Boiling point of alkyl halides

Explain why the boiling points of alkyl halides increase in order going down the column of halides in the periodic table, from fluorine through iodine. 

Boiling points of alkyl halides increase as the size (and polarizability) of X increases. Remember stronger intermolecular forces = higher boiling point. 

The low polarizability of fluorine and the relatively weak attractive forces between fluorine atoms suggest that the boiling points of fluorinated compounds should be lower than those of other halocarbons. Indeed, the boiling points of alkyl halides increase in the order H[Pg.94]

Ans. The higher boiling points (and also melting points) of alkyl halides arise from their higher molecular mass coupled with the presence of dipole-dipole attractive forces. The dipole-dipole attractions is a result of the polar C—Cl bond. Cl is more electronegative than C. The Cl end of the C—Cl bond is partially negative (5 — ) and the C end is partially positive (S -I-). Oppositely charged ends of adjacent dipoles attract each other as indicated by the dashed line ... 

Table 4 2 lists the boiling points of some representative alkyl halides and alcohols When comparing the boiling points of related compounds as a function of the alkyl group we find that the boiling point increases with the number of carbon atoms as it does with alkanes... 

For a discussion concerning the boiling point behavior of alkyl halides see the January 1988 issue of the Journal of Chemical Education pp 62-64... 

Boiling Points of Some Alkyl Halides and Alcohols... 

Solubility in Water Alkyl halides and alcohols differ markedly from one another m their solubility m water All alkyl halides are insoluble m water but low molecular weight alcohols (methyl ethyl n propyl and isopropyl) are soluble m water m all pro portions Their ability to participate m mtermolecular hydrogen bonding not only affects the boiling points of alcohols but also enhances their water solubility Hydrogen bonded networks of the type shown m Figure 4 5m which alcohol and water molecules asso ciate with one another replace the alcohol-alcohol and water-water hydrogen bonded networks present m the pure substances... 

This classical C-P bond-forming reaction (51) has seen limited application in the glyphosate arena, presumably for lack of suitable substrates that can tolerate the vigorous reaction conditions. Typically, C-P bond formation occurs when an alkyl halide reacts with excess neat trialkyl phosphite at temperatures exceeding 100 °C, near the boiling point of the phosphite. An Arbuzov-based strategy for glyphosate requires the synthesis of the... 

The physical properties of unsubstituted aryl halides are much like those of the corresponding alkyl halides. Thus, boiling points, melting points, and solubilities of aryl halides are very similar to those of alkyl halides containing the same number of carbon atoms. 

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. 

The carbon-halogen bond is slightly polar. Overall, however, an alkyl halide is not much more polar than an alkane, so the physical properties of an alkyl halide are not very different from those of an alkane of similar molecular weight. For example, the boiling point of 1-chlorobutane (MW = 92.5 g/mol) is 78°C, whereas that of hexane (MW = 86 g/mol) is 69°C. In general, alkyl halides are insoluble in water. Because of the presence of the more massive halogen atom, the alkyl halide may be more dense than water. For example, when dichloromethane, a common laboratory solvent, and water are mixed, two layers are formed, with dichloromethane as the lower layer. 

Alcohols and phenols are quite different from the hydrocarbons and alkyl halides we ve studied thus far. Not only is their chemistry much richer, their physical properties are different as well. Figure 17.1, which provides a comparison of the boiling points of some simple alcohols, alkanes, and chloroalkanes, shows that alcohols have much higher boiling points. For example, 1-propanol (MW = 60), butane (MW - 58), and chloroethane (MW = 65) have similar molecular weights, yet 1-propanol boils at 97"C, compared to 0.5°C tor the alkane and 12.5°C for the chloroalkane. 

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