Neopentane, boiling point

There are three forms of pentane the straight-chain normal pentane (a = 2), the branched isopentane (a = 1), and the doubly branched neopentane (a = 12) in the shape of a tetrahedron. Figure 4.37 shows that normal pentane has the highest density and boiling point, which can be attributed to the superior packing efficiency of linear molecules isopentane, in comparison, has lower density and lower melting and boiling points. Neopentane does not follow the usual rules it has the lowest... 

Determine which compound has the higher boiling point, neopentane or 3-hexanol ... 

Another interesting effect seen in alkanes is that increased branching lowers an alkane s boiling point. Thus, pentane has no branches and boils at 36.1 3C, isopentane (2-methylbutane) has one branch and boils at 27.85 °C, and neopentane (2,2-dimethylpropane) has two branches and boils at 9.5 °C. Similarly, octane boils at 125.7 °C, whereas isooctane (2,2,4-trimethylpentane) boils at 99.3 °C. Branched-chain alkanes are lower-boiling because they are more nearly spherical than straight-chain alkanes, have smaller surface areas, and consequently have smaller dispersion forces. 

These isomers are both nonpolar. Therefore, another factor, the shape of the molecule, influences the boiling point. The shape of n-pentane is rodlike, whereas that of neopentane is spherelike. Rods can touch along their entire length spheres touch only at a point. The more contact between molecules, the greater the London forces. Thus, the boiling point of n-pentane is higher. 

The total pore volume of the voids was determined from the amount of adsorbed water at saturation assuming that the water is present as a normal liquid with an average density assumed to be that of liquid water. Other adsorbates utilized in this study include the gases oxygen and nitrogen at their respective boiling points and n-butane and neopentane at room temperature. 

To predict relative boiling points, we should look for differences in (1) hydrogen bonding, (2) molecular weight and surface area, and (3) dipole moments. Except for neopentane, these compounds have similar molecular weights. Neopentane is the lightest, and it is a compact spherical structure that minimizes van der Waals attractions. Neopentane is the lowest-boiling compound. 

Problem 2.16 The boiling points of n-pentane and its isomer neopentane are 36.2 C and 9.5"C, respectively. Account for this difference (see Problem l.4 for the structural formulas.) -4... 

There are three different structures of pentane n-pentane, isopentane and neopentane. Their boiling points are 36°C, 28°C and 9.5°C respectively. Explain this difference. 

The shape of the molecule also affects the boiling point, the more symmetrical compounds having lower boiling points. In a series of unassociated molecules like K-pentane, isopentane, and neopentane (Table 1-5), the effect is probably a result of decreased van der Waals attraction in going from a straight chain to a spherical mole-... 

Each of the smaller alkanes, from methane through Ai-pentane and isopentane, can be obtained in pure form by fractional distillation of petroleum and natural gas neopentane does not occur naturally. Above the pentanes the number of isomers of each homolog becomes so large and the boiling point differences become so small that it is no longer feasible to isolate individual, pure compounds these alkanes must be synthesized by one of the methods outlined below. 

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. 

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