Chem241a_Lecture_10 - Organic Chemistry 241a Prof. Bogdan...

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Or ganic Chemistry 241a Prof. Bogdan Olenyuk Contact Information Office: Old Chemistry 309 olenyuk@email.arizona.edu Phone: (520) 626-0754 Course web site: http://www.chem.arizona.edu/courseweb/074/CHEM241A1_2/FrameSet.html
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Alkanes and Cycloalkanes • Conformations of alkanes • Cycloalkanes: naming and isomerism • Stereoisomerism of cycloalkanes • Strain and stability • Conformations of cycloalkanes Today we will cover: Reading Assignment: McMurry (the BOOK), Ch. 3.6 – 4.9
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Conformations of Ethane • Stereochemistry concerned with the 3-D aspects of molecules σ bonds are cylindrically symmetrical • Rotation is possible around C-C bonds in open-chain molecules
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Conformers Conformation - Different arrangement of atoms resulting from bond rotation • Conformations can be represented in 2 ways:
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Torsional Strain • We do not observe perfectly free rotation • There is a barrier to rotation, and some conformers are more stable than others Staggered - most stable: all 6 C-H bonds are as far away as possible Eclipsed - least stable: all 6 C-H bonds are as close as possible to each other
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Torsional Strain
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Conformations of Other Alkanes • The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH 3 interaction
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• Conformational situation is more complex for larger alkanes • Not all staggered conformations has same energy, and not all eclipsed conformations have same energy Conformations of Other Alkanes
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Conformations of Butane Anti conformation- methyl groups are 180˚ apart Gauche conformation- methyl groups are 60˚ apart Which is the most energetically stable?
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Steric Strain Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow
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Naming Cycloalkanes Cycloalkanes are saturated cyclic hydrocarbons • Have the general formula (C n H 2n )
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• Find the parent. # of carbons in the ring. • Number the substituents
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This note was uploaded on 08/23/2008 for the course CHEM 241A taught by Professor Olenyuk during the Fall '07 term at University of Arizona- Tucson.

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Chem241a_Lecture_10 - Organic Chemistry 241a Prof. Bogdan...

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