UConn CHEM 2443 - Chapter 8: Rotamers and Conformation

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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 containero Organic molecules constantly absorb E from their environmento Such molecular motion is one way in which molecules in soln transfer heato Also dissipate E by molecular vibration  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 moleculeo There may be many conformations for a given molecule, some higher in E and some lower in Eo 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 Co A special alkyl group—R1—is attached to C1o 1 is a Sawhorse Diagramo 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 Rgroups 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 othero 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 overlapo 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 othero Lower in E- Eclipsed: the R1 groups come close togethero They compete for the same space and repel one anothero Higher in E- The repulsion of groups that are close together in space is called steric hindranceo As the rotation continues around the bond, more E is required to bring together whenthey eclipse in order to force one past the othero 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 othero 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 o 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 examinedo 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 difficulto 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 outo @ Normal temps, there is plenty of E for rotationo 3, 5, and 7 H atoms on the front C are in b/w the H atoms on the back C atom They are staggered rotamerso 4, 6, and 8 the H atoms on the front C overlap (eclipse) those on the back C They are eclipsed rotamerso 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 Co Newman projections show the bond of interest head on so that one C is in front and the otherin the rear- Look @ 9 (3 from the previous illustration)o It is a staggered rotamero 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 othero The term “staggered” is used to denote the relationship of two atoms or groups in a rotamer that are not eclipsedo In an eclipsed rotamer, a H atom on the front C lines up w/ a H atom on the back C 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 rotamero 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—H and X—C—C—X

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UConn CHEM 2443 - Chapter 8: Rotamers and Conformation

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