carbons alkanes has anti relationships between carbons Hexane 3-19 3.4 The Shapes of Cycloalkanes: Planar or Nonplanar? 3-20 Adolf von Baeyer (19th century) Adolf von Baeyer (19th century) • assumed cycloalkanes are planar polygons • distortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane • Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor. • But Baeyer was incorrect in his belief that But cycloalkanes were planar. cycloalkanes 3-21 Types of Strain ••Torsional strain Torsional strain strain that results from eclipsed bonds strain that results from eclipsed bonds ••van der Waals strain (steric strain) van der Waals strain (steric strain) strain that results from atoms being too close strain that results from atoms being too close together together ••angle strain angle strain strain that results from distortion of bond strain that results from distortion of bond angles from normal values angles from normal values 3-22 Measuring Strain in Cycloalkanes Measuring Strain in Cycloalkanes Measuring Measuring • Heats of combustion can be used to compare stabilities of isomers. • But cyclopropane, cyclobutane, etc. are not But isomers. isomers. • All heats of combustion increase as the number of carbon atoms increase. 3-23 Measuring Strain in Cycloalkanes Measuring Strain in Cycloalkanes Measuring Measuring • Therefore, divide heats of combustion by number of carbons and compare heats of combustion on a "per CH2 group" basis. on 3-24 Heats of Combustion of Cycloalkanes Heats of Combustion of Cycloalkanes Cycloalkane kJ/mol Per CH2 Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Cyclononane Cyclodecane 2,091 2,721 3,291 3,920 4,599 5,267 5,933 6,587 697 681 658 653 657 658 659 659 3-25 Heats of Combustion of Cycloalkanes Heats of Combustion of Cycloalkanes Cycloalkane kJ/mol Per CH2 According to Baeyer, cyclopentane should have less angle strain than cyclohexane. Cyclopentane 3,291 658 Cyclohexane 3,920 653 The heat of combustion per CH2 group is lless for cyclohexane than for ess cyclopentane. cyclopentane. Therefore, cyclohexane has less strain Therefore, than than 3-26 Adolf von Baeyer (19th century) Adolf von Baeyer (19th century) • assumed cycloalkanes are planar polygons • distortion of bond angles from 109.5° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane • Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor. • But Baeyer was incorrect in his belief that But cycloalkanes were planar. cycloalkanes 3-27 3.5 Small Rings Cyclopropane Cyclopropane Cyclobutane 3-28 Cyclopropane Cyclopropane sources of strain: torsional strain angle strain 3-29 Cyclobutane Cyclobutane nonplanar conformation relieves some torsional strain angle strain present 3-30 3.6 Cyclopentane 3-31 Cyclopentane Cyclopentane all bonds are eclipsed planar conformation destabilized by torsional strain 3-32 Nonplanar conformations of cyclopentane Nonplanar conformations of cyclopentane Envelope Half-chair Relieve some, but not all, of the torsional strain. Envelope and half-chair are of similar stability and interconvert rapidly. 3-33 3.7 Conformations of Cyclohexane heat of combustion suggests that angle strain is unimportant in cyclohexane tetrahedral bond angles require tetrahedral nonplanar geometries nonplanar 3-34 Chair is the most stable conformation of cyclohexane Chair is the most stable conformation of cyclohexane All of the bonds are staggered and the bond All angles at carbon are close to tetrahedral. angles 3-35 Boat conformation is less stable than the chair Boat conformation is less stable than the chair 180 pm All of the bond angles are close to tetrahedral but close contact between flagpole hydrogens causes van der Waals strain in boat. 3-36 Boat conformation is less stable than the chair Boat conformation is less stable than the chair Eclipsed bonds bonds gives torsional strain