1559T_ch08_132-14710/30/0511:59Pa9e138 ⊕ EQA 138.Chapter 8 HYDROXY FUNCTIONAL GROUP:ALCOHOLS:PROPERTIES,PREPARATION,AND STRATEGY OF SYNTHESIS (b)Solubility decreases as the relative size of the nonpolar portion of a molecule increases. 25.Intram shown at right H a H So th ethanol should be more like that of 1.-ethanedio than 1.2-dichlorocthanc.in which hydrogen bonding is absent. H 26.(a)The diequatorial conformation of the diol is shown below.left.It is stabilized in two ways.relative form.First.the OH groups are larger than- andhcrcforc,stcnicsfa rs the ormation to form an internal hydre ().The of this conformation is further lowered by the strength of this hydrogen bond.about 2 keal mol- awi9teasa ()hof the dio in part (a)hodcnt icqutrilrin rale this point,re H osS92 OSi[CH(CH3)2ls Diequatorial OSi[CH(CHa)2 OSi[CH(CH3)2]s CH(CHs) H os OSi[CH(CH3)2l Diaxial 138 • Chapter 8 HYDROXY FUNCTIONAL GROUP: ALCOHOLS: PROPERTIES, PREPARATION, AND STRATEGY OF SYNTHESIS 24. (a) Ethanol hydrogen bonds to water. Chloroethane is attracted to water by dipole forces. Ethane is nonpolar and attracted least to water. (b) Solubility decreases as the relative size of the nonpolar portion of a molecule increases. 25. Intramolecular hydrogen bonding stabilizes the gauche conformation of 1,2-ethanediol, shown at right, but not the anti form. In 2-chloroethanol, similar hydrogen bonding can occur (although it is weaker because of poorer overlap between the large Cl 3p lone pair orbital with the small hydrogen): So the conformational ratio of 2-chloroethanol should be more like that of 1,2-ethanediol than 1,2-dichloroethane, in which hydrogen bonding is absent. 26. (a) The diequatorial conformation of the diol is shown below, left. It is stabilized in two ways, relative to the diaxial form. First, the OOH groups are larger than OH, and therefore sterics favors the diequatorial. Second, the two hydroxy groups are close enough in the trans-1,2-diequatorial con￾formation to form an internal hydrogen bond (below, right). The energy of this conformation is further lowered by the strength of this hydrogen bond, about 2 kcal mol
1 . (b) As the models of the diol in part (a) show, adjacent diequatorial substituents are in rather close proximity. A Newman projection view further illustrates this point, revealing that the groups are in a gauche relationship. Replacement of the two hydroxy hydrogens with very bulky silyl groups makes the diequatorial form less stable than the diaxial, because the 1,2-silyl-silyl steric interfer￾ence is greater than the alternative, two pairs of 1,3-silyl-hydrogen diaxial interactions. In addition, with the hydrogens gone from the oxygen atoms no hydrogen bonding is possible to help lower the energy of the diequatorial form. Compare the structures:  Diequatorial  Diaxial OSi[CH(CH3)2]3 H OSi[CH(CH3)2]3 H H H H H H H OSi[CH(CH3)2]3 OSi[CH(CH3)2]3 OSi[CH(CH3)2]3 OSi[CH(CH3)2]3 H H H OSi[CH(CH3)2]3 OSi[CH(CH3)2]3 H H H O O H H H OH OH H trans-1,2-Cyclohexanediol (both _ OH groups equatorial) O H H H H H Cl O H H H H H HO 1559T_ch08_132-147 10/30/05 11:59 Page 138