Conformations of acyclic compounds

Let us consider first the simple alkane ethane. Since both carbons have a tetrahedral array of bonds, ethane may be drawn in the form of a wedge-dot representation. 

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

The two representations shown here are actually two different conformers of ethane there will be an infinite number of such conformers, depending upon the amount of rotation about the C-C bond. Although there is fairly free rotation about this bond, there does exist a small energy barrier to rotation of about 12kJmol due to repulsion of the electrons in the C-H bonds. By inspecting the Newman projections, it can be predicted that this repulsion will be a minimum when the C-H bonds are positioned as far away from each other 

It follows that the preferred conformation of ethane is a staggered one but, since the energy barrier to rotation is relatively small, at room temperature there will be free rotation about the C-C bond. 

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. 

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Acyclic Compounds. Different conformations of acyclic compounds are best viewed by construction of ball-and-stick molecules or by use of Newman projections (see Fig. 1.2). Both types of representations are shown for ethane. Atoms or groups that are attached at opposite ends of a single bond should be viewed along the bond axis. If two atoms or groups attached at opposite ends of the bond appear one directly behind the other, these atoms or groups are described as eclipsed. That portion of the molecule is described as being in the eclipsed conformation. If not eclipsed, the atoms... 

However, note carefuUy that changing the conformation does not affect the spatial sequence about the chiral centres, i.e. it does not change the configuration at either chiral centre. This seems a trivial and rather obvious statement, and indeed it probably is in the case of acyclic compounds. It is when we move on to cyclic compounds that we need to remember this fundamental concept, because a common mistake is to confuse conformation and configuration (see Box 3.11). 

The 191 problems in this book cover most of the area of stereochemistry, including nomenclature, stereogenic elements (centers, axes, planes) and their descriptors, symmetry, inorganic stereochemistry, determination of enantiomer excess, conformation of acyclic and cyclic compounds, and more. The answers, in addition to providing solutions to the problems, frequently include additional explanations of the underlying principles. The problems are ordered more or less in order of increasing difficulty. (I had a hard time with some of the problems toward the end myself )... 

Eliel E, Wilen SH (1994) Stereochemistry of organic compounds. Conformation of acyclic molecules. Wiley, New York, pp 597-664, chap 10... 

You will see why such detailed conformational analysis of acyclic compounds is so important in Chapter 19 on eliminations where the products of the reactions can be explained only by considering the conformations of the reactants and the transition states. But first we want to use these ideas to explain another bl anch of organic chemistry—the conformation of ring structures. 

Among the most widespread classes of acyclic compounds to exhibit stereoelectronic control over conformation are acetals. Take the simple acetal of formaldehyde and methanol, for example what is its conformation An obvious suggestion is to draw it fully extended so that every group is fully anti-periplanar to every other—this would be the lowest-energy conformation of pentane, which you get if you just replace the Os with CH2S. 

The five coordinate spirophosphoranes have played an important role with respect to understanding the stereochemistry of four coordinate phosphorus reactions which proceed via phos-phorane intermediates. Variable temperature NMR studies have established the apicophilicities of a wide range of groups (i.e. relative preferences of a series of groups to occupy an apical position rather than an equatorial position of a trigonal bipyramidal (t.b.p.) conformation of a phosphorane). The spiro ring system has been used to stabilize transient reaction intermediates of acyclic compounds, for example hydrolysis intermediates such as hydroxyphosphoranes. 

In the main, the physical and chemical properties of saturated and partially unsaturated alicyclic compounds closely resemble those of the analogous acyclic compounds formally derived by cleavage of the carbon ring at a point remote from any functionality. Relatively small, but often significant, differences in properties arise from conformational effects, and from strain effects in small rings, and these differences can be striking in properties which are particularly sensitive to molecular shape. 

In the transition state, the torsional strain involving the partially formed bond between the nucleophile and the carbonyl group represents a substantial fraction of the total strain, even when the degree of bonding is low. Thus, in the case of acyclic carbonyl compounds, a staggered conformation is preferred in the transition state (Figure 6). 

However, steric factors are also important.74 Ruchardt and coworkers showed, for a series of acyclic alkyl derivatives, that a good correlation exists between kd and ground state strain.75,76 Additional factors are important for bicyclic and other conformationally constrained azo-compounds.49 51 77 Wolf78 has described a scheme for calculating kd based on radical stability (HOMO Jt-deloealization energies) and ground state strain (stcric parameters). 

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