to boat. 3-37 Skew boat is slightly more stable than boat Skew boat is slightly more stable than boat Boat Skew boat Less van der Waals strain and less torsional Less strain in skew boat. strain 3-38 The chair conformation of cyclohexane is the The chair conformation of cyclohexane is the most stable conformation and derivatives most stable conformation and derivatives oofcyclohexane almost always exist in the f off cyclohexane almost always exist in the o chair conformation cchair conformation hair chair 3-39 3.8 Axial and Equatorial Bonds in Cyclohexane 3-40 The 12 bonds to the ring can be divided into The 12 bonds to the ring can be divided into two sets of 6. two sets of 6. 3-41 6 bonds are axial 6 bonds are axial Axial bonds point "north and south" 3-42 6 bonds are equatorial 6 bonds are equatorial Equatorial bonds lie along the equator 3-43 3.9 Conformational Inversion Conformational (Ring-Flipping) in Cyclohexane (Ring-Flipping) 3-44 Conformational Inversion Conformational Inversion chair-chair interconversion (ring-flipping) rapid process (activation energy = 45 kJ/mol) all axial bonds become equatorial and vice all versa versa 3-45 3-46 Halfchair 3-47 Halfchair Skew boat 3-48 Halfchair Skew boat 3-49 Halfchair Skew boat 3-50 45 kJ/mol 45 kJ/mol 23 kJ/mol 3-51 3.10 Conformational Analysis of Monosubstituted Cyclohexanes most stable conformation is chair substituent is more stable when substituent equatorial equatorial 3-52 Methylcyclohexane Methylcyclohexane CH3 CH CH3 5% 95% Chair chair interconversion occurs, but at any instant Chair 95% of the molecules have their methyl group equatorial. equatorial. Axial methyl group is more crowded than an Axial equatorial one. equatorial 3-53 Methylcyclohexane Methylcyclohexane 5% 95% Source of crowding is close approach to axial Source hydrogens on same side of ring. Crowding is called a "1,3-diaxial repulsion" and is a type of van der Waals strain. and 3-54 Fluorocyclohexane Fluorocyclohexane F F 40% 60% Crowding is less pronounced with a "small" Crowding substituent such as fluorine. substituent Size of substituent is related to its branching. 3-55 tert-Butylcyclohexane tert-Butylcyclohexane C(CH3)3 C(CH C(CH3)3 Less than 0.01% Greater than 99.99% Crowding is more pronounced with a "bulky" Crowding substituent such as tert-butyl. tert tert-Butyl is highly branched. 3-56 tert-Butylcyclohexane tert-Butylcyclohexane van der Waals strain due to 1,3-diaxial repulsions 3-57 3.11 Disubstituted Cycloalkanes: Stereoisomers Stereoisomers are isomers that have Stereoisomers same constitution but different arrangement of atoms in space arrangement 3-58 Isomers Isomers Isomers Constitutional isomers Constitutional isomers Constitutional Stereoisomers Stereoisomers 3-59 1,2-Dimethylcyclopropane 1,2-Dimethylcyclopropane There are two stereoisomers of There 1,2-dimethylcyclopropane. 1,2-dimethylcyclopropane. They differ in spatial arrangement of atoms. 3-60 1,2-Dimethylcyclopropane 1,2-Dimethylcyclopropane cis-1,2-Dimethylcyclopropane has methyl groups on same side of ring. on trans-1,2-Dimethylcyclopropane has methyl groups trans on opposite sides. 3-61 Relative stabilities of stereoisomers may be Relative stabilities of stereoisomers may be determined from heats of combustion. determined from heats of combustion. 3-62 van der Waals strain makes cis stereoisomer less stable than trans 3371 kJ/mol 3366 kJ/mol 3-63 3.12 Conformational Analysis of Disubstituted Cyclohexanes 3-64 1,4-Dimethylcyclohexane stereoisomers 1,4-Dimethylcyclohexane stereoisomers CH3 H3C H H cis H H3C CH3 H trans 5219 kJ/mol 5212 kJ/mol less stable more stable Trans stereoisomer is more stable than cis, but Trans methyl groups are too far apart to crowd each other. other. 