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4.1 Conformation of Ethane

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4.1 Conformation of Ethane Ch.4 Stereochemistry of Alkanes and Cycloalkanes stereochemistry: 3-dimensional aspects of molecules 4.1 Conformation of Ethane conformation: the different arrangements of atoms that result from rotation about a single bond conformers: a specific conformation (conformational isomer); same connections of atoms H H H H C C rotate H H H H H C C HH H Ch.4 Stereochemistry of Alkanes and Cycloalkanes Sawhorse representation Newman projection back carbon H H H C H HH front carbon H C H H HH H Ch.4 Stereochemistry of Alkanes and Cycloalkanes Energy barrier for rotation: not perfectly free rotation about σ-bonds 4 kJ/mol (1.0 kcal/mol) H HH HHrotate 60o H H H 4 kJ/mol H HH 4 kJ/mol H staggered conformation eclipsed conformation rotational barrier: 12kJ/mol (2.9 kcal/mol) Ch.4 Stereochemistry of Alkanes and Cycloalkanes - the 12 kJ/mol (2.9 kcal/mol) of extra energy present in the eclipsed conformation of ethane is called torsional strain • Torsional strain is due to repulsion between electron clouds in the C-H bonds as they pass close by each other in the eclipsed conformer HH H H HH eclipsed conformation Ch.4 Stereochemistry of Alkanes and Cycloalkanes A graph of potentiol energy versus bond rotation in ethane HH HH H HH H H H H H H HH H HH H H H HH H o o 0o 120o 240 360 gy r 12 kJ/mol Ene o o 60o 180 300 H H H H H H H HH H H H H H H H H H Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.2 Conformation of Propane 6 kJ/mol (1.4 kcal/mol) CH3 H3CH H H rotate 60o H H H 4.0 kJ/mol H HH H 4.0 kJ/mol staggered conformation eclipsed conformation rotational barrier: 14 kJ/mol (3.4 kcal/mol) Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.3 Conformation of Butane CH3 CH3 H H H H H CH3 CH3 H3C H H H H H H CH3 H anti gauche anti conformation: two large groups are in the opposite side eclipsed conformations gauche conformation: two large groups are 60o apart Ch.4 Stereochemistry of Alkanes and Cycloalkanes Eclipsed conformations 6.0 kJ/mol H3CH H C 6.0 kJ/mol 3H H H 4.0 kJ/mol total cost: 16 kJ/mol (3.8 kcal/mol) eclipsed Ch.4 Stereochemistry of Alkanes and Cycloalkanes Eclipsed conformations 11 kJ/mol (2.6 kcal/mol) H3CCH3 H H H 4.0 kJ/mol H 4.0 kJ/mol total cost: 19 kJ/mol (4.6 kcal/mol) least stable eclipsed Ch.4 Stereochemistry of Alkanes and Cycloalkanes Gauche conformation: 3.8 kJ/mol (0.9 kcal/mol) unstable due to steric strain between two methyl groups 3.8 kJ/mol (0.9 kcal/mol) H H H H CH3 H CH3 H H H H H H H H gauche H - hydrogen atoms on methyl groups interact steric strain: repulsive interaction that occurs when atoms are too closer Ch.4 Stereochemistry of Alkanes and Cycloalkanes CH3 CH3 Four possible H H H3C H conformations of n-butane H H HH CH3 H anti gauche staggered H C 3 H H3CCH3 H3C H H HH H HH eclipsed eclipsed Ch.4 Stereochemistry of Alkanes and Cycloalkanes A graph of potentiol energy versus bond rotation in buthane Energy 19 kJ/mol 16 kJ/mol 3.8 kJ/mol 180o 120o 60o 0o 60o 120o 180o CH 3 CH CH3 CH3 H CH 3 H3CCH HCH H H 3 3 3 H H3C H H CH3 H H H H H3C HH H H H CH3 H H CH H HH H H HH H 3 H H CH3 anti gauche gauche anti Ch.4 Stereochemistry of Alkanes and Cycloalkanes Energy costs for interactions in alkane conformers 4.0 kJ/mol 6.0 kJ/mol (1.0 kcal/mol) (1.4 kcal/mol) HH H3CH torsional strain mostly torsional strain 11 kJ/mol 3.8 kJ/mol (2.6 kcal/mol) (0.9 kcal/mol) CH3 H3CCH3 CH3 torsional + steric strain steric strain Ch.4 Stereochemistry of Alkanes and Cycloalkanes • the most stable conformation of any alkanes has the C-C bonds in staggered arrangements and large substituents arranged anti to each other HHHHHHHHHH HH H H HHHH H H zig-zag conformation • At room temperature, enough thermal energy is present to cause rotation around σ-bonds to occur rapidly so that all conformations are in equilibrium. At any given time, however, a larger percentage of molecules will be found in a more stable conformation than in a less stable one. Ch.4 Stereochemistry of Alkanes and Cycloalkanes Practice Conformation of 1-chloropropane Newman projections H C 3 Cl CH CH 3 3 H3CH H3CCl HHH Cl H H H H H H H Cl H H Cl H HH Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.4 Stability of Cyclohexanes: The Baeyer Strain Theory Angle strain (Baeyer strain): the strain induced in a molecule when a bond angle deviates from the ideal tetrahedral value bond angles for hypothetical planar cycloalkane structures; 108o 120o o 60 90o 109o (tetrahedral angle) Ch.4 Stereochemistry of Alkanes and Cycloalkanes Heat of Combustion of Cycloalkanes ring strain: total energy of the compound- the energy of a strain-free reference compound heat of combustion: the amount