Butane, chiral conformations

Organic compounds commonly exist as rapidly interconverting chiral conformations. Even ethane is chiral in its skew conformations. When we speak of chirality, however, we intend to focus on observable, persistent properties rather than transient conformations. For example, butane exists in gauche conformations that are chiral, but... 

Even a simple molecule such as //-butane has many chiral conformations. When rotation about the C-C bond is fixed, enantiomers of n-butane exist (Figure 3.1a). Chiral n-butane, even if it could be isolated, readily racemizes through C-C bond rotation with a barrier of around 3 kcal/mol. The corresponding half-life of racemization is expected to be shorter than 10-9 s at —78°C. The molecule behaves as an achiral molecule beyond this time scale. 2,2/-Dimethylbiphenyl is another example of a chiral molecule in a limited time (Figure 3.1//). Enantiomers with axial chirality exist when rotation of the C(1)-C(T) bond is restricted. Racemization takes place through C-C bond... 

It may be observed that the gauche conformation of butane (L) or any other similar molecule is chiral. The lack of optical activity in such compounds arises from the fact that L and its mirror image are always present in equal amounts and interconvert too rapidly for separation. 

The addition of the sodium salt (Nal) to a solution of the camphoroquinone anion radical sodium salt results in enhanced ion association. This association pulls the spin density onto the oxygen. Stevenson et al. (1998) used the camphoroquinone R- or S-conformer for anion radical preparation in the chiral solvent [SS- or RR-2,3-dimethoxy-l,4-bis(dimethy-lamino)butane]. In this case, the interaction of the chiral-solvated cation with the chiral an-... 

These descriptors can be used to specify enantiomeric conformers, such as gauche butane, and the absolute configuration of stereogenic axes and planes. Prelog and Helmchen have recommended the use of P,M instead of R,S for specifying the absolute configuration of planes and axes of chirality [86]. See also ref. [101], Ch. 14. 

Figure 1. Top row staggered conformation of ethane row B the three conformers of -butane (only carbon atoms are shown in this and subsequent drawings) row C the four orientations of vectors (grid coordinates a-d) around the tetrahedral central carbon atom of neopentane row D one of the chiral gauche rotamers, and the achiral anti rotamer of -butane row E chair-cyclohexane, and its Cartesian coordinates X, y, z.

Stereoisomers are either diastereomers or enantiomers therefore, we also can apply these terms to conformational isomers. The gauche and anti forms of butane are diastereomers because they are not mirror images. The two gauche forms of butane are enantiomers because they are mirror images and not superposable (see the reflection in the mirror given). Hence, these forms of butane are chiral. However, butane is not a chiral molecule because these three isomers interconvert very rapidly at room temperature and because they interconvert through the intermediacy of the anti isomer, which is achiral (refer back to Figure 2.9 to see the interconver-sion). The anti isomer is achiral because there is a plane of symmetry when all four carbons are planar. 

Show that butane in a gauche conformation is chiral. Do you expect that resolution of butane at room temperature is possible ... 

There are many chiral molecules for which enantiomeric forms can be interconverted by a rotation about a single bond. The enantiomeric conformations of gauche butane provide an example, where rapid rotation interconverts the two under most conditions. If the rotation that interconverts a pair of such enantiomers is slow at ambient temperature, however, the two enantiomers can be separated and used. Recall from our first introduction of isomer terminology (Section 6.1) that stereoisomers that can be interconverted by rotation about single bonds, and for which the barrier to rotation about the bond is so large that the stereoisomers do not interconvert readily at room temperature and can be separated, are called atropisomers. One example is the binaphthol derivative shown in the margin. It is a more sterically crowded derivative of the biphenyl compound discussed previously as an example of a chiral molecule with no "chiral center". A second example is frans-cyclooctene, where the hydrocarbon chain must loop over either face of the double bond (Eq. 6.4). This creates a chiral structure, and the enantiomers interconvert by moving the loop to the other side of the double bond. 

These two conformations are nonsuperimposable mirror images of each other, and their relationship is therefore enantiomeric. Nevertheless, butane is not a chiral compound. It is optically inactive, because these two conformations are constandy interconverting via single-bond rotation (which occurs with a very low energy barrier). The temperature would have to be extremely low to prevent interconversion between these two conformations. In contrast, a chirality center cannot invert its configuration via single-bond rotations. (i )-2-Butanol cannot be converted into (5)-2-butanol via a conformational change. 

FIGURE 4.30 The gauche form of butane is chiral The mirror images are not superimposable. However, rapid rotation around carbon-carbon single bonds interconverts these two conformational enantiomers. ... 

Prepare models of the four isomeric butenes, C4H8. Note that the restricted rotation about the double bond is responsible for the cis-trans stereoisomerism. Verify this by observing that breaking the n bond of cw-2-butene allows rotation and thus conversion to rran5 -2-butene. Is any of the four isomeric butenes chiral (nonsuperposable with its mirror image) Indicate pairs of butene isomers that are structural (constitutional) isomers. Indicate pairs that are diastereomers. How does the distance between the Cl and C4 atoms in mw5 -2-butene compare with that of the anti conformation of butane Compare the Cl to C4 distance in cw-2-butene with that in the conformation of butane in which the methyls are eclipsed. 

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