Organic Chemistry I
Organic Chemistry I CHEM 2010
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4 Stereochemistry of Alkanes and Cycloalkanes Based on McMurry s Organic Chemistry 6th edition Chapter 4 The Shapes of Molecules n The threedimensional shapes of molecules result from many forces a A molecule may assume different shapes called conformations that are in equilibrium at room temperature the conformational isomers are called conformers emphasis on the first syllable El The systematic study of the shapes molecules and properties from these shapes is stereochemistry n The field of stereochemistry is one of the central parts of organic chemistry and includes many important topics 2 41 Conformations of Ethane Conformers interconvert rapidly and a structure is an average of conformers a Molecular models are three dimensional objects that enable us to visualize conformers n Representing three dimensional conformers in two dimensions is done with standard types of drawings Conformations of Ethane staggered conformation eclipsed conformation Conformations of Ethane Staggered conformation eclipsed conformation Representing Conformations n Sawhorse representations show molecules at an an le n showing a molecular mo el 11x yquot I quotII 1 I CC bonds are at an angle to the edge of the page It and all CH bonds are If 9 S h 0W n Sawhorse n Newman projections show representatiun how the CC bond would project endon onto the paper Bonds to front carbon are lines going to the center I Bonds to rear carbon are lines going to the edge of the circle Newman projection Newman Projections A MIL mrhnn H H Fm Newman limitsEl n Ethane s Conformations Ethane staggered conformation 2004 Thomson BrooksICole 40 kJ mol 4 HH Rotate rear carbon 60quot H H EH 40 Umol 40 Hlmol Ethane eclipsed conformation Ethane s Conformations El There barrier to rotation between conformations is small 12 kJmol 29 kcalmol The most stable conformation of ethane has all six C H bonds away from each other staggered n The least stable conformation has all six C H bonds as close as possible eclipsed in a Newman projection energy due to torsional strain Eclipserl Eglrill m mers ll kanmnl I n H I u lllI II I w Il 39 39 wags quot iig g 9 Ethane s Conformations Eclipsediggfmmers quota x quotH H 391 h H h HH HHV 0 60 120 180 240 300 3600 Ethane s Conformations Eclipsed conformers Energy N H H H H H H H H H H u Hg A H H i H H A 11 HH HH HH BE ME HH H H H H H H H H H H H H I I I I 0 60 120 180 240 300 360 1 1 3114 Thnttsnnl mols Cole 42 Conformations of Propane El Propane C3H8 torsional barrier around the carbon carbon bonds 14 kJmol El Eclipsed conformer of propane has two ethane type H H interactions and an interaction between C H and C C bond ill H Htaggcrr i J WIPE39HE CH3 M kJrJ39mn39 I i I II it e L J 40 kJi39mnl Eclipsed pmpanc Propane conformations CH3 60 kJmol CH3 H H H Rotate rear carbcm 60 H H H H H H H 40 kJmok 40 kJmol Staggered propane Eclipsed propane 2004 Timmnl moks Cole 43 Conformations of Butane n anti conformation has two methyl groups 180 away from each other El Rotation around the C2 C3 gives eclipsed conformation El Staggered conformation with methyl groups 60 apart is gauche conformation in In 3 l3 J39l l ii GEUEI39IE Mast stable eclipsed Staggered Conformations of Butane ng 3 Anti Gauche Least Mable E ll Conformations of Butane Conformations of Butane Energy mm 1 mhlfmnl m Eagi 130 a fmm