3_stereochemistry_post

3_stereochemistry_post - Volhardt: 2-7, 2-8 Conformations...

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3. Stereochemistry 1 Conformations Groups bonded by only sigma bonds can undergo rotation about that bond with respect to each other and f orces within a molecule make some make spatial arrangement of the molecule energetically more favorable than others. Conformations (or rotamers) are the temporary molecular shapes that result from rotation about single bonds. Conformers are each possible structure. Conformational analysis is the study of the energy changes associated with a molecule undergoing rotation about single bonds. Volhardt: 2-7, 2-8 Newman projection
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3. Stereochemistry 2 Conformations of open-chained compounds Newman Projections C C H H H H H H saw horse looking down C – C bond C C H H H H H H H HH H H H staggered conformation C C H H H H H H looking down C – C bond C H H H H H H H H H H H H eclipsed conformation σ bond rotation Ball and stick models Newman Projections Stability of conformations
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3. Stereochemistry 3 Stability of conformations Better overlap of σ and σ * orbitals. Hyperconjugation: The phenomenon of electron delocalization via orbital overlap. This is not a true resonance structure. This concept will be very important later
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3. Stereochemistry 4 Free rotation around the single bond is hindered due to repulsion of atoms in the eclipsed conformations. Steric interaction raises the energy barrier to rotation. CH 3 CH 2 CH 2 CH 3 C C H 3 C CH 3 H H H H σ bond rotation C C H 3 C CH 3 H H H H greatest repulsion or steric hindrance
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3. Stereochemistry 5 Rotation around the C 2 –C 3 sigma bond of butane C C H 3 C CH 3 H H H H σ bond rotation CH 3 H H H H CH 3 Greatest steric interaction
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3. Stereochemistry 6 Cycloalkanes - Ring strain For cyclic alkanes confined to a specific geometry the bond angle may not correspond to the desired 109.5 o of an sp 3 carbon and cause ring strain. Therefore cycloalkanes do not all have the same relative stability. This can be demonstrated experimentally determining the heat of combustion for each compound. Lets first look at generally at heats of combustion and what it means. CH 3 CH 2 CH 2 CH 3 + 6.5 O 2 4 CO 2 + 5 H 2 O Δ H= -2877 kJ/mol + 6.5 O 2 4 CO 2 + 5 H 2 O Δ H= -2838 kJ/mol Since butane liberates more heat on combustion than isobutane, it must contain relatively more potential energy. Isobutane must be more stable. Heats of combustion per CH 2 unit reveal cyclohexane has no ring strain and other cycloalkanes have some ring strain CH 3 CHCH 3 CH 3 Volhardt: Chapter 4
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3. Stereochemistry 7 Ring Strain in Cycloalkanes : Angle Strain and Tortional Strain Angle strain is caused by bond angles different from 109.5 o Torsional strain is caused by eclipsing C-H bonds on adjacent carbons Cyclopropane has both high angle and torsional strain There is no available bond rotation to achieve a staggered conformation.
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3_stereochemistry_post - Volhardt: 2-7, 2-8 Conformations...

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