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Chem 341 • Organic Chemistry I Lecture Summary 10 • September 14, 2007

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Chem 341 • Organic Chemistry I Lecture Summary 10 • September 14, 2007 Chem 341 • Organic Chemistry I Lecture Summary 10 • September 14, 2007 Chapter 4 - Stereochemistry of Alkanes and Cycloalkanes Conformations of Cycloalkanes Cyclic compounds contain something we call Ring Strain. There are three things that contribute to ring strain. Torsional strain (electron repulsion in eclipsing bonds), steric strain (atoms bumping into each other) and angle strain. Angle Strain: the strain due to bond angles being forced to expand or contract from their ideal. Sp3 hybridized atoms want to have bond angles of 109.5°. However, if the rings are very small or very large, there is no way to accommodate this angle. Thus, this increases the energy of the molecule. Heat of Combustion: the amount of heat (energy) released when a molecule burns completely with oxygen. By comparing the heat of combustion of different sized cycloalkanes, their relative energies can be obtained. The fact that the size of the ring has an influence on the total energy of the molecule indicates that there is some degree of instability associated with constraining the rings. This added energy (in addition to what would be expected from carbon and hydrogen combustion per mole) can be attributed to ring strain. n i a r t S g n i R 3 4 5 6 7 8 9 10 11 12 13 14 Ring Size Conformations of Cyclopropane Cyclopropane has a high degree of angle strain due to the highly distorted bond angles. The angle between the carbons is 60°. This cannot be accommodated by sp3-hybridized atoms. Thus, the 60° bonds actually are bent sigma bonds. ©2007 Gregory R. Cook page 1 Chem 341 North Dakota State University Cyclopropane only has three atoms in the ring, so it must be a flat structure. Indeed, it is the only cycloalkane that is completely flat. This causes a high degree of torsional strain because all the bonds are eclipsed. Conformations of Cyclobutane Cyclobutane is almost as high in ring strain energy as cyclopropane, but being a bigger ring, it has just enough flexibility to pucker slightly. This helps to alleviate a little bit of the torsional strain. The bonds are not quite eclipsed. Conformations of Cyclopentane Cyclopentane has even more flexibility and can bend more. This removes a large amount of the angle strain and the torsional strain. Note that at any given time cyclopentane has some bonds more eclipsed than others. Conformations of Cyclohexane Cyclohexane has no ring strain. In the chair conformation, all the bonds are staggered and all the angles are ideal. Note that the bonds that point straight up or down from the plane of the ring are in axial positions and the bonds that point out away from the plane of the ring are called equatorial positions. ©2007 Gregory R. Cook page 2 Chem 341 North Dakota State University Cyclohexanes are flexible enough to undergo a ring flip from one chair conformation to another at room temperature (and below). When the chair flips, all the equatorial positions become axial and all the axial positions become equatorial. If the cyclohexane ring is substituted with subsituents like a methyl group, the two chair conformations are no longer equal in energy (like the all H cyclohexane). The conformation with the methyl group in the axial position is higher in energy because the methyl group has steric interactions with the other axial groups (1,3-diaxial interactions). 1,3-diaxial interaction axial methyl H CH3 H ring flip equatorial methyl H CH H H H 3 H 1.8 kcal/mol more H H H stable conformation axial methylcyclohexane side and top views ©2007 Gregory R. Cook page 3 Chem 341 North Dakota State University equatorial methylcyclohexane side and top views The chair conformations of disubstituted cyclohexanes will have different interactions depending on where on the ring they are substituted and their stereochemistry. In cis-1,2-dimethylcyclohexane one can draw both chair conformations. In each conformation, one methyl is in an axial position and one is in an equatorial position. Thus, both conformations have identical energy. In the trans stereoisomer, there is a clear difference in the two conformations. One places both methyl groups in the more crowded axial positions and one in the more stable (lower energy) equatorial positions. cis-1,2-dimethylcyclohexane ax ax CH3 CH3 ring flip CH3 1 eq 1 1 CH3 H 2 2 2 CH3 H CH3 eq H H same interactions in both conformations -- equal in energy trans-1,2-dimethylcyclohexane ax CH3 H CH 3 1 ring flip eq 1 1 CH3 H 2 2 2 H CH eq 3 CH3 H CH3ax different interactions in both lower in conformations -- NOT equal in energy energy For on-line models of alkane and cycloalkane conformations, see the Models Page on the web. For a good site for drawing chairs, see: http://www.csub.edu/Chemistry/331/cychex/chair.html ©2007 Gregory R. Cook page 4 Chem 341 North Dakota State University Quiz of the day ©2007 Gregory R. Cook page 5 Chem 341 North Dakota State University.
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