qt HYquot lmh H u Gmnma Gamma I l l l T 6W W m D wdu angab meeunw wlgnmpa Eclipsed Conformations of Butane Cost 60 kJInol H30 H Total cost 16 kJmol Cost 60 kJmol HCHB HH Coat 40 kJmol Cost 11 kJmol HBC CH3 A Total cost 19 Mind H H Cost 40 kJmol H H 7 Cost 40 kJmol 2004 Thomson BrooksCole Gauche conformation steric strain 2004 TMI I39BONEI OOKS Cale Sources of strain caused by rotation TABLE 41 Energy Costs for Interactions in Alkane Conformers Energy cost Interaction Cause kl lmol kcal mol H lt gt H eclipsed Torsional strain 40 10 H lt CH3 eclipsed Mostly torsional strain 60 14 CH3 lt gt CH3 eclipsed Torsional plus steric strain 11 26 CH3 lt gt CH3 gauche Steric strain 38 09 2004 Thomson Brookstole 20 1 Ch10ropropane CH3 Most stable staggered 2004 Thomson BrooksCole H3001 All H H Least stable eclipsed 21 Hydrocarbon Chains Staggered HHHHHHHHHH a 2004 Tmmsonl mols Gala 22 44 Stability of Cycloalkanes The Baeyer Strain Theory Baeyer 1885 since sp3 carbon prefers to have bond angles of approximately 109 ring sizes other than five and six may be too strained to exist Rings from 3 to 30 C s do exist but are strained due to bond bending distortions and steric interactions Strain energy LkJII39mulj I 39 BE I a I I I ll is a 21 1 I 5 IS 7 8 9 IIIIIIZIE M Ringsi 23 Baeyer s hypothesis angle strain 109quot tetrahedral 19 AQi wax E00 90 H l Cyclopropane Cyclobutane Cyclopentane 2004 Thomson BrooksCole 24 Heats of Combustion CH2n 3 11 02 gt n 002 n H20 2004 Thomson BrooksC 25 Stability of Cycloalkanes M MTS Strain Energy Minna a 3 was 15 7 33991 1mm 112113514 mugging 26 45 The Nature of Ring Strain El Rings larger than 3 atoms are not flat planar El Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring puckering El Larger rings have many more possible conformations than smaller rings and are more difficult to analyze 27 Angle Strain 109 tetrahedral 19 w49 1 amp w x 90 Cyclopropane Cyclobutane Cyclopentane 2004 Thomson 5 BrooksCole 28 Torsional Strain H H H H H H Ethana staggered conformation 2004 Thomson Bronkleole 40 kJmol I L HH Rotate rear gt carbon 60quot H H i H 40 kJmol 40 nal Ethane eclipsed conformation 29 Steric Strain Gauche 30 Strain Energies TABLE 41 Energy Costs for Interactions in Alkane Conformers Energy cost Interaction Cause klmoi kcalmol H lt gt H eclipsed Torsional strain 40 10 H lt gt CH3 eclipsed Mostly torsional strain 60 14 CH3 9 CH3 eclipsed Torsional plus steric strain 11 26 CH3 lt gt CH3 gauche Steric strain 38 09 2004 Thomson Brookstole 31 Summary Types of Strain nAngle strain expansion or compression of bond angles away from most stable nTorsional strain eclipsing of bonds on neighboring atoms nSteric strain repulsive interactions between nonbonded atoms in close proximity 32 46 Cyclopropane An Orbital View n 3membered ring must have planar structure a Symmetrical with C C C bond angles of 60 El Requires that sp3 based bonds are bent and weakened i All CH bonds are eclipsed H H H I Ct39limul H n l39lcli lht tl 33 Bent Bonds of Cyclopropane El Structural analysis of cyclopropane shows that electron density of C C bondis displaced outward from the internuclear axis 34 Bent bonds in cyclopropane less than maximum orbital overlap a A typical alkane C C band A bent cyclopropane 043 band 2004 Thomson Brookstole L l l I I l l 35 47 Conformations of Cyclobutane and Cyclopentane El Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens El Cyclobutane is