《有机化学》课程教学资源（参考书籍）Solvents and Solvent Effects in Organic Chemistry, Third, Updated and Enlarged Edition, 2003, Christian Reichardt.pdf_P26-P30

similarity is present, the solution of the two components will usually have a structure similar to that of the pure materials (e.g. alcohol-water mixtures [19]). This rule of thumb has only limited validity, however, since there are many examples of solutions of chemically dissimilar compounds. For example, methanol and benzene, water and N,Ndimethylformamide, aniline and diethyl ether, and polystyrene and chloroform, are all completely miscible at room temperature. On the other hand, insolubility can occur in spite of similarity of the two partners. Thus, polyvinyl alcohol does not dissolve in ethanol, acetyl cellulose is insoluble in ethyl acetate, and polyacrylonitrile is insoluble in acrylonitrile [20]. Between these two extremes there is a whole range of possibilities where the two materials dissolve each other to a limited extent. The system water/diethyl ether is such an example. Pure diethyl ether dissolves water to the extent of 15 mg/g at 25 C, whereas water dissolves diethyl ether to the extent of 60 mg/g. When one of the two solvents is in large excess a homogeneous solution is obtained. Two phases occur when the ratio is beyond the limits of solubility. A more recent example of a rea‰rmation of the old ‘‘like dissolves like’’ rule is the di¤erential solubility of fullerene (C60), consisting of a three-dimensional delocalized 60p-electron system, in solvents such as methanol (s ¼ 0:01 mg/mL) and 1-chloronaphthalene (s ¼ 50 mg/mL) [311]. However, rather than the ‘‘like dissolves like’’ rule, it is the intermolecular interaction between solvent and solute molecules that determines the mutual solubility. A compound A dissolves in a solvent B only when the intermolecular forces of attraction KAA and KBB for the pure compounds can be overcome by the forces KAB in solution [21]. The sum of the interaction forces between the molecules of solvent and solute can be related to the so-called polarity*) of A and B. Denoting compounds with large interactions A A or B B, respectively, as polar, and those with small interactions as nonpolar, four cases allowing a qualitative prediction of solubility can be distinguished (Table 2-1). An experimental verification of these simple considerations is given by the solubility data in Table 2-2. Table 2-1. Solubility and polarity [22]. Solute A Solvent B Interaction A A B B A B Solubility of A in B Nonpolar nonpolar weak weak weak can be higha) Nonpolar polar weak strong weak probably lowb) Polar nonpolar strong weak weak probably lowc) Polar polar strong strong strong can be higha) a) Not much change for solute or solvent. b) Di‰cult to break up B B. c) Di‰cult to break up A A. * For a more detailed definition of solvent polarity, see Sections 3.2 and 7.1. 8 2 Solute-Solvent Interactions

10 2 Solute-Solvent Interactions respectively.is a solvent property which measures the work necessary to separate the solvent molecules(ie.disruption and reorganization of solvent/solvent interactions)to create a suitably sized cavity,large enough to accommodate the solute.Accordingly. highly ordered self-associated solvents exhibit relatively large 6 values(=0 for the gas phase).As a rule,it has been found that a good solvent for a certain non-electrolyte has a value close to that of the solute [20.24.25:cf.Table 3-3 in Section 3.2 for a collec. tion of values.Often a mixture of two solvents.one having a value higher and the other having a value lower than that of the solute is a better solvent than each of the two solvents separately 24:cf.also Section 3.2. a nice example demonstrating mutual insolubility due to different values has been described by Hildebrand and was later improved A system of eight miscible liauid lave ructed The eight in of in sities are oil w nilin a s gallium and me rature is r 、lH6 ad ha with mehh and pho pho ss liquid pha of m eral (pa afin oil,methyl silico an oro(N or anot 10 can co our some of the five layers 2.2 Intermolecular Forces 26,27,182-184 ntermolecular for ces are thos These are o calle van der Waals forces,since van der Waals recognized them as the t real gases.Intermole irectiona induct on,and dispersion forces,which are non-specific and cannot be completely satu rated (just as Coulomb forces between ions cannot).The second group consists of hydrogen-bonding forces,and charge-transfer or electron-pair donor-acceptor forces The latter group are specific,directional forces,which can be saturated and lead to stoi chiometric molecular compounds.For the sake of completeness,in the following the Coulomb forces between ions and electrically neutral molecules (with permanent dipole moments)will be considered first,even though they do not belong to the intermolecular forces in the narrower sense. 2.2.1 Ion-Dipole Forces [28,185] Electrically neutral molecules with an unsymmetrical charge distribution possess a per- manent dipole moment 4.If the magnitude of the two equal and opposite charges of this molecular dipole is denoted by g.and the distance of separation l,the dipole moment is given by u=g.1.When placed in the electric field resulting from an ion,the dipole will orient itself so that the attractive end(the end with charge opposite to that of the ion) will be directed toward the ion,and the other repulsive end directed away.The potential energy of an ion-dipole interaction is given by

