12th_Lecture_Ch3_9_26_2011

12th_Lecture_Ch3_9_26_2011 - AxialandEquatorialPositions...

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Axial and Equatorial Positions • Each face of the cyclohexane chair has 3 axial and 3 equatorial hydrogens in an alternating rearrangement
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1 2 1 2 Both up/down and Equatorial/axial information Only up/down information
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Conformational Mobility of Cyclohexane • Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip
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The Ring Flip
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Conformations of  Monosubstituted Cyclohexanes • Which is more stable ??? H CH 3 H H 1 2 H H H H CH 3 H H H H H H H 1 2 Ring flip The  up  and  down  positions of all substitutents remain  unchanged The  equatorial  and  axial  positions of all substituents  are switched
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• Which is more stable ??? CH 3 CH 3 The  up  and  down  positions of all substitutents remain  unchanged The  equatorial  and  axial  positions of all substituents  are switched
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Which of them is more stable? 1 2 3 13% 6% 81% 1. A 2. B 3. I don’t know. CH 3 CH 3 A B
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Conformations of  Monosubstituted Cyclohexanes • The equatorial conformation of methyl cyclohexane is more stable than the  axial one by  7.6 kJ/mol , which implies over  95%  of methylcyclohexane has  the methyl group equatorial at room temp. H CH 3 H H 1 2 H H H H CH 3 H H H H H H H 1 2 Ring flip
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1,3-Diaxial Interactions • The difference between axial and equatorial conformers is due to steric strain (hindrance) caused by 1,3-diaxial interactions. • Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogens 3 carbons away on C3 and C5, resulting in 7.6 kJ/mol of steric strain. CH 3 H H H CH 3 H ring-flip steric interference 1 2 3 4 5 6 1 6 5 4 3 2
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The larger the group, the worse the 1,3-diaxial interaction
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Relationship to Gauche Butane Interactions Gauche butane is less stable than anti butane by 3.8 kJ/mol because of steric interference between the 2 Me groups. • The 4 C fragment of axial
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This note was uploaded on 02/15/2012 for the course ORGO 101 taught by Professor Staff during the Fall '11 term at Syracuse.

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12th_Lecture_Ch3_9_26_2011 - AxialandEquatorialPositions...

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