Molecular polarity boiling point effect

The concept of asphaltenes is rooted in the solubility behavior of high-boiling hydrocarbonaceous materials in benzene and low-molecular-weight n-paraffin hydrocarbons. This behavior is a result of physical chemistry effects that are caused by a spectrum of chemical properties. This chapter has pointed out that by considering molecular weight and molecular polarity as separate properties of molecules, the solvent-precipitation behavior of materials derived from various carbonaceous sources can be understood. Future quantification of this approach probably can be achieved by developing a polarity scale based on solubility parameter. 

The general properties of the significantly covalent behavior of metal alkoxides (both homo- and heterometallic), in spite of the polar character of the M5+—O5- bond, were already dealt with in some detail (6). The effect of steric and inductive factors on the extent of polarization of the M6+—O5- bond as well as the consequent degree of association and volatility can be exemplified by the boiling points (under 1-mm pressure) and the observed degrees of their association (given in parentheses) by the three isomeric butoxides of zirconium Zr(0-n-Bu)4 ( 250°C 3.5) Zr(0-sec-Bu)4 ( 150°C 2.0 Zr(0-/-Bu)4 ( 50°C 1.0). However, the similarities in the molecular association of the neopentyloxides of Ti, Zr, and A1 to the secondary rather than primaiy amy-loxides have been adduced to indicate the higher predominance of steric rather than inductive factors in the above directions. Similarly, the insolubility and... 

Knowing the shape of a substance s molecules is a key to understanding its physical and chemical behavior. One of the most important and far-reaching effects of molecular shape is molecular polarity, which can influence melting and boiling points, solubility, chemical reactivity, and even biological function. 

To see the effect of dipole—dipole forces, we compare the boiling points of two compounds of similar molecular weight acetonitrile (CH3CN, MW 41 amu, bp 355 K) and propane (CH3CH2CH3, MW 44 amu, bp 231 K). Acetonitrile is a polar molecule, with a dipole moment of 3.9 D, so dipole—dipole forces are present. However, propane is essentially nonpolar, which means that dipole—dipole forces are absent. Because acetonitrile and propane have similar molecular weights, dispersion forces are similar for these two molecules. Therefore, the higher boiling point of acetonitrile can be attributed to dipole-dipole forces. 

In the experiment that follows, the hydration of a terminal alkyne is illustrated by the conversion of 2-methyl-3-butyn-2-ol (3) to 3-hydroxy-3-methyl-2-butanone (4), as shown in Equation 11.13. The presence of a hydroxyl group in 3 has little effect on the chemical properties of the carbontriple bond. Rather, the main effect of the polar hydroxyl group is on the physical properties of the molecule, with the boiling point of 3 being considerably higher than those of other acetylenic hydrocarbons having the same molecular weight. 

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