respectively. d is a solvent property which measures the work necessary to separate the solvent molecules (i.e. disruption and reorganization of solvent/solvent interactions) to create a suitably sized cavity, large enough to accommodate the solute. Accordingly, highly ordered self-associated solvents exhibit relatively large d values (d ¼ 0 for the gas phase). As a rule, it has been found that a good solvent for a certain non-electrolyte has a d value close to that of the solute [20, 24, 25]; cf. Table 3-3 in Section 3.2 for a collection of d values. Often a mixture of two solvents, one having a d value higher and the other having a d value lower than that of the solute is a better solvent than each of the two solvents separately [24]; cf. also Section 3.2. A nice example demonstrating mutual insolubility due to di¤erent d values has been described by Hildebrand [180], and was later improved [181]. A system of eight non-miscible liquid layers was constructed. The eight layers in order of increasing densities are para‰n oil, silicon oil, water, aniline, perfluoro(dimethylcyclohexane), white phosphorus, gallium, and mercury. This system is stable indefinitely at 45 C; this temperature is required to melt the gallium and phosphorus [181]. A simplified, less harmful version with five colourless liquid phases consists of mineral (para‰n) oil, methyl silicon oil, water, benzyl alcohol, and perfluoro(N-ethylpiperidine) (or another perfluoroorganic liquid), in increasing order of density [317]. Addition of an organic-soluble dye can colour some of the five layers. 2.2 Intermolecular Forces [26, 27, 182–184] Intermolecular forces are those which can occur between closed-shell molecules [26, 27]. These are also called van der Waals forces, since van der Waals recognized them as the reason for the non-ideal behaviour of real gases. Intermolecular forces are usually classified into two distinct categories. The first category comprises the so-called directional, induction, and dispersion forces, which are non-specific and cannot be completely saturated ( just as Coulomb forces between ions cannot). The second group consists of hydrogen-bonding forces, and charge-transfer or electron-pair donor–acceptor forces. The latter group are specific, directional forces, which can be saturated and lead to stoichiometric molecular compounds. For the sake of completeness, in the following the Coulomb forces between ions and electrically neutral molecules (with permanent dipole moments) will be considered first, even though they do not belong to the intermolecular forces in the narrower sense. 2.2.1 Ion-Dipole Forces [28, 185] Electrically neutral molecules with an unsymmetrical charge distribution possess a permanent dipole moment m. If the magnitude of the two equal and opposite charges of this molecular dipole is denoted by q, and the distance of separation l, the dipole moment is given by m ¼ q l. When placed in the electric field resulting from an ion, the dipole will orient itself so that the attractive end (the end with charge opposite to that of the ion) will be directed toward the ion, and the other repulsive end directed away. The potential energy of an ion-dipole interaction is given by 10 2 Solute-Solvent Interactions

2.2 Intermolecular Forces 11 Uion-dipole = 13e…4cos0 4·6 (2-2)*割 where so is the permittivity of a vacuum.ze the charge on the ion.r the distance from the ion to the center of the dipole,and 0 the dipole angle relative to the line r joining the ion and the center of the dipole.Cos=1 for 0=0,ie.in this case the dipole nositioned next to the ion in such a way that the ion and the senarated chars of the dipole are linearly arranged or e).Equation (2-2)gives the free energy for the interaction of an ionie charge z.e and a s -called‘poi t-dipole (for which /=0)in vacuun For typical interatomic ngsr≈300-400 th dinole intera ch str the the k.T at 300 K.Fc the novalent sodiun +1.radius =95 dius≈140p =5.9.10-30Cm.th maximum inte energy calculated by Eq.(2-2) ts to U=39k.To 96 kI m01at300K26b ould he called dinalar es p Apart fro a few d h etrachl 31 10-0m ent pos perm en m g Apper -0 trian mide is the on e hig. en 6 follo ane ).The ropyle 67. arge pol mome as the ong. compo syanones p-155 ropyl)syd hig C/3 T )w 6 at 2: C),has a dipo ment of u Cm 10.7D318 peculiar physical propertie such room temperature liquid sydnones make them to good nonaqueous dipolar sol- vents for many ionophores (electrolytes lon-dipole forces are important for solutions of ionic compounds in dipolar sol vents. where solvated species such as Na(OH)and CIH2O)(for solutions of NaC in H2O)exist.In the case of some metal ions,these solvatec table to be considered as discrete species.such as Co(NH or Ag(CH.CN For a comprehensive review on ion/solvent interactions,see reference 241. 2.2.2 Dipole-Dipole Forces 29] Directional forces depend on the electrostatic interaction between molecules possessing a permanent dipole moment u due to their unsymmetrical charge distribution.When two dipolar molecules are optimally oriented with respect to one another at a distance r as shown in Fig.2-3a,then the force of attraction is proportional to 1/r3.An alternative arrangement is the anti-parallel arrangement of the two dipoles as shown in Fig.2-3b. It should be noted that Eas.(2-2)to (2-6)are valid only for an exact application to solu- tions is not possible.Furthermore,Eqs.(2-2)to (2-6)are restricted to cases with