of heat released when a compound burns completely with oxygen ; used for determination of starin energies of cycloalkanes ; the more energy (strained) a compound contains, the more energy (heat) is released on combistion 3n (CH ) + O nCO2 + nH2O + heat 2 n 2 2 Ch.4 Stereochemistry of Alkanes and Cycloalkanes cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane ring is strain-free. • the strain molecules (cyclopropane, cyclobutane) are unstable and highly reactive • cyclopentane, cyclohexane are srtain free o •medium rings C7-C13 are srtained: 115-120 • larger rings are unstrained ≥C14 Ch.4 Stereochemistry of Alkanes and Cycloalkanes cycloalkane strain energies Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.5 The Nature of Ring Strain • rings are not flat; 3-dimensional conformations • torsional strain due to eclipsed C-H bonds in ring systems The conformation of cyclopropane, showing the eclipsing of neighboring C-H bonds that give rise to torsional strain. eclipsed H H H HH H H H H H HH eclipsed Ch.4 Stereochemistry of Alkanes and Cycloalkanes Cycloalkanes adopt their minimum-energy conformation for the combination of three reasons: • Angle strain: the strain due to expansion or compression of bond angles • Torsional strain: the strain due to eclipsing of bonds on neighboring atoms • Steric strain: the strain due to repulsive interactions when atoms approach each other too closely Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.6 Cyclopropane: An Orbital View colorless gas (bp= -33 oC); first prepared by reaction of sodium with 1,3- dibromopropane 2 Na Br Br + 2 Na Br bent bond: C C poor overlap C C C C 109o A typical alkane C-C bond A bent cyclopropane C-C bond Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.7 Conformations of Cyclobutane and Cyclopentane Cyclobutane less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens ; total strain of cyclobutane- 110.4 kJ/mol (26.4 kcal/mol) ; total strain of cyclopropane- 115 kJ/mol (27.5 kcal/mol) ; not flat but puckered → increase angle strain but decrease torsional strain not quite eclipsed H H H H H H H H H H H H H H H H not quite eclipsed Ch.4 Stereochemistry of Alkanes and Cycloalkanes Cyclopentane total strain of cyclopentane- 26.0 kJ/mol (6.2 kcal/mol) ; not planar but envelop conformation H H H H H H H H H H envelop Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.8 Conformations of Cyclohexane • unstrained chair conformation: bond angle 111.5o (close to the ideal 109.5o tetrahedral angle) H H H H H H H H CH2 H H H H H HCH2 H H H H H H chair all staggered conformation Ch.4 Stereochemistry of Alkanes and Cycloalkanes • drawing chair conformation: draw parallel three bonds pairs this bond is in back this bond is in front Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.9 Axial and Equatorial Bonds H axial H H H H H equatorial H H H H H H H H H H H H H H H top view Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.10 Conformational Mobility of Cyclohexane ring-flip the energy barrier for the ring flipping is small: fast ring flipping is observed ( two conformers are in fast equilibrium) Ch.4 Stereochemistry of Alkanes and Cycloalkanes ring flipping H Br H ring-flip Br Ch.4 Stereochemistry of Alkanes and Cycloalkanes Hax Ea= 45 kJ/mol H Heq eq chair Hax - rapid interconversion at 25 ℃ (Ea= 45 kJ/mol (10.8 kcal/mol), 20 kcal/mol available at 25 ℃) -Hax and Heq are indistinguishable by 1H NMR at 25 ℃ o - at < -70 C, Hax and Heq are distinguishable by 1H NMR Ch.4 Stereochemistry of Alkanes and Cycloalkanes 4.11 Conformation of Monosubstituted Cyclohexanes • two conformers of a monosubstituted cyclohexane are in fast equilibrium at room temperature but not equally stable CH3 ring-flip CH3 ∆ E = - RT ln K Ch.4 Stereochemistry of Alkanes and Cycloalkanes CH3 CH3 ring-flip Ch.4 Stereochemistry of Alkanes and Cycloalkanes A B energy barrier vs rate constant energy barrier at RT 5 kcal/mol 109 sec-1 10 kcal/mol 105 sec-1 15 kcal/mol 102 sec-1 > 20 kcal/mol Ch.4 Stereochemistry of Alkanes and Cycloalkanes A B ∆ G = - RT ln K free energy vs % of isomer more stable free energy isomer(%) 1.0 kcal/mol 80% 1.3 kcal/mol 90% 2.3 kcal/mol 98% 4.1 kcal/mol >99.9% Ch.4 Stereochemistry of Alkanes and Cycloalkanes 1,3-diaxial interaction: steric strain, butane gauche interaction H CH H 3 H 7.6 kJ/mol H CH3 (1.8 kcal/mol) 7.6 kJ/mol ~ 95% equatorial methyl preference Hax Hax Hax Heq Heq Heq Heq H H ax ax CH3 Ch.4 Stereochemistry of Alkanes and Cycloalkanes butane gauche interactions 3.8 kJ/mol (0.9 kcal/mol) CH3 butane gauche H CH3 H H H H H H H H CH3 H CH3 H CH3 1,3-diaxial interaction: butane gauche interaction (x2) 3.8 x 2 = 7.6 kJ/mol Ch.4 Stereochemistry of Alkanes and Cycloalkanes
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