Boiling points of the alkanes

For example, the melting and boiling points of the alkanes shown in Table 14.1 gradually increase. This is due to an increase in the intermolecular forces (van der Waals forces) as the size and mass of the molecule increases (Chapter 3, p. 49). 

The boiling points of the alkanes and alkenes follow a similar pattern, but the melting points are slightly different. Can you offer any possible explanation You might like to compare the melting and boiling points of... 

Using data In the following table, plot a graph of the number of carbon atoms contained In the straight chain alkane (x axis) against boiling point of the alkane y axis). Draw a curve to Join up the points. 

As you can see fromTable 3.4, each CH2 group added to the carbon chain of an alkane increases the boiling point of the alkane. The increase is greater going... 

A (a) How do the melting points and boiling points of the alkanes vary with molecular weight (b) Do you expect them to vary in this order (c) Why or why not Use intermolecular forces to explain your answer. 

As the chain length increases, the boiling point of the alkane increases. 

The regular increase in boiling point of the alkanes with increasing chain length is of immense commercial importance. It is the basis of the initial refinery process of fractional distillation whereby crude oil is separated into useful fractions (Figures 10.10 and 10.11). Crude oil (or petroleum) is the major commercial source of hydrocarbons for a variety of uses as fuels and chemical feedstock. Figure 10.11 shows the components present in the major fractions obtained by distillation at an oil refinery. 

Fig. 3.12 Boiling points of alkanes (a), perfluoroalkanes (b), and the difference between the boiling points (boiling point of the alkane minus that of the perfluo-roalkane) (c). (From Ref. 19. Reproduced by permission of The Macmillan Co.)... 

FIGURE 2 12 Boiling points of unbranched alkanes and their 2 methyl branched iso mers (Temperatures in this text are expressed in de grees Celsius C The SI unit of temperature is the kelvin K To convert degrees Celsius to kelvins add 273 15 )... 

This will generally be tr-ue as we proceed to look at other alkanes as the number of carbon atoms increases, so does the boiling point. All the alkanes with four car bons or less are gases at room temperature and atmospheric pressure. With the highest boiling point of the three, propane is the easiest one to liquefy. We are all faniliar- with propane tanks. These are steel containers in which a propane-rich mixture of hydrocar bons called liquefied petroleum gas (LEG) is maintained in a liquid state under high pressure as a convenient clean-burning fuel. 

FIGURE 18.4 The melting and boiling points of the unbranched alkanes from CH4 to C]()H, 4. 

Table 1.10 compares the boiling points of an alkane, an alcohol, and an aldehyde with the same number of carbon atoms. You can see that the boiling point of an alcohol is much greater than the boiling point of an alkane or an aldehyde. 

The central dogma of chemistry is, that the structure of a molecule correlates with its physico-chemical properties. This is usually illustrated using the correlation between the boiling point of n-alkanes and the number of carbon atoms... 

Fig. 1 The correlation between the boiling points of n-alkanes and number of carbon atoms in the molecule...

Boiling points of unbranched alkanes are related to their size the greater their size the greater their boiling point. The relative increase in size per CH2 group is greatest between CH4 and... 

O-H-O bonds are stronger than N-H - N bonds, which explains the order of the last two compounds. No hydrogen bonding is possible in the first two compounds, but C-O bonds are polar, giving the ether a higher boiling point than the alkane. The actual boiling points are pentane, 36°C methyl propyl ether, 39°C 1-aminobutane, 78°C and 1-butanol, 118°C. 

Alkane boiling points. The boiling points of the unbranched alkanes (blue) are compared with those of some branched alkanes (green). Because of their smaller surface areas, branched alkanes have lower boiling points than unbranched alkanes. 

Comparison of the Boiling Points of Ethers, Alkanes, and Alcohols of Similar MolecularWeights... 

An explanation of the general rule that the boiling point falls with increasing degree of branching follows directly from this consideration, in that the clew becomes denser so that the boiling point of the normal compound is always the highest (Table 35). In the above formula, B is equal to —12500 for 2-methyl-alkanes. 

By conducting a calibration curve based on the boiling points of n-alkanes, it is possible to estimate the distillation temperatures according to the ASTM D2887 method. Figures 4 and 5 show the calibration and the... 

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