Uion-dipole ¼ 1 4p e0 z e m cos y r2 ð2-2Þ*) where e0 is the permittivity of a vacuum, z e the charge on the ion, r the distance from the ion to the center of the dipole, and y the dipole angle relative to the line r joining the ion and the center of the dipole. Cos y ¼ 1 for y ¼ 0, i.e. in this case the dipole is positioned next to the ion in such a way that the ion and the separated charges of the dipole are linearly arranged ( or ). Equation (2-2) gives the free energy for the interaction of an ionic charge z e and a so-called ‘point-dipole’ (for which l ¼ 0) in vacuum. For typical interatomic spacings (rA300–400 pm), the ion-dipole interaction is much stronger than the thermal energy k T at 300 K. For the monovalent sodium cation (z ¼ þ1, radius ¼ 95 pm) near a water molecule (radiusA140 pm; m ¼ 5:9 1030 Cm), the maximum interaction energy calculated by Eq. (2-2) amounts to U ¼ 39k T or 96 kJ mol1 at 300 K [26b]. Only molecules possessing a permanent dipole moment should be called dipolar molecules. Apart from a few hydrocarbons (n-hexane, cyclohexane, and benzene) and some symmetrical compounds (carbon disulfide, tetrachloromethane, and tetrachloroethene) all common organic solvents possess a permanent dipole moment of between 0 and 18 1030 Cm (i.e. Coulombmeter). Among the solvents listed in the Appendix, Table A-1, hexamethylphosphoric triamide is the one with the highest dipole moment (m ¼ 18:48 1030 Cm), followed by propylene carbonate (m ¼ 16:7 1030 Cm), and sulfolane (m ¼ 16:05 1030 Cm). The largest dipole moments amongst fluids are exhibited by zwitterionic compounds such as the sydnones (i.e. 3-alkyl-1,2,3- oxadiazolium-5-olates). For example, 4-ethyl-3-(1-propyl)sydnone, a high-boiling liquid (tbp ¼ 155 C/3 Torr) with a large relative permittivity (er ¼ 64:6 at 25 C), has a dipole moment of m ¼ 35:7 1030 Cm (¼10.7 D) [318]. The peculiar physical properties of such room temperature liquid sydnones make them to good nonaqueous dipolar solvents for many ionophores (electrolytes). Ion-dipole forces are important for solutions of ionic compounds in dipolar solvents, where solvated species such as Na(OH2) l m and Cl(H2O)m n (for solutions of NaCl in H2O) exist. In the case of some metal ions, these solvated species can be su‰ciently stable to be considered as discrete species, such as [Co(NH3)6] 3l or Ag(CH3CN )l 2...4. For a comprehensive review on ion/solvent interactions, see reference [241]. 2.2.2 Dipole-Dipole Forces [29] Directional forces depend on the electrostatic interaction between molecules possessing a permanent dipole moment m due to their unsymmetrical charge distribution. When two dipolar molecules are optimally oriented with respect to one another at a distance r as shown in Fig. 2-3a, then the force of attraction is proportional to 1=r3. An alternative arrangement is the anti-parallel arrangement of the two dipoles as shown in Fig. 2-3b. * It should be noted that Eqs. (2-2) to (2-6) are valid only for gases; an exact application to solutions is not possible. Furthermore, Eqs. (2-2) to (2-6) are restricted to cases with rg l. 2.2 Intermolecular Forces 11