slightly bent out of plane one carbon atom is about 25 above I The bend increases angle strain but decreases torsional strain Nut quite CIlIJHCIJ 391 NM quilc L l39lill st Il 36 Cyclobutane a 2004 Tromsonl moks Gale b H I 1 Not quite eclipseQ41 H H HE Not quite eclipsed 37 Cyclopentane n Planar cyclopentane would have no angle strain but very high torsional strain El Actual conformations of cyclopentane are nonplanar reducing torsional strain El Four carbon atoms are in a plane I The fifth carbon atom is above or below the plane looks like an envelope 38 Cyclopentane Observer 2004 Thomnnamole Cole 39 48 Conformations of Cyclohexane El Substituted cyclohexanes occur widely in nature I The cyclohexane ring is free of angle strain and torsional strain a The conformation has alternating atoms roughly in a common plane and tetrahedral angles between all carbons n This is called a chair conformation Chair Conformations H 1 u 1 Uhservvnr 41 Cholesterol three Chair conformations HO Cholesterol 2004 Thomson BrooksCole 42 HOW to Draw Cyclohexane STEP 1 Draw twn parallel linesg slanted l tllnwnwartl and slightly nf mt t mm each other This means that tour atquot 39 the cyclahcxanc carbutm atnms lie in a plane l STEP 2 Locate the tmpm ost cathan atam above and to the right of the plane df If the other Fuur and waned the hunch l S39I EI 3 Lucate the huttumnmust tarbnn quotI nth h eluw and m the left If the Vquot plane of the middlle Fumr and I canntwt the hands Nate that HM handh tn the hnttmnmast EHIII hD 39 atmm are parallel tn the hands tn the tupmnst carhmm 43 49 Axial and Equatorial Bonds in Cyclohexane I The chair conformation has two kinds of positions for substituents on the ring axial positions and equatorial positions a Chair cyclohexane has six axial h drogens perpen icular to the rin parallel to the ring axis and six equatorial throgens near the plane 0 the ring Ring axis Ring equator 44 Axial and Equatorial Bonds Ring axis 7 7 nRing equator 45 Axial and Equatorial Positions El Each carbon atom in cyclohexane has one axial and one equatorial hydrogen El Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement I iquulnrial AlXi l ll Drawing the Axial and Equatorial Hydrogens Axial hands The all axlal J ban Ila an an 2th carbam quot are parallel and alternate up dmvm liqualurial hunrls The ll HII l tl1ll39i lll hands an an each aarham aama in WEE Eels m l39wa parallel llnag Each sat is aaa parallel la lwa ring banana Equatorial hands allar nam hamcan sidca araun ll ring Lianlphtlad cyclalIaaaaa 47 Axial and Equatorial Hydrogens M 410 Conformational Mobility of Cyclohexane El Chair conformations readily interconvert resulting in the exchange of axial and equatorial positions by a ringflip Move this carbon clown Move this carbon up u Ringflip 49 Conformational Mobility Mqu this calbun Clown Move this carbon up u Ring ip Iii l l H 139 50 Bromocyclohexane I When bromocyclohexane ring flips the bromine s position goes from equatorial to axial and so on I At room temperature the ring flip is very fast and map the structure is seen as Br the weighted average LET Axial bromocyclohexane Equatorial bromocyclohexane Thomson Brooks Cnie 51 Bromocyclohexane Ring ip gt mm Axial bromocyclohexane Equatorial bromocyclohexane Thomson Brooks Cole 52 411 Conformations of Monosubstituted Cyclohexanes n The two conformers of a monosubstituted cyclohexane are not equal in energy I The