Ethane conformational isomers

Figure 11.18 Two conformational isomers of ethane, C2H6. (a) Sawhorse representation and (b) Newman projections.

The staggered and eclipsed forms of ethane are conformational stereoisomers (conformational isomers, conformers) because they have the same molecular formulas and sequences of bonded elements but different spatial arrangements due to rotations around single bonds. (Actually there are an infinite number of conformational isomers (also called conformations) because there are an infinite number of degrees of rotation around the bond, but normally one only needs to be concerned with energy minima and maxima.)... 

Potential applications of molecular visualization include structures and processes not accessible by experiment. For example, [Co(en)3]3+ (en = ethane-1,2-diamine) can adopt four conformational isomers, lel3, lel2ob, ob2lel and ob3 (Fig. 6.1). Crystal... 

Conformational isomers of alkanes arise from the rotation of C-C single bonds. Different shapes of alkanes can be adopted by a molecule like ethane by rotation around the C-C bond. The most distinctive ones (Fig.H) are I and II are known as staggered and eclipsed respectively. In conformation I, the C-H bonds on carbon... 

Because of the cylindrical symmetry of cr bonds (Section 3.8), orbital overlap in the C-C single bond of ethane is exactly the same regardless of the geometric relationships among other atoms attached to the carbons (Figure 4.1). The different arrangements of atoms that result from rotation about a single bond are called conformations, and a specific conformation is called a conformer (conformational isomer). Unlike constitutional isomers, which have different connection. of atoms, different conformers have... 

Conformational isomers differ only in rotations about single bonds. The eclipsed and staggered conformational isomers of ethane, CH3CH3, are shown in Figure 1.11. Although single bonds are drawn as if they were frozen in space, rotation usually has no significant barrier at room temperature (free rotation). Ethane s rotational barrier is tiny, only 3 kcal/mol (13 kJ/mol). One kilocalorie/mol is equal to 4.184 kilojoules/mol. 

Figure 1.11 Views of two conformational isomers of ethane, staggered and eclipsed. The end-on view is called a Newman projection. Below them is a plot of the energy as the C-C bond rotates.

Conformational isomers of ethane (H3C-CH3). Eclipsed and staggered conformations for ethane are possible by virtue of the unrestricted rotation about the carbon-carbon single bond. There is a potential energy difference between the two forms, the staggered form being at the minimum and the eclipsed form at the maximum. 

If two conformational isomers are cooled to a temperature low enough to prevent their interconversion by bond rotation or other motion, then they can be described as stereoisomers structures with the same connectivity but with different atomic arrangements in three dimensions. For example, the conformers in (d) are nonsuperimposable mirror images (enantiomers) if bond rotation is not allowed to occur (make a model ). Those in (g) are not enantiomers, but they are still stereoisomers. The temperatures required to freeze out conformational interconversion are very low on the order of -200°C for substituted ethanes and — 100°C for cyclohexanes. Conformers of these types are occasionally referred to as interconvertible stereoisomers. 

Figure 2.6 shows a ball-and-stick model and a Newman projection for an eclipsed conformation of ethane. In this conformation, the three C—H bonds on one carbon are as close as possible to the three C—H bonds on the adjacent carbon. In other words, hydrogen atoms on the back carbon are eclipsed by the hydrogen atoms on the front carbon (just as the sun is eclipsed when the moon passes in front of it). Different conformations are often called conformational isomers or conformers. 

Even a molecule as simple as ethane, the two-carbon alkane, has a complex structure For example, staggered and eclipsed conformational isomers exist, separated by a low barrier to rotation ( 3 kcal/mol). Because this barrier is so low, there is rapid rotation around the carbon-carbon bond. AH six hydrogens in ethane are equivalent there is only one ethane and only one kind of ethyl derivative. You may need a ball-and-stick model set to convince yourself... 

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). 

IR studies together with 1H NMR have been of importance in deciding the conformations of l,2-di(iV-piperidyl)ethane bis-lV-oxide (270) in its mono- and diprotonated forms (79JST(52)77). In aqueous methylcellosolve solutions the cis isomers (271) and (272) predominate, while in water the equilibrium shifts in favor of the trans isomers (273) and (274). 

Kao and Huang employed molecular mechanics and ab initio molecular orbital (MO) theory to determine the structure, energies, and conformations of the (Z)- (40, R = H) and ( )- (12, R1 - R4 = H) isomers. For the (Z)-isomer, an unsymmetrical conformation (67) is favored by at least 6 kcal over symmetrical chair, boat, and twist forms, which suffer from ethane-type and other H-H repulsions. In the case of ( )-isomer, the twist... 

---