Boiling points of n-alkanes

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...

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... 

Equivalent Carbon Number Index—an index based on the boiling point of a chemical normalized to the boiling point of n-alkanes or its retention time in a boiling point gas chromatographic column (GC). 

Fig. 11.4 Boiling point of n-alkanes against number of carbon atoms...

TABLE 3.1 Critical properties and normal boiling points of n-alkanes and some selected nonhydrocarbons ... 

Balaban, A.T., N. Joshi, L.B. Kier, and L.H. Hall. 1992a. Correlations between chemical structure and normal boiling points of halogenated alkanes Q - C4. J. Chem. Inf. Comput. Sci. 32 233-37. 

Balaban, A.T., Joshi, N., Kier, L.B. and Hall, L.H. (1992). Correlations Between Chemical Structure and Normal Boiling Points of Halogenated Alkanes C1-C4. J.Chem.lnf.Comput.ScL, 32, 233-237. 

Butane and all succeeding members of the alkanes exhibit structural isomerism. Recall from Section 19.4 that structural isomerism occurs when two molecules have the same atoms but different bonds. For example, butane can exist as a straight-chain molecule (normal butane, or n-butane) or with a branched-chain structure (called isobutane), as shown in Fig. 21.4. Because of their different structures, these molecules exhibit different properties. For example, the boiling point of n-butane is -0.5°C, whereas that of isobutane is -12°C. 

Figure 10.13. Typical variations in surface tension and boiling point for n-alkanes as a function of carbon number.

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. 

The green line in Figure 3-3 represents the boiling points of some branched alkanes. In general, a branched alkane boils at a lower temperature than the n-alkane with the same number of carbon atoms. This difference in boiling points arises because branched alkanes are more compact, with less surface area for London force interactions. 

Most physical properties of alkenes are similar to those of the corresponding alkanes. For example, the boiling points of but-l-ene, m-but-2-ene, fran.v-but-2-ene, and n-butane... 

Alkanes and alkenes are nonpolar compounds whose corresponding structures have almost identical molecular weights. Boiling points of alkenes are close to those of alkanes and similarly have 20° increments per C atom. Both are soluble in nonpolar solvents and insoluble in water, except that lower-molecular-weight alkenes are slightly more water-soluble because of attraction between the n bond and H20. 

Randic, M. and Trinajstic, N. (1994). Isomeric Variations in Alkanes Boiling Points of Nonanes. NewJ.Chem., IS, 179-189. 

FIGURE 7.4 The melting and boiling points of the straight-chain alkanes increase with chain length n. Note the alternation in the melting points Alkanes with n odd tend to have lower melting points because they are more difficult to pack into a crystal lattice. 

In general, the n-alkanes from Cj to C4 are gases, those from C5 to C16 are liquids, and those above are solids. The melting points and boiling points of some of the solid alkanes in tobacco and smoke are summarized in Table I.A-6. 

Table Boiling point relationships for members of the normal alkane series. Here n is the number of carbon atoms in the molecule, i is the boiling point of the n-th member, At the difference in boiling point temperature between the n th and (n + l)-th member of the series, D is density, and a, and 7 are constants.

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