CHAPTER FOUR Alcohols and Alkyl Halides ygen and chlorine, as well as their unshared electron pairs, contribute to the con- ation of negative charge on these atoms Relatively simple notions of attractive forces between opposite charges are suffi cient to account for many of the properties of chemical substances. You will find it help- ful to keep the polarity of carbon-oxygen and carbon-halogen bonds in mind as we develop the properties of alcohols and alkyl halides in later sections 4.5 PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES NTERMOLECULAR FORCES Boiling Point. When describing the effect of alkane structure on boiling point in Sec tion 2.14, we pointed out that the forces of attraction between neutral molecules are of three types listed here. The first two of these involve induced dipoles and are often referred to as dispersion forces, or London forces. nduced-dipole/induced-dipole forces 2. Dipole/induced-dipole forces 3. Dipole-dipole forces Induced-dipole/induced-dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes. In addition to these forces, polar mol ecules engage in dipole-dipole and dipole/induced-dipole attractions. The dipole-di attractive force is easiest to visualize and is illustrated in Figure 4.3. Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other. As its name implies the dipole/induced-dipole force combines features of both the induced-dipole/induced- dipole and dipole-dipole attractive forces. a polar region of one molecule alters the elec- tron distribution in a nonpolar region of another in a direction that produces an attrac tive force between them Because so many factors contribute to the net intermolecular attractive force, it is not always possible to predict which of two compounds will have the higher boiling point. We can, however, use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features hat influence them Consider three compounds similar in size and shape: the alkane propane, the alco- hol ethanol, and the alkyl halide fluoroethane. CH3CH,CH CH3,OH CH3CH,F Propane(u=0 D) Ethanol (u= 1.7 D) Fluoroethane (u= 1.9 D) p:-42C bp:78°C p:-32C Both polar compounds, ethanol and fluoroethane, have higher boiling points than the nonpolar propane. We attribute this to a combination of dipole/induced-dipole and dipole-dipole attractive forces that stabilize the liquid states of ethanol and fluoroethane, FIGURE 4.3 A dipole-dipole out that are absent in propane. attractive force. Two mole The most striking aspect of the data, however, is the much higher boiling point of ules of a polar substance are ethanol compared with both propane and fluoroethane. This suggests that the attractive oriented so that the posi- forces in ethanol must be unusually strong Figure 4.4 shows that this force results from tively polarized region of a dipole-dipole attraction between the positively polarized proton of the -OH group of one and the negatively po arized region of the other one ethanol molecule and negatively polarized oxygen of another. The term attract each other hydrogen bonding is used to describe dipole-dipole attractive forces of this type. The Back Forward Main Menu Study Guide ToC Student OLC MHHE Websiteto oxygen and chlorine, as well as their unshared electron pairs, contribute to the con￾centration of negative charge on these atoms. Relatively simple notions of attractive forces between opposite charges are suffi- cient to account for many of the properties of chemical substances. You will find it help￾ful to keep the polarity of carbon–oxygen and carbon–halogen bonds in mind as we develop the properties of alcohols and alkyl halides in later sections. 4.5 PHYSICAL PROPERTIES OF ALCOHOLS AND ALKYL HALIDES: INTERMOLECULAR FORCES Boiling Point. When describing the effect of alkane structure on boiling point in Sec￾tion 2.14, we pointed out that the forces of attraction between neutral molecules are of three types listed here. The first two of these involve induced dipoles and are often referred to as dispersion forces, or London forces. 1. Induced-dipole/induced-dipole forces 2. Dipole/induced-dipole forces 3. Dipole–dipole forces Induced-dipole/induced-dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes. In addition to these forces, polar mol￾ecules engage in dipole–dipole and dipole/induced-dipole attractions. The dipole–dipole attractive force is easiest to visualize and is illustrated in Figure 4.3. Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other. As its name implies, the dipole/induced-dipole force combines features of both the induced-dipole/induced￾dipole and dipole–dipole attractive forces. A polar region of one molecule alters the elec￾tron distribution in a nonpolar region of another in a direction that produces an attrac￾tive force between them. Because so many factors contribute to the net intermolecular attractive force, it is not always possible to predict which of two compounds will have the higher boiling point. We can, however, use the boiling point behavior of selected molecules to inform us of the relative importance of various intermolecular forces and the structural features that influence them. Consider three compounds similar in size and shape: the alkane propane, the alco￾hol ethanol, and the alkyl halide fluoroethane. Both polar compounds, ethanol and fluoroethane, have higher boiling points than the nonpolar propane. We attribute this to a combination of dipole/induced-dipole and dipole–dipole attractive forces that stabilize the liquid states of ethanol and fluoroethane, but that are absent in propane. The most striking aspect of the data, however, is the much higher boiling point of ethanol compared with both propane and fluoroethane. This suggests that the attractive forces in ethanol must be unusually strong. Figure 4.4 shows that this force results from a dipole–dipole attraction between the positively polarized proton of the OH group of one ethanol molecule and the negatively polarized oxygen of another. The term hydrogen bonding is used to describe dipole–dipole attractive forces of this type. The Ethanol (  1.7 D) bp: 78°C CH3CH2OH Fluoroethane (  1.9 D) bp: 32°C CH3CH2F Propane (  0 D) bp: 42°C CH3CH2CH3 130 CHAPTER FOUR Alcohols and Alkyl Halides   FIGURE 4.3 A dipole–dipole attractive force. Two mole￾cules of a polar substance are oriented so that the posi￾tively polarized region of one and the negatively po￾larized region of the other attract each other. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website