Sawhorse representations

The formalism that we have set up to describe chain flexibility readily lends itself to the problem of hindered rotation. Figure 1.8a shows a sawhorse representation of an ethane molecule in which the angle of rotation around the bond is designated by electron repulsion between the atoms bonded to... 

Figure 3.6 A sawhorse representation and a Newman projection of ethane. The sawhorse representation views the molecule from an oblique angle, while the Newman projection views the molecule end-on. Note that the molecular model of the Newman projection appears at first to have six atoms attached to a single carbon. Actually, the front carbon, with three attached green atoms, is directly in front of the rear carbon, with three attached red atoms.

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.

In the sawhorse representation, the molecule is viewed from an oblique angle, and all bonds can be seen. 

Let us now consider rotation about the central C-C bond in butane. Rotation about either of the two other C-C bonds will generate similar results as with ethane above. Wedge-dot, Newman, and sawhorse representations are all shown use the version that appears most logical to you. 

Generally we shall avoid such drawings and suggest that it is much better to learn to draw molecules in as nearly correct perspective as possible. Once the sawhorse representation of cyclohexane is mastered, it is almost as easy to draw 14 as 13, and 14 is much more informative about the shape of the molecule ... 

Redraw the perspective drawings a, b, and c as Fischer projection formulas, leaving the configuration at the chiral centers unchanged. Similarly, redraw d and e in perspective, using a staggered sawhorse representation for e. 

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 12-4 Extreme boat form of cyclohexane showing interfering and eclipsed hydrogens. Top, space-filling model center, ball-and-stick models bottom, sawhorse representations...

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.

A sawhorse representation and a Newman projection of ethane. The sawhorse projection views the molecule from an oblique angle, while the Newman projection views the molecule end-on. 

In these "sawhorse representations of stereochemistry, the part of the molecule on the lower left is in front of the part on the upper right. 

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. 

FIGURE 8.1 Sawhorse representations of site-controlled isotactic (a), chain-end-controUed isotactic (b), and stereoblock-isotactic (c) polypropylene sequences. 

FIGURE 8.2 Sawhorse representations of an ideal isotactic/syndiotactic stereoblock polypropylene juncture (a), and of stereodefective isotactic, and syndiotactic polypropylene sequences (b-f). 

FIGURE 8.3 Sawhorse representations of highly isotactic (a), poorly isotactic ( isotactoid ) (b), and syndiotactic (c) seqnences/blocks fonnd in polypropylenes produced with heterogeneous Ziegler-Natta catalysts. 

FIGURE 8.13 Sawhorse representation of two isotactic polypropylene blocks with same (a) or opposite (b) relative configurations separated by a short stereoirregular sequence. 

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