Boiling points of haloalkanes

Toropov A, Toropova A (2004) Nearest neighboring code and hydrogen bond index in labeled hydrogen-filled graph and graph of atomic orbitals Application to model of normal boiling points of haloalkanes. J. Mol. Struct. (Theochem) 711 173-183. 

The method of Lydersen [28] is a GCM of this type to estimate the critical temperature, Tc. Other approaches to non-linear GCMs include the model of Lai et al. [29] for the boiling point, Tby and the ABC approach [30] to estimate a variety of thermodynamic properties. Further, artificial neural networks have been used to construct nonlinear models for the estimation of the normal boiling point of haloalkanes [31] and the boiling point, critical point, and acentric factor of diverse fluids [32]. 

Balaban, A. T., et al., Correlation Between Structure and Normal Boiling Points of Haloalkanes C1-C4 Using Neural Networks. J. Chem. Inf. Comput. Sci., 1994 34, 1118-1121. 

Basak, S. C., B. D. Gute, and G. D. Grunwald, Estimation of Normal Boiling Points of Haloalkanes Using Molecular Similarity. Croat. Chim. Acta, 1966 69, 1159-1173. 

Basak, S.C., Gute, B.D. and Grunwald, G.D. (1996b). Estimation of the Normal Boiling Points of Haloalkanes Using Molecular Similarity. Croat.Chem.Acta, 69,1159-1173. 

In Section 6-1 we invoked the polarity of the haloaUcanes to explain why their boiling points are higher than those of the corresponding nonpolar alkanes. The polarity of alcohols is similar to that of the haloalkanes. Does this mean that the boiling points of haloalkanes and alcohols correspond Inspection of Table 8-1 shows that they do not Alcohols have unusually high boiling points, much higher than those of comparable alkanes and haloalkanes. 

The polarizability of an atom (Section 2.8) is a measure of the ease with which its electrons can be distorted in an electric field. The polarizabihty of the halogen atoms increases as we move down the periodic table F < Cl < Br < I. Highly polarizable atoms interact more strongly by van der Waals forces than less polarizable atoms. Therefore, intermolecular forces for haloalkanes increase in the order RF < RCI < RBr < RJ. The effect of intermolecular forces is reflected in the boiling points of haloalkanes, which increase in the same order as the polarizability of their halogen components. 

The boiling points of Isomeric haloalkanes decrease with increase In branching (Unit 12, Class XI). For example, 2-bromo-2-methylpropane has the lowest boiling point among the three Isomers. 

It is interesting to note that boiling points of alcohols and phenols are higher in comparison to other classes of compounds, namely hydrocarbons, ethers, haloalkanes and haloarenes of comparable molecular masses. For example, ethanol and propane have comparable molecular masses but their boiling points differ widely. The boiling point of methojqmiethane is intermediate of the two boiling points. 

The reactivity of haloalkanes in alkylation reactions also decreases with increasing chain length. In general, syntheses of salts with short alkyl substituents are more complex due to the low boiling points of the haloalkanes. The most frequently used halide salt in this field, l-ethyl-3-methylimidazolium chloride ([EMIM] Cl), is typically synthesized in an autoclave with the chloroethane cooled to below its boiling point (12 °C) before addition. 

Boiling points of several low-molecular-weight haloalkanes and the alkanes from which they are derived are given in Table 8.3. Several trends are to be noticed from these data. 

I Table 8.3 Boiling Points of Some Low-Molecular-Weight Alkanes and Haloalkanes... 

Sulfur, because of its large size, its diffuse orbitals, and the relatively nonpolarized S-H bond (Table 1-2), does not enter into hydrogen bonding very efiSciently. Thus, the boiling points of thiols are not as abnormally high as those of alcohols rather, their volatilities lie close to those of the analogous haloalkanes (Table 9-2). 

Table 9-2 Comparison of the Boiling Points of Thiols, Haloalkanes, and Alcohols...

Melting points and boiling points of alkanes Increase with molecular weight the intermolecular interactions are dispersion forces, and these are greatest for linear, unsubstituted species. Intermolecular forces in haloalkanes are dipole-dipole in nature. 

The physical properties of the haloalkanes are quite distinct from those of the corresponding alkanes. To understand these differences, we must consider the size of the halogen substituent and the polarity of the carbon-halogen bond. Let us see how these factors affect bond strength, bond length, molecular polarity, and boiling point. 

The polarity of the C-X bond affects the physical properties of the haloalkanes in predictable ways. Their boiling points are generally higher than those of the corresponding alkanes (Table 6-2), largely because of coulombic attraction between the 8 and 8 ends of C-X bond dipoles in the liquid state (dipole-dipole interaction Figure 2-6). 

In Summary The halogen orbitals become increasingly diffuse along the series F, Cl, Br, 1. Hence, (1) the C-X bond strength decreases (2) the C-X bond becomes longer (3) for the same R, the boiling points increase (4) the polarizability of X becomes greater and (5) London interactions increase. We shall see next that these interrelated effects also play an important role in the reactions of haloalkanes. 

Boiling Points and Densities of Haloalkanes Boiling Points and Solubilities of Alcohols ... 

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