Sawhorse structures

Sawhorse structure (Section 3.6) A manner of representing stereochemistry that uses a stick drawing and gives a perspective view of the conformation around a single bond. 

The interesting eclipsed or "sawhorse" structure (C2 ) of the S2042 ion can be shown as... 

Exercise 9-33 The proton-proton coupling in meso-2,3-dibromobutanedioic acid (determined by the same procedure as for 1,1,2,2-tetrachloroethane, see Exercise 9-32) is 11.9 Hz. Write a sawhorse structure for the preferred conformation of this molecule. 

Draw all possible staggered and eclipsed conformations of l-bromo-2-chloroethane (see the followingball-and-stick model), using Newman projections. Underneath each structure, draw the corresponding dash-wedge and sawhorse structures. Rank the conformations in order of decreasing stability. 

One can spot some cases for which little or no rearrangement is required, which would result in facile association reactions. This is the case for the high-spin d complexes with sawhorse structure, which have been discussed in a previous section association of one ligand to give a trigonal bipyramidal complex, or of two ligands to form an octahedral derivative. 

It is no accident that sections of our chaii cyclohexane drawings resemble sawhorse projections of staggered confonnations of alkanes. The same spatial relationships seen in alkanes cany over to substituents on a six-membered ring. In the structure... 

Pyrazines and substituted pyrazines have long been known to act as exo-bidentate ligands to linearly bridge metal ions, and then have also been employed for the self-assembly of polymeric materials. With pyrazine (prz) varying the reaction conditions and stoichiometry, ID, 2D, and 3D structures containing silver(I) atoms with several coordination modes have been prepared.606-609 A 3D racemate is present in [Ag2(2,3-Me2prz)3](SbF6)2.610 A sawhorse connection has been achieved in the compound [Ag(N02)(pyz)] .611 With 2,2 -bipyrazine an infinite loop has been obtained.612... 

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. 

Visualization of conformers There are four conventional methods for visualization of three-dimensional structures on paper. These are the ball and stick method, the sawhorse method, the wedge and broken line method and the Newman projection method. Using these methods, the staggered and eclipsed conformers of ethane can be drawn as follows. 

We have indicated how the enantiomers of 2-butanol differ by drawing their structures 5 and 6 (Section 5-ID) in perspective to show the tetrahedral configuration of substituents at the chiral carbon. This configuration also can be represented by the sawhorse or Newman formulas using any one of the several possible staggered conformations such as 5a and 6a or 5b and 6b ... 

Write structural formulas for substances (one for each part) that fit the following descriptions. Make sawhorse drawings of the substances forwhich conformational problems are involved. 

Whenever a ring has to be constructed, you should consider the possibility of cycloaddition reactions, especially [4 + 2] cycloaddition by the Diels-Alder reaction. A first glance at 9, written in the usual sawhorse-perspective formula, might lead to overlooking the possibility of constructing the skeleton by [4 + 2] addition, because the compound seems only to be made up of five-membered rings. If the structure is rewritten as 10, the six-membered ring stands out much more clearly ... 

Other common methods for representing the three-dimensional structures of molecules include Newman projections for showing conformational relationships and sawhorse figures. Newman projections look down a carbon-carbon bond so that the front carbon, designated by a circle, obscures the carbon directly behind it. Valences (bonds) to the front carbon extend to the center of the circle, while bonds to the rear carbon stop at the circle. Sawhorse projections have the carbon-carbon bond at oblique angles, which attempts to represent a perspective drawing of the molecule. Thus for 2-chloro butane, if one chooses to examine the 2,3 bond, then the sawhorse and Newman projections would be... 

R,R)-TA crystallizes in different enantiomorphous superstructures on Cu( 110), but at a coverage of 0.25 molecules per substrate atom, the monotartrate species forms an achiral c(4 x 2) or (4 0,2 1) structure [71]. In contrast to the bitartrate in its sawhorse geometry, only a single molecular site is connected to the substrate and chirality is not transferred into the lattice structure. Under these conditions, chiral resolution cannot be expected (see below) [72],... 

Draw the structure of raeso-tartaric acid (2,3-dihydroxybutanedioic acid) with an anticlinal conformation in a sawhorse projection. 

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