Pentane molecular shape

Figure 12.16 Molecular shape and boiling point. Spherical neopentane molecules make less contact with each other than do cylindrical n-pentane molecules, so neopentane has a lower boiling point.

FIGURE 11.6 Molecular shape affects intermoiecuiar attraction. Molecules of n-pentane make more contact with each other than do neopentane molecules. Thus, n-pentane has stronger intennolecular attractive forces and a higher boiling point. 

For nonpolar substances with the same molar mass, the strength of the dispersion forces is influenced by molecular shape. Shapes that allow more points of contact have more area over which electron clouds can be distorted, so stronger attractions result. Consider the two five-carbon alkanes, pentane (also called n-pentane) and 2,2-dimethylpropane (also called neopentane). These isomers have the same molecular formula (C5H,2) but different shapes. n-Pentane is shaped like a cylinder, whereas neopentane has a more compact, spherical shape, as shown in Figure 12.16. Thus, two n-pentane molecules make more contact than do two neopentane molecules. Greater contact allows the dispersion forces to act at more points, so n-pentane has a higher boiling point. Figure 12.17 summarizes how to analyze the intermolecular forces in a sample. 

A. FIGURE 11.5 Dispersion Force and Molecular Shape (a) The straight shape of n-pentane... 

Dispersion forces also depend on molecular shape. For example, the boiling points of K-pentane and eo-pentane are 36 °C and 10 °C, respectively. With rod-shaped molecules, the dipoles are able to get closer to each other and have a stronger interaction than with essentially spherical entities (Figure 2.9). 

The strength of the London interaction also depends on the shapes of the molecules. Both pentane (8) and 2,2-dimethylpropane (9), for instance, have the molecular formula C5H12, so they each have the same number of electrons. 

O II ch3ch2ch2ch Butanal -99° Butanal has the same molecular mass and same general shape as pentane. It is higher melting than pentane because the dipole-dipole forces of the polar C=0 group hold the molecules together more strongly. 

The early work of Kiselev (1957) revealed that the adsorption isotherms of n-pentane and n-hexane on non-porous quartz were intermediate in character between Types II and m. Values of C(BET) <10 were obtained and the differential enthalpies of adsorption decreased steeply at low surface coverage. More recently, the isotherms of isobutane (at 261 K) and neopentane (at 273 K) on TK800 have been found to be of a similar shape (Carrott et al., 1988 Carrott and Sing, 1989). Unlike those of benzene, these alkane isotherms do not undergo a pronounced change of shape as a result of surface dehydroxylation. This is consistent with the non-specific nature of their molecular interactions (see Chapter 1). 

Despite having the same molecular weight, the rod-shaped pentane molecules have more surface area available for contact between them than the spherical 2,2-dimethylpropane molecules. Pentane molecules, therefore, experience more van der Waals attractions (hence, higher boiling point) than do 2,2-dimethylpropane molecules. 

Both molecules have the same molecular formula, but they have different structures. The London (dispersion) forces between pentane molecules in the liquid or solid states are stronger because the linear molecules have a larger surface area for interaction. In contrast, its isomer, 2,2 dimethylpropane, is more compact (adopting a roughly spherical shape) owing to its extensive branching, and hence has a smaller surface area for interaction (Figure 4.73). 

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