Conformations of Other Alkanes

Propane, the next higher member in the alkane series, also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

The notion of assigning definite energ values to specific interactions within a molecule is a very useful one that weTl return to in the next chapter. A summary of what we ve seen thus far is given in Table 3.5. 

One final point saying that one particular conformer is more stable than another doesn t mean the molecule adopts and maintains only the more stable conformation. At room temperature, rotations around a bonds occur so rapidly that all conformers are in equilibrium. At any given instant, however, a larger percentage of molecules will be found in a more stable conformation than in a less stable one. 

Sighting along the C1-C2 bond of 1-chloropropane, draw Newman projections of the most stable and least stable conformations. 

Problem 3.15 Make a graph of potential energy versus angle of bond rotation for propane, and assign values to the energy ma.xima. 

FIGURE 3.8 Newman projections of propane showing staggered and eclipsed conformations. The staggered con-former is lower in energy by 14 kj/mol. 

CHAPTER 3 ORGANIC COMPOUNDS ALKANES AND THEIR STEREOCHEMISTRY 

As bond rotation continues, an energy minimum is reached at the staggered conformation where the methyl groups are 60° apart. Called the gauche 

The eclipsed coiiformer of propane has three interactions— two ethane-type hydrogen-hydrogen interactions and one additional hydrogen-methyl interaction. Since each eclipsing He—interaction is the same as that in ethane and thus has an energy cost of 4.0 kJ/mol, we can assign a value of 14 (2 X 4.0) = 

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Analysis of the conformations of other alkanes can be done in a similar manner. For any linear alkane the most stable conformation is the so-called zigzag conformation, which is anti about all of the carbon-carbon bonds. The zigzag confonnation for hexane follows ... 

A modification of the united-atom approach, called the anisotropic united-atom (AUA) model was the focus of extensive work by Karabomi et al. [362-365]. As in the other models of hydrocarbon chains described so far, the AUA approach to monolayers was preceded by work on alkanes [367]. hi the AUA model the interaction site is located at the geometrical mean of the valence electrons of the atoms it represents, while the pseudoatom itself is located at the carbon atom position. The movement of each interaction center depends on the conformation of the molecule as a whole. 

The elution volumes for n-hydrocarbons show a straight line relationship vs the logarithms of their molar volumes. Molar volumes, calculated from the densities of compounds other than n-hydrocarbons, must be modified to have the elution volumes of these compounds conform to the same calibration line (elution volume vs log molar volume) as that for the n-hydrocarbons. W. W. Schulz (1 ) related the elution behavior of branched alkanes in the range of Cy-C] ] to the average numbers of gauche arrangements (Zg) which the molecule can assume. 

The configuration-bias Monte Carlo (CB-MC) technique (112) has also been extensively applied to characterize the sorption of alkanes, principally in silicalite (111, 156, 168-171) but also in other zeolites (172-174). Smit and Siepmann (111, 168) presented a thorough study of the energetics, location, and conformations of alkanes from n-butane to n-dodecane in silicalite at room temperature. A loading of infinite dilution was simulated, based on a united-atom model of the alkanes and a zeolite simulation box of 16 unit cells. Potential parameters were very similar to those used in the MD study of June et al. (85). As expected, the static properties (heat of adsorption, Henry s law coefficient) determined from the CB-MC simulations are therefore in close agreement with the values of June et al. The... 

A series of other condis crystals are the larger cyclo-alkanes, analyzed by Gross-mann 171). Figure 23 illustrates the transition behavior of cyclotetracosane and cyclo-hexanonacontane. The melting transition of cyclotetracosane has only about 1/4 the heat of transition at the disordering to the condis phase. The condis phase has a much higher symmetry and fewer X-ray diffraction lines. Infrared and Raman spectroscopy indicate that practically no additional conformational isomers are introduced on... 

There is free rotation around carbon-carbon single bonds. Therefore, alkane chains are quite flexible and can adopt a large number of conformations. For example, two conformations of butane are shown in Figure 11.5. The first structure converts into the second by rotation around the C2-C3 bond. These two structures are not isomers of each other because it is not possible to separate them. 

Many applications have been reported in the field of biomolecular NMR spectroscopy which use RDCs for the refinement of three-dimensional structures. The approach is quite powerful and can also be applied to smaller molecules whenever the conformation of a molecule is important, as for example in the case of rational drug design. Traditionally, NMR in liquid crystals is applied on a multitude of small organic compounds to obtain their fully characterized structure. Most examples are measured on all kinds of aromatic systems as reported in refs. 204—212 other recent examples deal with substituted alkanes, aldehydes216,217 or bridged systems like norbomadiene.218 In general, these very detailed studies can be applied to molecules with up to 12 protons. 

Triplet excimers of aromatic hydrocarbons have proved very difficult to detect and hence their role in deactivation of excited states is largely speculative. However, on the basis of emission experiments (Subudhi and Lim, 1976 Okajima et al, 1977 Chandra and Lim, 1977 Webster et ai, 1981), it has been suggested that some di(l-naphthyl)alkanes form such species. It is suggested that the favoured conformation of the triplet excimer does not have the two naphthalene rings lying parallel to each other. 

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