CHAPTER FOUR Alcohols and Alkyl Halides With respect to the halogen in a group of alkyl halides, the boiling point increases as one descends the periodic table; alkyl fluorides have the lowest boiling points, alkyl iodides the highest. This trend matches the order of increasing polarizability of the halo- gens. Polarizability is the ease with which the electron distribution around an atom is distorted by a nearby electric field and is a significant factor in determining the strength of induced-dipole/induced-dipole and dipole/induced-dipole attractions. Forces that depend on induced dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the halogen is a nonpolarizable fluorine The boiling points of the chlorinated derivatives of methane increase with the num- ber of chlorine atoms because of an increase in the induced-dipole/induced-dipole attrac tive forces CHCI CH,CI ne Dichloromethane Trichloromethane Tetrachloromethane ethyl chloride) ( methylene dichloride)(chloroform)(carbon tetrachloride) boiling -24°C 40°C 61°C 77°C Fluorine is unique among the halogens in that increasing the number of fuorines does not produce higher and higher boiling points CH3 CHF CH CHF CH3CF3 CF,CF Fluoroethane 1. 1-Difluoroethane 11.1-Trifluoroethane Hexafluoroethane -25°C 47°C crease with Thus, although the difluoride CH3 CHF2 boils at a higher temperature than CH3CH,F, the trifluoride CH3CF3 boils at a lower temperature than either of them. Even more striking is the observation that the hexafluoride CF3 CF3 is the lowest boiling of any of the fluo- rinated derivatives of ethane. The boiling point of CF3CF3 is, in fact, only 11 higher than that of ethane itself. The reason for this behavior has to do with the very low polar inability of fluorine and a decrease in induced-dipole/induced-dipole forces that accom- panies the incorporation of fluorine substituents into a molecule. Their weak intermole- cular attractive forces give fluorinated hydrocarbons (fluorocarbons) certain desirable physical properties such as that found in the"no stick"Teflon coating of frying pans Teflon is a polymer(Section 6.21)made up of long chains of -CF2CF Solubility in Water. Alkyl halides and alcohols differ markedly from one another in their solubility in water. All alkyl halides are insoluble in water, but low-molecular- weight alcohols(methyl, ethyl, n-propyl, and isopropyl) are soluble in water in all pro- portions. Their ability to participate in intermolecular hydrogen bonding not only affects the boiling points of alcohols, but also enhances their water solubility. Hydrogen-bonded networks of the type shown in Figure 4.5, in which alcohol and water molecules asso- ciate with one another, replace the alcohol-alcohol and water-water hydrogen-bonded networks present in the pure substances Higher alcohols become more hydrocarbon-like" and less water-soluble 1-Octanol, for example, dissolves to the extent of only 1 mL in 2000 mL of water. As the alkyl chain gets longer, the hydrophobic effect(Section 2. 14)becomes more impor- tant, to the point that it, more than hydrogen bonding, governs the solubility of alcohols Density. Alkyl fluorides and chlorides are less dense, and alkyl bromides and iodides Back Forward Main Menu Study Guide ToC Student OLC MHHE WebsiteWith respect to the halogen in a group of alkyl halides, the boiling point increases as one descends the periodic table; alkyl fluorides have the lowest boiling points, alkyl iodides the highest. This trend matches the order of increasing polarizability of the halo￾gens. Polarizability is the ease with which the electron distribution around an atom is distorted by a nearby electric field and is a significant factor in determining the strength of induced-dipole/induced-dipole and dipole/induced-dipole attractions. Forces that depend on induced dipoles are strongest when the halogen is a highly polarizable iodine, and weakest when the halogen is a nonpolarizable fluorine. The boiling points of the chlorinated derivatives of methane increase with the num￾ber of chlorine atoms because of an increase in the induced-dipole/induced-dipole attrac￾tive forces. Fluorine is unique among the halogens in that increasing the number of fluorines does not produce higher and higher boiling points. Thus, although the difluoride CH3CHF2 boils at a higher temperature than CH3CH2F, the trifluoride CH3CF3 boils at a lower temperature than either of them. Even more striking is the observation that the hexafluoride CF3CF3 is the lowest boiling of any of the fluo￾rinated derivatives of ethane. The boiling point of CF3CF3 is, in fact, only 11° higher than that of ethane itself. The reason for this behavior has to do with the very low polar￾izability of fluorine and a decrease in induced-dipole/induced-dipole forces that accom￾panies the incorporation of fluorine substituents into a molecule. Their weak intermole￾cular attractive forces give fluorinated hydrocarbons (fluorocarbons) certain desirable physical properties such as that found in the “no stick” Teflon coating of frying pans. Teflon is a polymer (Section 6.21) made up of long chains of ±CF2CF2±units. Solubility in Water. Alkyl halides and alcohols differ markedly from one another in their solubility in water. All alkyl halides are insoluble in water, but low-molecular￾weight alcohols (methyl, ethyl, n-propyl, and isopropyl) are soluble in water in all pro￾portions. Their ability to participate in intermolecular hydrogen bonding not only affects the boiling points of alcohols, but also enhances their water solubility. Hydrogen-bonded networks of the type shown in Figure 4.5, in which alcohol and water molecules asso￾ciate with one another, replace the alcohol–alcohol and water–water hydrogen-bonded networks present in the pure substances. Higher alcohols become more “hydrocarbon-like” and less water-soluble. 1-Octanol, for example, dissolves to the extent of only 1 mL in 2000 mL of water. As the alkyl chain gets longer, the hydrophobic effect (Section 2.14) becomes more impor￾tant, to the point that it, more than hydrogen bonding, governs the solubility of alcohols. Density. Alkyl fluorides and chlorides are less dense, and alkyl bromides and iodides more dense, than water. 1,1-Difluoroethane 25°C CH3CHF2 1,1,1-Trifluoroethane 47°C CH3CF3 Hexafluoroethane 78°C CF3CF3 Fluoroethane 32°C CH3CH2F Boiling point: Dichloromethane (methylene dichloride) 40°C CH2Cl2 Trichloromethane (chloroform) 61°C CHCl3 Tetrachloromethane (carbon tetrachloride) 77°C CCl4 Chloromethane (methyl chloride) 24°C CH3Cl Boiling point: 132 CHAPTER FOUR Alcohols and Alkyl Halides These boiling points illus￾trate why we should do away with the notion that boiling points always in￾crease with increasing molec￾ular weight. 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