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Chapter 8 Rotamers and Conformation Collisions of molecules will dissipate excess E to another molecule to the solvent or to the sides of the container o Organic molecules constantly absorb E from their environment o o Also dissipate E by molecular vibration Such molecular motion is one way in which molecules in soln transfer heat One vibration mode changes the shape of the molecule by stretching covalent bonds bending bonds or internal rotation around single covalent bonds Rotation about C C bonds positions atoms and groups in the molecules diff relative positions in space Some of these positions may lead to interactions b w atoms or groups induced by those units being too close together known as steric hindrance Conformation the overall shape for a molecule o There may be many conformations for a given molecule some higher in E and some lower in E o A molecule will spend most of its time in these low E conformations o Conformation of a molecule is due to rotation about C C single bonds In 1 there are two C atoms C1 to the front and C2 to the back where alkyl subs R are attached to each C o A special alkyl group R1 is attached to C1 o o 1 is a Sawhorse Diagram 2 is a Newman Projection The C in the rear C2 is represented as a circle w three R groups attached to it The front C is represented as a dot in the center of the circle also w three R groups attached to it The R1 groups on the front C appears to rotate in a clockwise direction thru 360 degrees 60 degrees a time This is monitored by keeping track of the position of the R1 group relative to the R groups Why 60 degrees o The positions of the three R groups on the rear C or the front C arranged to the corners of a tetrahedron o Bond rotations of 120 degrees or half that amount 60 degrees conveniently show if groups are closer to or further away from each other o Note the arrangement of R groups in both 1 and 2 represents as tetrahedral array b c each C is tetrahedral Both the sawhorse diagram and the Newman projection are used to show the spatial relationship of the R groups and the R1 group when there s rotation about the bond Rotamers the structures that result from rotation about C C bonds Beginning w 2 rotation by 60 degrees will generate a diff looking structure 3 where all of the bonds to groups overlap o Another 60 degree rotation gives the structure 4 that looks like 2 but the R1 group is in a diff spatial position There are an infinite of possible rotations b c the size of the rotation angle can be any but the 60 degree rotations generate structure where the R groups are as closer together or as far apart as they can be Staggered the R1 groups is in b w other R groups far away from each other o Lower in E Eclipsed the R1 groups come close together o They compete for the same space and repel one another o Higher in E o o The repulsion of groups that are close together in space is called steric hindrance o As the rotation continues around the bond more E is required to bring together when they eclipse in order to force one past the other If the steric repulsion is too great it constitutes a barrier that inhibits rotation This E barrier is usually low enough that rotation continues but it is hindered slowed down as the R and R1 groups sweep past each other o These two cases close together eclipsed or far apart staggered represent the max and min interactions for those groups There is continuous rotation around each single bond as long as there s enough E in the environment to induce the rotation If each structure could be frozen a particular rotation angle slight differences in the spatial relationships of the atoms relative positions in the 3D space can be examined o Rotation about C C single bonds can therefore generate many rotamers o Conformation a particular arrangement of all the atoms in space ID those rotamers where interactions of the various atoms attached to the C s will raise their E relative to others Some conformations are lower in E than others and thus more abundant By lowering the temp of the environment the E available to the molecule is diminished and rotation becomes increasingly difficult o If the temp is lowered so that the available E is less than the steric interaction mentioned earlier rotation about 360 degrees will stop and the rotamer will be frozen out o Normal temps there is plenty of E for rotation 3 5 and 7 H atoms on the front C are in b w the H atoms on the back C atom o o They are staggered rotamers They are eclipsed rotamers 4 6 and 8 the H atoms on the front C overlap eclipse those on the back C o This sawhorse diagram shows the molecule is tilted an angle to show both C atoms as well as the groups and atoms attached to each C o Newman projections show the bond of interest head on so that one C is in front and the other in the rear Look 9 3 from the previous illustration It is a staggered rotamer o o When the H on the front and top is compared w the H in the back and on the bottom it s apparent that they are 180 degrees from each other o The term staggered is used to denote the relationship of two atoms or groups in a rotamer that are not eclipsed In an eclipsed rotamer a H atom on the front C lines up w a H atom on the back C o These atoms are close together in space B c each bond is made up of two electrons it s reasonable to assume that there will be electronic repulsion like charges repel Torsional Strain repulsive E Eclipsed rotamers are higher in E than staggered rotamers b c they have torsional strain o So rotamer 9 is lower in E than 10 Eclipsed rotamers are less abundant b c they are higher in E making it difficult for them to exist when compared to the lower E anti rotamers o They will thus spend most of their time as anti rotamers If sufficient E is present to cause the molecule to rotate freely around the C C bond both eclipsed and staggered rotamers will be present but the eclipsed rotamers are higher in E To go from one staggered rotamer to the next during the rotation the molecule must pass thru an eclipsed rotamer o B c the eclipsed rotamer is higher in E there s an E barrier to rotation o Rotation slows down when eclipsed rotamers are encountered b c each presents a barrier to rotation Rotamers of substituted alkanes can be categorized into two fundamental types X C C …

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UConn CHEM 2443 - Chapter 8: Rotamers and Conformation

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