Ethane molecules

Figures 2-24 and 2-25 present the structure of the ethanal molecule and a corresponding Molfile, respectively. The file was extracted from the Enhanced...

Figure 2-25. Wolflle representing the ethanal molecule shown in Figure 2-24.

Schematic illustration of the arrangements of ethane molecules in slits of varying sizes. In the slit of width ochJ tich methyl group is able to occupy a potential minimum from the pore (middle). 

All bonds between equal atoms are given zero values. Because of their symmetry, methane and ethane molecules are nonpolar. The principle of bond moments thus requires that the CH3 group moment equal one H—C moment. Hence the substitution of any aliphatic H by CH3 does not alter the dipole moment, and all saturated hydrocarbons have zero moments as long as the tetrahedral angles are maintained. 

The formalism that we have set up to describe chain flexibility readily lends itself to the problem of hindered rotation. Figure 1.8a shows a sawhorse representation of an ethane molecule in which the angle of rotation around the bond is designated by electron repulsion between the atoms bonded to... 

Figure 1.8 Hindered rotation around a carbon-carbon bond, (a) The definition of (p (from 0 = 0) in terms of the ethane molecule, (b) The potential energy as a function of (p. (c) Here (p is shown (from (p = 0) for a carbon-carbon bond along a polyethylene backbone, (d) The potential energy for case (c) shown as a function of (p. [Panels (b) and (d) reprinted with permission from W. J. Taylor, J.Chem.Phys. 16 257 (1948).]...

Type 4A sieves. A crystalline sodium aluminosilicate with a pore size of about 4 Angstroms, so that, besides water, ethane molecules (but not butane) can be adsorbed. This type of molecular sieves is suitable for drying chloroform, dichloromethane, diethyl ether, dimethylformamide, ethyl acetate, cyclohexane, benzene, toluene, xylene, pyridine and diisopropyl ether. It is also useful for low pressure air drying. The material is supplied as beads, pellets or powder. 

Despite what we ve just said, we actually don t observe perfectly free rotation in ethane. Experiments show that there is a small (12 kj/mol 2.9 kcal/mol) barrier to rotation and that some conformers are more stable than others. The lowest-energy, most stable conformer is the one in which all six C-H bonds are as far away from one another as possible—staggered when viewed end-on in a Newman projection. The highest-energy, least stable conformer is the one in which the six C-H bonds are as close as possible—eclipsed in a Newman projection. At any given instant, about 99% of ethane molecules have an approximately staggered conformation... 

The fact that both heats of formation and equilibrium pressures of the hydrates of spherical molecules correctly follow from one model must mean that the L-J-D theory gives a good account of the entropy associated with the motions of these solutes in the cavities of a clathrate. That the heat of formation of ethane hydrate is predicted correctly, whereas the theoretical value of its vapor pressure is too low, is a further indication that the latter discrepancy must be ascribed to hindered rotation of the ethane molecules in their cavities. 

Ethanol, CH3CH2OH (4), the alcohol of beer and wine, is an ethane molecule in which one H atom has been replaced by an —OH group, and CH3OH (5) is the toxic alcohol called methanol, or wood alcohol. 

What are the symmetry groups of the possible conformations of the ethane molecule ... 

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. 

Unless the temperature is extremely low (-250 °C), many ethane molecules (at any given moment) will have enough energy to surmount this barrier. 

In terms of a large number of ethane molecules, most of the molecules (at any given moment) will be in staggered or nearly staggered conformations. 

This barrier (torsional strain) causes the potential energy of the ethane molecule to rise to a maximum when rotation brings the hydrogen atoms into an eclipsed conformation. 

It has been reported that the degree of dissociation of unsymmetri-cal hexaarylethanes is usually somewhat less than the average of the related symmetrical ethanes.21-22 Unfortunately, the available ethanes do not include any with extremely electronegative and electropositive substituents in opposite parts of the ethane molecule. A marked difference in electronegativity of the two radicals might increase the strength of the central bond by increasing its ionic character. 

The simplest molecule where we can discuss rotation armed a carbon-carbon single bond is ethane. Using the ball and stick model, the ethane molecule can be represented as ... 

The ethane molecule can be constructed by union of two pyramidal methyl radical fragments. The interaction diagram is shown in Fig. 16 and the key stabilizing orbital interactions are depicted below. 

The sigma nonbonded interaction between the two substituents fall into pattern d of Scheme 1. Here, unlike the case of 1,2-difluoroethane, we conclude that there will be a preference for the syn conformation due to the sigma nonbonded interaction of the pi systems of the substituents. This will be counteracted by the inherent preference of any ethane molecule for the staggered geometry and a compromise is expected to be reached in the gauche conformation, barring adverse steric effects. 

On the basis of the above discussion, we are led to the conclusion that sigma nonbonded attractive interaction in the form of a hydrogen bond will tend to favor a syn conformation opposing the inherent preference of ethane molecules for a staggered conformation. A compromise is expected to be reached in the gauche conformation. However, severe steric effects may force an anti conformational preference. 

The importance of hydrogen bonding in determining the preferred conformation of 1,2-disubstituted ethane can be appreciated by reference to the calculations of Pople and his collaborators68. Representative systems were examined and in all cases the most stable conformer was calculated to be the one involving hydrogen bonding between the two vicinal substituents of the ethane molecule. Two typical examples of such structures are shown below. 

Thus methyl radicals are consumed by other methyl radicals to form ethane, which must then be oxidized. The characteristics of the oxidation of ethane and the higher-order aliphatics are substantially different from those of methane (see Section HI). For this reason, methane should not be used to typify hydrocarbon oxidation processes in combustion experiments. Generally, a third body is not written for reaction (3.85) since the ethane molecule s numerous internal degrees of freedom can redistribute the energy created by the formation of the new bond. 

Suppose that we replace one of the hydrogen atoms of methane with a methyl group, —CH3. The resultant molecule, ethane, has the composition C2H6, with molecular mass 30. There are several other simple ways to model the ethane molecule, as well as several rather complex and elegant ways to do so. Let s consider three simple ones. At one extreme, we can write out the bonding pattern in detail and show ethane as... 

The elegant models of three-dimensional protein structures, such as those shown in figure 11.3, fail in one respect they provide a sense of a static molecule in space. As we learned from very simple structures such as ethane, molecules are dynamic, changing conformations in space rapidly. This is surely true for proteins as well... 

A Newman projection is obtained by viewing a molecule along a bond. Take the ethane (or substituted ethane) molecule represented below (a). This is seen in perspective (b). 

Adsorbed ethyl radicals are formed by the dissociative adsorption of ethane. Every ethyl radical may either leave the surface with a deuterium atom to form an ethane molecule or lose one of the three hydrogen atoms of the methyl group to form adsorbed ethylene. The chances of these two events are 1/(1 + P) and P/(l -f P), respectively, P being a constant for a given catalyst. Equal chance is assumed for the loss of each of the three hydrogen atoms of the methyl group in the second process. 

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