Bond rotation, alkanes butane

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. 

In addition to the locations of the double bonds, another difference of alkenes is the molecule s inability to rotate at the double bond. With alkanes, when substituent groups attach to a carbon, the molecule can rotate around the C-C bonds in response to electron-electron repulsions. Because the double bond in the alkene is composed of both sigma and pi bonds, the molecule can t rotate around the double bond (see Chapter 6). What this means for alkenes is that the molecule can have different structural orientations around the double bond. These different orientations allow a new kind of isomerism, known as geometrical isomerism. When the non-hydrogen parts of the molecule are on the same side of the molecule, the term cis- is placed in front of the name. When the non-hydrogen parts are placed on opposite sides of the molecule, the term trans- is placed in front of the name. In the previous section, you saw that the alkane butane has only two isomers. Because of geometrical isomerism, butene has four isomers, shown in Figure 19.12. 

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

Conformational isomerism in butane Butane is a four-carbon- (sp -hybridized) atom-containing linear alkane. All are tetrahedrally arranged. When a hydrogen atom of propane is replaced by a methyl (CH3) group, we have butane. There is rotation about two C-C cr bonds, but the rotation about C2-C3 is the most important. 

Butane (C4H10) can exist in two different isomeric forms, e.g. n-butane and isobutane (2-methylpropane). Open chain alkanes have free rotation about their C—C bonds, hut cycloalkanes cannot undergo free rotation, so substituted cycloalkanes can give rise to cis and trans isomers (see Section 3.2.2). 

Th conformational aituation becomes more complex a the alkane becomes larger. In butane, for nstarH e. a plot of potential energy versus rotation about the C2-C3 bond is shown in Fignre 4.6. 

Butane and higher molecular weight alkanes have several carbon-carbon bonds, all capable of rotation. 

FIGURE 4.1 3D shapes of some alkanes using balls (for atoms) and sticks (for bonds) to depict (a) ethane, (b) propane, and (c) butane, (d) The dynamics of ethane as it rotates around the C—C bond and changes the relative orientations of the C—H bonds. The variable location of the distinctly colored H emphasizes the rotation. 

Figure 2.1 Potential energy for rotation about a C—C bond in an alkane. The minima correspond togauche ((f> = —120°), trans ((/> = 0°) andgauche (0 = 120°) conformers, as represented by the Newman projections shown for butane...

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