3-65 Conformational analysis of H3C Conformational cis-1,4-dimethylcyclohexane cisH CH3 H CH3 CH3 H H CH3 H3C H H Two equivalent conformations; each has one axial methyl group and one equatorial methyl group methyl 3-66 Conformational analysis of Conformational trans-1,4-dimethylcyclohexane trans- H H3C CH3 H H CH3 H H3C CH3 H H3C H Two conformations are not equivalent; most stable Two conformation has both methyl groups equatorial. 3-67 1,2-Dimethylcyclohexane stereoisomers 1,2-Dimethylcyclohexane stereoisomers CH3 CH3 H H H CH3 cis H3C H trans 5223 kJ/mol 5217 kJ/mol less stable more stable Analogous to 1,4 in that trans is more stable than cis. 3-68 CH3 Conformational analysis of Conformational cis-1,2-dimethylcyclohexane cis- H H CH3 CH3 CH3 H CH3 H CH3 H H Two equivalent conformations; each has one axial Two methyl group and one equatorial methyl group methyl 3-69 CH3 Conformational analysis of Conformational trans-1,2-dimethylcyclohexane trans- H H3C CH3 H H H CH3 H CH3 H3C H Two conformations are not equivalent; most stable Two conformation has both methyl groups equatorial. 3-70 1,3-Dimethylcyclohexane stereoisomers 1,3-Dimethylcyclohexane stereoisomers CH3 H H H3C CH3 H H3C H cis 5212 kJ/mol more stable trans 5219 kJ/mol less stable Unlike 1,2 and 1,4; cis-1,3 is more stable than Unlike trans. trans. 3-71 Conformational analysis of Conformational cis-1,3-dimethylcyclohexane cis- CH3 CH H H H3C CH3 CH3 H H CH3 H3C H H Two conformations are not equivalent; most stable conformation has both methyl groups equatorial. 3-72 CH3 Conformational analysis of Conformational trans-1,3-dimethylcyclohexane transH3C H H H CH3 H H3C H CH3 H3C H Two equivalent conformations; each has one axial Two and one equatorial methyl group. 3-73 Table 3.2 Heats of Combustion of Table Isomeric Dimethylcyclohexanes Isomeric Compound Orientation -∆ H° cis-1,2-dimethyl trans-1,2-dimethyl ax-eq eq-eq 5223 5217* cis-1,3-dimethyl trans-1,3-dimethyl eq-eq ax-eq 5212* 5219 cis-1,4-dimethyl trans-1,4-dimethyl ax-eq eq-eq 5219 5212* *more stable stereoisomer of pair 3-74 3.13 Medium and Large Rings 3-75 Cycloheptane and Larger Rings Cycloheptane and Larger Rings More complicated than cyclohexane. Common for several conformations to be of Common similar energy. similar Principles are the same, however. Minimize total strain. 3-76 3.14 Polycyclic Ring Systems Contain more than one ring….. Contain more than one ring….. bicyclic, tricyclic, tetracyclic, etc. bicyclic, tricyclic, tetracyclic, etc. 3-77 Number of rings Number of rings equals minimum number of bond disconnections required to give a noncyclic species 3-78 Monocyclic Monocyclic requires one bond disconnection requires 3-79 Bicyclic Bicyclic Bicyclic requires two bond disconnections 3-80 Bicyclic Bicyclic requires two bond disconnections 3-81 Types of ring systems Types of ring systems spirocyclic fused ring bridged ring 3-82 Spirocyclic Spirocyclic one atom common to two rings Spiro[4.5]decane Spiro[4.5]decane 3-83 Fused ring Fused ring adjacent atoms common to two rings two rings share a common side Bicyclo[4.3.0]nonane Bicyclo[4.3.0]nonane 3-84 Bridged ring Bridged ring nonadjacent atoms common to two rings Bicyclo[3.2.1]octane 3-85 Steroids Steroids carbon skeleton is tetracyclic 3-86 3.15 Heterocyclic Compounds 3-87 Heterocyclic Compound Heterocyclic Compound a cyclic compound that contains an atom other cyclic than carbon in the ring than (such atoms are called heteroatoms) typical heteroatoms are N, O, and S 3-88 Oxygen-containing heterocycles Oxygen-containing heterocycles O Ethylene oxide oxide O O Tetrahydrofuran O Tetrahydropyran 3-89 Nitrogen-containing heterocycles Nitrogen-containing heterocycles N H N H Pyrrolidine Piperidine 3-90 Sulfur-containing heterocycles Sulfur-containing heterocycles S S O Lipoic acid CH2CH2CH2CH2COH CH S S Lenthionine S S S 3-91 End of Chapter 3 3-92 ...
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