CHAPTER 2 Bonding and Molecular Structure 23 Attractive forces betv such molecules can mutually mix and solution is easy.The attractive forces between polarH.orC.H.OH mol. ecules are strong H-bonds.Most nonpolar molecules cannot overcome these H-bonds and therefore do not dis- solve in such polar protic solvents. Problem 2.19 Explain why CH,CH,OH is much more soluble in water than is CH (CH,),CH,OH. Problem 2.20 Explain why NaCl dissolves in water Water.a protie solvent,helps separate the strongly attracting ions of the solid salt by solvation.Several negative ends o Hrcmdn CI-- Nat. and H Problem 2.21 Compare the ways in which NaCl dissolves in water and in dimethyl sulfoxide. also solvates positive ions by an ion-dipole attraction;the Oo the group is d to is surrounded hy the methyl o and cannot get close enough to solvate the anion The bare negative ions discussed in Problem 2.21 have a greatly enhanced reactivity.The small amounts of salts that dissolve in nonpolar or weakly polar solvents exist mainly as ion-pairs or ion-clusters.where the oppo sitely charged on ose to eacn other and move a as units.I etween ion-pairs are separated by a small number of s hloanpiteheenosolemtmolecules 2.8 Resonance and Delocalized x Electrons Resonance theory describes species for which a single Lewis electron structure cannot be written.As an example,consider dinitrogen oxide,N,O: 放==0： resonance N=N-0: 0.1200.115 0.1100.147 Observed 0.1120.119 0.1120.119 Bond Length on of the calculated and observed bond leng s show that neithe ture is tributing (resonance s tell us th ance hybrid ha character between Nand and some triple-bond character between Nand N.This state of affairs is described by the non-Lewis structure: :=i6Attractive forces between nonpolar molecules such as mineral oil and n-hexane are very weak. Therefore, such molecules can mutually mix and solution is easy. The attractive forces between polar H2 O or C2 H5 OH mol￾ecules are strong H-bonds. Most nonpolar molecules cannot overcome these H-bonds and therefore do not dis￾solve in such polar protic solvents. Problem 2.19 Explain why CH3CH2OH is much more soluble in water than is CH3(CH2)3CH2OH. The OH portion of an alcohol molecule tends to interact with water—it is hydrophilic. The hydrocarbon portion does not interact. Rather, it is repelled—it is hydrophobic. The larger the hydrophobic portion, the less soluble in water is the molecule. Problem 2.20 Explain why NaCl dissolves in water. Water, a protic solvent, helps separate the strongly attracting ions of the solid salt by solvation. Several water molecules surround each positive ion (Na+) by an ion-dipole attraction. The O atoms, which are the negative ends of the molecular dipole, are attracted to the cation. H2O typically forms an H-bond with the negative ion (in this case Cl). CHAPTER 2 Bonding and Molecular Structure 23 Problem 2.21 Compare the ways in which NaCl dissolves in water and in dimethyl sulfoxide. The way in which NaCl, a typical salt, dissolves in water, a typical protic solvent, was discussed in Problem 2.20. Dimethyl sulfoxide also solvates positive ions by an ion-dipole attraction; the O of the SO group is attracted to the cation. However, since this is an aprotic solvent, there is no way for an H-bond to be formed and the negative ions are not solvated when salts dissolve in aprotic solvents. The S, the positive pole, is surrounded by the methyl groups and cannot get close enough to solvate the anion. The bare negative ions discussed in Problem 2.21 have a greatly enhanced reactivity. The small amounts of salts that dissolve in nonpolar or weakly polar solvents exist mainly as ion-pairs or ion-clusters, where the oppo￾sitely charged ions are close to each other and move about as units. Tight ion-pairs have no solvent molecules between the ions; loose ion-pairs are separated by a small number of solvent molecules. 2.8 Resonance and Delocalized π Electrons Resonance theory describes species for which a single Lewis electron structure cannot be written. As an example, consider dinitrogen oxide, N2 O: A comparison of the calculated and observed bond lengths show that neither structure is correct. Neverthe￾less, these contributing (resonance) structures tell us that the actual resonance hybrid has some double-bond character between N and O, and some triple-bond character between N and N. This state of affairs is described by the non-Lewis structure: