o em37 gLiweg Solutions to Problems (e)1-Ethylcyclohutaol,3 ()(-Hmyclodecanol, j)(R-2-Ch 22.CHSiCH.CH.OH △Y CH
Solutions to Problems • 137 lems that follow. The practice will not only help you develop and improve your ability to analyze synthesis problems, it will also help you become more and more familiar with all the reactions and reagents. You will have to know them in the end. SUMMARY CHART Synthesis of alcohols from carbonyl compounds Alcohol product from reaction with Carbonyl compounds NaBH4 or LiAlH4 R''Li or R''MgX Formaldehyde methanol 1° alcohol (HCHO) (CH3OH) (R''CH2OH) Aldehyde 1° alcohol (RCHO) (RCH2OH) Solutions to Problems 21. (a) 2-Butanol, 2° (b) 5-Bromo-3-hexanol, 2° (c) 2-Propyl-1-pentanol, 1° (d) (S)-1-Chloro-2-propanol, 2° (e) 1-Ethylcyclobutanol, 3° (f ) (1R,2R)-2-Bromocyclodecanol, 2° (g) 2,2-Bis(hydroxymethyl)-1,3-propanediol, 1° [“Bis” is used as the prefix instead of “di” when the name that follows is complicated enough to be in parentheses.] (h) meso-1,2,3,4-Butanetetraol, 1° on C1 and C4, 2° on C2 and C3 (i) (1R,2R)-2-(2-Hydroxyethyl)cyclopentanol, 2° on ring, 1° on side chain (j) (R)-2-Chloro-2-methyl-1-butanol, 1° 22. (a) (CH3)3SiCH2CH2OH (b) CH(CH3)2 CH2CH2CH3 A (c) CH3CHOHCHCH2CH2CH3 (d) HOOOH CH3 (e) 23. (a) Cyclohexanol chlorocyclohexane cyclohexane (polarity) (b) 2-Heptanol 2-methyl-2-hexanol 2,3-dimethyl-2-pentanol (branching) OH Br Br CH3 OH COH R R R 3 alcohol CHOH R R 2 alcohol CO R R Ketone CHOH R R 2 alcohol 1559T_ch08_132-147 10/30/05 11:59 Page 137
n of 1.2-et edio 9 ip. er th H OSiCHCHh
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 conformation 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 mol1 . (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 interference 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