equatorial conformer of methyl cyclohexane is more stable than the axial by 76 kJmol 53 Methylcyclohexane 54 Energy and Equilibrium D The relative Energy differenceikcalimul amounts of the mquot D i two conformers J 39 39 39 depend on their Iun mlalr iillll39l difference in an energy AE RT an in I R is the gas constant 8315 3940 JKomol T is the Kelvin 20 em II39 E L39Clll temperature firLess stable isomer and K is the D I I I I I I u I I equilibrium 5 m 15 CO nsta nt Energy difference kailmuli between isomers 55 13 Diaxia1 Interactions a Difference between axial and equatorial conformers is due to steric strain caused by 13diaxial interactions El Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogens three carbons away on C3 and C5 resulting in 76 kJmol of steric strain FStcric interference 1n 56 13 Diaxial Interactions f Stcric interference CH3 3F 7 a 7 4 h H i 6 Jam 57 Relationship to Gauche Butane Interac ons fllj El Gauche butane is less quot317 II 39 stable than anti butane g 39 39 by 38 kJmol because of u u 3 steric interference II J l a between hydrogen atoms Gmmmhm 0 on the two methyl l lkJmulslfraiml groups El The fourcarbon Q u H CH fragment of axial J 3 39 H n methylcyclohexane and a i g gauche butane have the H H same steric interaction 1 u u H n In general equatorial Mia positions give more stable isomer methyI cyclnlmxane V76 lk39Jmmll strain 58 Gauche Butane Interactions CH3 H3c H H H H Gauche butane 33 kJJ mml strain H H CH3 H H H H H H H Axial methyIcyclnhexana 76 Mlmul strain 59 Monosubstituted Cyclohexanes TABLE 42 Steric Strain in Monosu bstituted Cyclohexanes Strain of one H Y 13diaxial interaction Y kJ mol kcalmol F 05 012 CI 10 025 Br 10 025 0H 21 05 CH3 38 09 CHQCH3 40 095 CHCH32 46 11 CCH33 114 27 C5H5 63 15 C02H 29 07 CN 04 01 2004 Thomson BrooksCole 412 Conformational Analysis of Disubstituted Cyclohexanes IIIn disubstituted cyclohexanes the steric effects of both substituents must be taken into account in both conformations IThere are two isomers of 12 dimethylcyclohexane cis and trans 61 412 Conformational Analysis of Disubstituted Cyclohexanes cis1Z Dl39lmcthyllcyclohcxanc One gauche interaction 08 kJmull 39l uu L39H3 H tli dxitll El In the cis isomer both iuncramimmn kJI mnlI methyl groups same face l otal strain 3876 114 kJmul of the ring and compound u in 11 199 239 39 can exist in two chair conformations El Consider the sum of all interactions I In cis 12 both conformations are equal in energy interaction 38 kJmoli Two CH3H dinxial interactions 76 kJmol 39l utul strain 33 76 l 4 lemul Cz395 1 Z dimethylcyclohexane ciaLla im hylcyglnh mnt ue gmhi inlwm hm 1L8 kmm fqu Hjallzlimiul il 39M Wi l l 176 kqlfmwln Tall strain 3 H a 4 anml 63 Cz395 1 Z dimethylcyclohexane LLB Ma me I m Cl H diza39xiul immmiuns WI HimDH THEM mm 439 75131 1 L4 Myriam 64 Tram 1 2 Dimethy1cyclohexane n Methyl groups are on translZsDim etm39lcyclluhexane opposite faces of the ring 0 intrraction 68 Mmull El One trans conformation has both methyl groups equatorial and only a gauche butane interaction between methyls 38 kJmol and no 13 diaxial interactions n lfnurt IH Ildimial conformation has both immnus mm methyl groups axial with four 13diaxial interactions 65 Tram 1 Z Dimethylcyclohexane El Steric strain of 4 x 38 kJmol 152 kJmol itquot2quot IE l li quot quote makes the diaxial inln39actionll kJr mnl conformation 114 kJmol quot v fi m may 1 less favorable than the