12 2 Solute-Solvent Interactions 66⊙66⊙ (a) b Unless the dipole molecules are very voluminous,the second arrangement is the more stable one.The two situations exist only when the attractive energy is larger than the thermal energies.Therefore,the thermal energy will normally prevent the dipoles from optimal orientation.If all possible orientations were equally probable,that is,the dipoles correspond to freely rotating molecules,then attraction and repulsion would compensate each other.The fact that dipole orientations leading to attraction are sta tistically favored leads to a net attraction,which is strongly temperature dependent according to Eq.(2-3)(kB Boltzmann constant;T=absolute temperature)[29]. 1 Udipole-dipoe (2-3) As the temperature increase the angle-averaged dipole/dipole interaction energy tive unti t very all di pole onentat s are equa and potentia energ. zm averaged ipole/dipol rac s usually re rred to as the or Ke eraction 29 According 3.3.10 -0. m at 25 This is clearly smaller than the average molar kinetic energy of 3/2 k.T=3.7 kJ.m at the same temperature [26d]. Among other interaction forces,these dipole-dipole interactions are mainly responsible for the association of dipolar organic solvents such as dimethyl sulfoxide [30] or N,N-dimethylformamide [31]. It should be mentioned that dipoles represent only one possibility for the charge arrays in electric multipoles(n-poles).n-Poles with an array of point charges with an n-pole moment (but no lower moment)are n-polar.Thus,a monopole (n=1)is a point charge and a monopole moment represents an overall charge(e.g.of an ion Nat or Cl-).A dipole (n =2;e.g.H2O,H3C-CO-CH3)is an array of partial charges with no monopole moment (i.e.no charge).A quadrupolar molecule(n=4;e.g.CO2,CsH6) has neither a net charge nor a dipole moment and an octupolar molecule (n=8:ea CH4,CCl4)has neither charge nor a dipole or quadrupole moment.In addition to dipole/dipole interactions,in solution there can also exist such higher intermolecular multipole/multipole interactions.Therefore,to some degree,octupolar tetrachloro- methane is also a kind of polar solvent.However,the intermolecular interaction energy rapidly falls off at higher orders of the multipole [26d).The anomalous behaviour of the

Unless the dipole molecules are very voluminous, the second arrangement is the more stable one. The two situations exist only when the attractive energy is larger than the thermal energies. Therefore, the thermal energy will normally prevent the dipoles from optimal orientation. If all possible orientations were equally probable, that is, the dipoles correspond to freely rotating molecules, then attraction and repulsion would compensate each other. The fact that dipole orientations leading to attraction are statistically favored leads to a net attraction, which is strongly temperature dependent, according to Eq. (2-3) (kB ¼ Boltzmann constant; T ¼ absolute temperature) [29]. Udipole-dipole ¼ 1 ð4p e0Þ 2 2m2 1 m2 2 3kB T r6 ð2-3Þ As the temperature increases, the angle-averaged dipole/dipole interaction energy becomes less negative until at very high temperatures all dipole orientations are equally populated and the potential energy is zero. This Boltzmann-averaged dipole/dipole interaction is usually referred to as the orientation or Keesom interaction [29]. According to Eq. (2-3), for pairs of dipolar molecules with m ¼ 3:3 1030 Cm (¼1 D), at a separation of 500 pm, the average interaction energy is about 0.07 kJ mol1 at 25 C. This is clearly smaller than the average molar kinetic energy of 3/2 k T ¼ 3:7 kJ mol1 at the same temperature [26d]. Among other interaction forces, these dipole-dipole interactions are mainly responsible for the association of dipolar organic solvents such as dimethyl sulfoxide [30] or N,N-dimethylformamide [31]. It should be mentioned that dipoles represent only one possibility for the charge arrays in electric multipoles (n-poles). n-Poles with an array of point charges with an n-pole moment (but no lower moment) are n-polar. Thus, a monopole (n ¼ 1) is a point charge and a monopole moment represents an overall charge (e.g. of an ion Naþ or Cl). A dipole (n ¼ 2; e.g. H2O, H3CaaCOaaCH3) is an array of partial charges with no monopole moment (i.e. no charge). A quadrupolar molecule (n ¼ 4; e.g. CO2, C6H6) has neither a net charge nor a dipole moment, and an octupolar molecule (n ¼ 8; e.g. CH4, CCl4) has neither charge nor a dipole or quadrupole moment. In addition to dipole/dipole interactions, in solution there can also exist such higher intermolecular multipole/multipole interactions. Therefore, to some degree, octupolar tetrachloromethane is also a kind of polar solvent. However, the intermolecular interaction energy rapidly falls o¤ at higher orders of the multipole [26d]. The anomalous behaviour of the Fig. 2-3. (a) ‘‘Head-to-tail’’ arrangement of two dipole molecules; (b) Antiparallel arrangement of two dipole molecules. 12 2 Solute-Solvent Interactions