Chem 34 Organic Chemistry I Lecture Summary 0 September 4, 007 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. Sp 3 hybridized atoms want to have bond angles of 09.5. owever, 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. eat 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. Ring Strain 3 4 5 6 7 8 9 0 3 4 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 sp 3 -hybridized atoms. Thus, the bonds actually are bent sigma bonds. 60 007 Gregory R. Cook page Chem 34
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 ial positions and the bonds that point out away from the plane of the ring are called uatorial positions. 007 Gregory R. Cook page Chem 34
Cyclohexanes are flexible enough to undergo a from one chair conformation to another at room temperature (and below). When the chair flips, all the uatorial positions become ial and all the ial positions become uatorial. If the cyclohexane ring is substituted with subsituents like a methyl group, the two chair conformations are no longer ual in energy (like the all cyclohexane). The conformation with the methyl group in the ial position is higher in energy because the methyl group has steric interactions with the other ial groups (,3-diial interactions).,3-diial interaction ial methyl C 3 uatorial methyl C 3.8 kcal/mol more stable conformation ial methylcyclohexane side and top views 007 Gregory R. Cook page 3 Chem 34
uatorial 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-,-dimethylcyclohexane one can draw both chair conformations. In each conformation, one methyl is in an ial position and one is in an uatorial 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 ial positions and one in the more stable (lower energy) uatorial positions. cis-,-dimethylcyclohexane C 3 C 3 trans-,-dimethylcyclohexane C 3 C 3 C 3 C 3 C 3 C 3 C 3 C 3 same interactions in both conformations -- ual in energy different interactions in both conformations -- NOT ual in energy C 3 C 3 lower in 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/33/cychex/chair.html 007 Gregory R. Cook page 4 Chem 34
Quiz of the day 007 Gregory R. Cook page 5 Chem 34