Conformations Sawhorse” representations

Conformational isomers are represented in two ways, as shown in Figure 3.6. A sawhorse representation views the carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-Tl bonds. A Newman projection views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle. Bonds attached to the front carbon are represented by lines to the center of the circle, and bonds attached to the rear carbon are represented by lines to the edge of the circle. 

Figure 11.18 Two conformational isomers of ethane, C2H6. (a) Sawhorse representation and (b) Newman projections.

Exercise 9-32 The proton-proton coupling in 1,1,2,2-tetrachloroethane cannot be observed directly because the chemical shift is zero. However, measurements of the splittings in 13CCI2H—12CCI2H show that the proton-proton coupling in CHCI2CHCI2 is 3.1 Hz. Explain how you can use this information to deduce the favored conformation of CHCI2CHCI2. Draw a sawhorse representation of the preferred conformation. 

Figure 12-5 Chair form of cyclohexane showing equatorial and axial hydrogens. Top, space-filling model center, ball-and-stick model bottom, sawhorse representation. Notice that all the axial positions are equivalent and all the equatorial positions are equivalent. By this we mean that a substituent on any one of the six axial positions gives the same axial conformation, whereas a substituent on any one of the six equatorial positions gives the same equatorial conformation.

Figure 12-18 Most stable conformation of cyclodecane Dale and sawhorse representations. The shaded area in the sawhorse convention indicates substantial nonbonded H - - H interactions.

The different arrangements of atoms caused by rotation about a single bond are called conformations. A conformer (or conformational isomer) is a compound with a particular conformation. Conformational isomers can be represented by Sawhorse representations or Newman projections. 

Ethane can exist in an infinite number of conformations. The conformation in which the hydrogen atoms and the bonding electrons are the farthest away firom one another has the lowest energy. This conformation is sta ered. The conformation in which the hydrogen atoms are closest to one another has the highest energy. This conformation is eclipsed. In the echpsed conformation, each C—H bond on one carbon atom lines up with a C—H bond on another carbon atom, as the moon sometimes eclipses the sun. Sawhorse representations of the conformations of ethane are shown in Figure 4.2. These representations are three-dimensional and show the carbon—carbon bond as well as all of the C—H bonds. 

Below are Newman and sawhorse representations of the two conformations of (S)-2-bromobutane that can lead to E2 reactions, along with the products of these E2 reactions. 

Complete the following Newman and sawhorse representations of (S)-3-bromohexane showing a conformation that would give rise to frawy-3-hexene. Also draw tra s -3-hexene, and show the meehanism of formation using curved arrows. 

Draw a sawhorse representation of the conformation that would give rise to each of these two products. 

Could E2 reaction with this configurational stereoisomer give rise to either of the two uncircled alkene products If so, show the sawhorse representation of the conformation giving rise to each product. 

The above quasi three-dimensional representations are known as sawhorse and Newman projections, respectively. The eclipsed and staggered forms, and the infinite variety of possible structures lying between them as extremes, are known as conformations of the ethane molecule conformations being defined as different arrangements of the same group of atoms that can be converted into one another without the breaking of any bonds. 

The sawhorse or Newman representations of 2-butanol, 5a and 5b and 6a and 6b, are excellent for showing the arrangements of the atoms in conformations, but are needlessly complex for representing the stereochemical configuration. Fischer projection formulas are widely used to show configurations and are quite straightforward, once one gets the idea of what they represent. 

Exercise 5-9 Draw a staggered conformation in both the sawhorse and Newman representations that corresponds to the configurations shown in the projection for-... 

If you view the second formula from the top, you will see that it is just a three-dimensional representation of the Fischer projection. Horizontal groups at each stereogenic center come up toward you, and vertical groups recede away from you. The second formula represents an eclipsed conformation of D-threose. The third and fourth formulas represent sawhorse and Newman projections, respectively, of a staggered conformation of D-threose. 

The most useful way to depict the various conformational states of an acyclic sugar chain is by means of a formalized drawing that shows the relative dihedral angles of the substituents at each end of a carbon-carbon bond. Two such types of representation are the sawhorse and Newman (Boeseken) formulas, as illustrated for erythritol. The zigzag type of representation shown has the advantage that it can be employed to display the relative orientations of groups along a chain of several atoms that lie approximately in a plane (for example, the plane of the paper). 

Figure 4.6. Sawhorse and Newman representations for staggered and eclipsed conformers of ethane (CH3CH3).

Fischer projections do not attempt to represent energy minimum conformations. Instead, they are designed to help distinguish the stereoisomers. An analogy would be the representation of substituted ethanes in an eclipsed conformation, called the sawhorse stmcture (p. 65). Fischer projections are used because they are simple to draw and read. We will see soon how to translate them into more accurate representations of molecules. 

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