quotfluquot quot diequatorial conformation imam v El trans 12 I f392 lea39l39 M dimethylcyclohexane WI IL alggggg exist almost exclusively quotU a ii gt99 in the diequatorial quot conformation i 222liff 39 i lffilml 66 Tram 1 Z Dimethylcyclohexane Emma I Ja immhj my nhcmm ne 333mm imon iun I J Milan Ringamp 67 Tram 1 Z Dimethylcyclohexane F Mir EHJHH diaminl in l racli ng i 51 Minna 68 Axial Equatorial Relationships TABLE 43 Axial and Equatorial Relationships in Cis and TransDisubstituted Cyclohexanes Cistrans substitution pattern 12Cis disubstituted 12Trans disu bstituted 13Cis disubstituted 13Trans disubstituted 14Cis disubstituted 14Trans disubstituted Axialequatorial relationships ae or ea aa or ae aa or ee ae or ea ae or ea aa or ee 2004 Thomson BrooksCole 69 t Butyl Groups H3Cl H30 2 x 10 20 kJmol static strain 2004 Thomson Brmkleole Ring ip H30 0 H H H H Cl 2 x 114 228 kJmol static strain 70 t Butyl Groups Br cis1 me4tertbntylcycluhexane axial bromine EDIIM Thnmsnn Brmksf nle 71 t Butyl Groups trans1BromoAtertbutylcycluheme equatorial bromine 39Br 72 Prob 440 Most stable conformation of Mentholgt CH3 CH3ZCH Solution CH3 OH more stable H3C CH3 74 Problem 439 Galactose has an axial OH group at C4 Draw the Chair HO O CHZOH Galactose O OH OH 2004 Thomson BrooksCole Solution OH CHZOH HO OH HO Galactose 413 Boat Cyclohexane n Cyclohexane flips through a Slug 31min aflwdmgen boat conformation MU anti El Less stable than chair 51 7 391 cyclohexane due to steric quot and torsional strain El C 2 3 5 6 are in a plane I H on C 1 and C 4 approach each other closely enough to produce considerable H H quotismYr steric strain a Four eclipsed H pairs on C 2 3 5 6 produce torsional strain 29 kJmol 70 kcalmol less stable than chair Torsional strai n 77 I 139I6Mkcalmml e Boat amp Twist boat conformations Boat cyclohexane Twistboat cyclohexane 29 kJmol strain 23 kJmol strain 2004 mm moks Cole 79 414 Conformations of Polycyclic Molecules El Decalin consists of two cyclohexane rings joined to share two carbon atoms the bridgehead carbons C1 and C6 and a common bond ll CH2 CH2 10 2 H2C Cl EH2 9 1 3 H20 8 6 4 CH2 l CH2 7 5 H Decalin two fused cyclohexane rings 80 2004 Thomson BrooksCole Decalin 1 CH2 CH2 10 2 H2 I IEHZ 9 1 3 H2O C CH2 8 6 4 CH2 CH2 7 5 H Decalin two fused cyclohexane rings 2004 Thomson BrooksJCole 81 414 Conformations of Polycyclic Molecules a Two isomeric forms of decalin trans fused or cis fused n In cisdecalin hydrogen atoms at the bridgehead carbons are on the same face of the rings a In transdecalin the bridgehead hydrogens are on opposite faces El Both compounds can be represented using chair cyclohexane conformations n Flips and rotations do not interconvert cis and trans 82 Cz s and z mm decalins tramsDecalin H IrisDecalin 83 Steroids Cholesterol H0 Cholesterol 2004 Thomson BrooksICole 85 Testosterone CH CH3 H 3 0H Testosterone a steroid 2004 Thomson BrooksCole 86 Bicyclic Compounds A l carbon bridge A 2carbon bridge Bridgehead carbons Norbornane Bicyclo221heptane 2004 Thomson Brookleole 87 Camphor H30 CH3 CH3 Camphor 2004 Thomson gt Br kleo e Morphine and Opium Alkaloid Morphine mmTlLornsuna39 moks Gala 89 Morphine 2004 Thomson BrooksCole e morphine rule an aromatic ring ttached to a quaternary carbon matched to two more carbons ttached to a tertiary amine 2004 Thomson BrooksCole Codeine Heroin CH3 CH CBH5 N 3 02H5020 N CH3 CH3 Methadone Meperidine Demerol