Butane conformational isomers

Small molecules may also form condis crystals, provided they posses suitable conformational isomers, It is of interest to note that several of the organic molecules normally identified as plastic crystals are probably better described as condis crystals. Their motion was, as already shown in Sect. 5.2.2, not the complete reorientation of the presumed rigid molecule, but rather an exchange between a limited number of conformational isomers. The examples treated in Sect. 5.2.2 are 2,3-dimethyl-butane, cyclohexanol and cyclohexane. 

Considerations of minimum overlap of radii of nonbonded substituents on the polymer chain are useful in understanding the preferred conformations of macromolecules in crystallites. The simplest example for our purposes is the polyethylene (1-3) chain in which the energy barriers to rotation can be expected to be similar to those in /i-butane. Figure 4-2 shows sawhorse projections of the conformational isomers of two adjacent carbon atoms in the polyethylene chain and the corresponding rotational energy barriers (not to scale). The angle of rotation is that between the polymer chain substitutents and is taken here to be zero when the two chain segments are as far as possible from each other. 

In Sec. 3.5, we saw that there are several different staggered conformations of /i-butane, each of which lies at the bottom of an energy valley—at an energy minimum—separated from the others by energy hills (see Fig. 3.4, p. 79). Different conformations corresponding to energy minima are called conformational isomers, or conformerS. Since conformational isomers differ from each other only in the way their atoms are oriented in space, they, too, are stereoisomers. Like stereoisomers of any kind, a pair of conformers can either be mirror images of each other or not. 

Butane exists as three conformational isomers, one anti and two gauche (Sec. 3.5). The gauche conformers, II and III, are mirror images of each other, and hence are (conformational) enantiomers. Conformers I and II (or I and III) are not mirror images of each other, and hence are (conformational) diastereomers. 

If we managed to slow down the rapid interconversions in butane (by cooling to very low temperature, for example), we would be able to isolate the three stable conformations—the anti-periplanar and the two synclinal conformations. These different stable conformations of butane are some sort of isomers. They are called conformational isomers or conformers for short. 

Fig. 2.4 Conformational isomers of n-butane (lower row dimethylcyclohexane (only hydrogen atoms adjacent to...

Conformational isomers of butane. The hydrogen atoms are much more crowded in the conformation depicted in (b) compared with the conformation shown in (a). The form shown in (a) is energetically favored. 

Single bonds between C, H, O, N, S, P, and halogen are free to rotate. (Exceptions to this generalization will be discussed as the need arises.) This results in different conformations or conformational isomers for the same molecule. The following are two of the different conformations of the compound butane (C4H,o) ... 

The special change that occurs with the longer molecules (n > 4) is connected with their flexibility, i.e., the possible rotation about the interior bonds of the molecule. It becomes first possible in butane with its three conformational isomers, as illustrated in Fig. 1.37. Figure 5.123 shows that for the paraffins with even n from hexane (n = 6) to dodecane (n = 12), there is an increase in AS 20 J K mol for each pair of additional bonds around which rotation is possible. Similar observations can be made for the paraffins with odd numbers of carbon atoms. The difference in absolute level of AS between the odd and even series arises from differences in crystal packing as was analyzed with thermodynamic parameters in Fig. 4.52 for decane. The few paraffins with exceptionally low AS are, at present, not fully understood. Their low AS may be caused by experimental error caused by incomplete crystallization, mobility in the crystal through mesophase formation, or by differences in the packing in the crystal or stmcture in the melt. 

Rotamers Conformational isomers that can be interconverted by rotation about one or more single bonds (e.g., gauche and anti butane). 

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. 

Fig. 2.31) provides a nice example of conformational analysis, the study of the relative energies of conformational isomers. Let s begin this analysis by constructing a Newman projection by looking down the C(2)—C(3) bond of butane... 

Stereoisomers also include conformational isomers, in which different isomers are generated through rotations about bonds. Conformational isomers are often called con-formers. Eclipsed and staggered ethane are typical examples. Note that a conformational isomer need not be an energy minimum— the eclipsed conformation of ethane is an energy maximum, for example. Conformational isomers can be either enantiomeric or diastereomeric.The two gauche forms of butane are conformational enantiomers, but the gauche and the anti forms of butane are conformational diastereomers (Rg. 4.57). 

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

Although the —CH2— group could be inserted in other places, the free rotation about single C—C bonds in hydrocarbons allows the resulting molecules to be twisted into one or the other of these two isomers. Both compounds are gases, but butane (24) condenses at —1CC, whereas methylpropane (25) condenses at — 12°C. Two molecules that differ only by rotation about one or more bonds may look different on paper, but they are not isomers of each other they are different conformations of the same molecule. Example 18.3 illustrates how to tell if two molecules are different isomers or different conformations of the same isomer. 

A very common approximation goes by the name of rotational isomeric state model (RIS). With such a model it is assumed that the molecular population is placed exclusively in a few energy minima, always according to the Boltzmann distribution. The conformations corresponding to the minima are called conformers, or rotational isomers, and are indicated by one or more letters (G, T) for butane three conformers are considered (G, T, G ), for pentane five (TT, G T, TG, G G, G G ). A more detailed examination of pentane reveals the existence of two further wells (fl, = 65 , 2 = 260° and 0, = 100°, 02 = 295°), sometimes indicated by G G and G G , respectively, close to the forbidden G G" conformation, but with a much tower energy (157, 158). 

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