Solutions 80 CHAPTER a) trans b) not stereoisomeric c) trans d) trans e) trans f) not stereoisomeric g) cis

Transcription

1 80 CAPTE 5 killbuilder 5.9 Assigning configuration from a Fischer projection AIG TE CFIGUATI F TE CIALITY CETE I TE FLLWIG CMPUD C 2 olutions 5.1. trans not stereoisomeric trans trans trans f) not stereoisomeric g) cis CCC 2 C 2 C 2 CC 2 = either double bond exhibits stereoisomerism, so this compound does not have any stereoisomers All chirality centers are highlighted below:

2 CAPTE 5 81 f) 5.5. chirality center 5.6. The phosphorus atom has four different groups attached to it (a methyl group, an ethyl group, a phenyl group, and a lone pair). This phosphorous atom therefore represents a chirality center. This compound is not superimposable on its mirror image, as can be seen clearly by building and comparing molecular models C3 f) g) 5.8.

3 82 CAPTE f) P specific rotation = [] = specific rotation = [] = c l ( º ) ( g / ml) (1.00 dm) = c l ( 2.99º ) (0.095 g / ml) (1.00 dm) = specific rotation = [] = c l = ( º ) (0.260 g / ml) (1.00 dm) = This compound does not have a chirality center, because two of the groups are identical: Accordingly, the compound is achiral and is not optically active.

4 CAPTE [] = c l = [] c l = (+13.5)(0.100 g / ml)(1.00 dm) = º % ee = observed [ ] [ ] of pure enantiomer = ( - 37 ) ( ) = 94 % % ee = observed [ ] [ ] of pure enantiomer = ( ) ( ) = 95 % % ee = observed [ ] [ ] of pure enantiomer = ( 85 ) ( 92 ) = 92 %

5 84 CAPTE bserved [] = c l = ( º ) (0.350 g / ml) (1.00 dm) = +2.2 % ee = observed [ ] [ ] of pure enantiomer = ( 2.2 ) ( 2.8 ) = 79 % enantiomers diastereomers diastereomers diastereomers diastereomers f) enantiomers There are three chirality centers, and only one of these chirality centers has a different configuration in these two compounds. The other two chirality centers have the same configuration in both compounds. Therefore, these compounds are diastereomers yes yes no yes yes f) no f has three planes of symmetry f)

6 CAPTE Each of these compounds is a meso compound and does not have an enantiomer There are only four stereoisomers: not a chirality center (see problem 5.5) meso meso not a chirality center (see problem 5.5) 5.29.

7 86 CAPTE C 2 C 2 C C 2 C 2 C F C Paclitaxel has eleven chirality centers. The enantiomer of paclitaxel is shown below:

8 CAPTE trans not stereoisomeric not stereoisomeric enantiomers same compound constitutional isomers constitutional isomers diastereomers f) same compound g) enantiomers h) diastereomers i) same compound j) same compound k) same compound l) same compound f) Et F F f) g) C 3 h) C 2 i) j) k) l)

9 88 CAPTE Et 2 Et F f) g) h) i) % ee diastereomers diastereomers enantiomers same compound enantiomers f) diastereomers g) enantiomers h) diastereomers i) enantiomers j) same compound k) enantiomer l) diastereomers % ee = observed [ ] [ ] of pure enantiomer = ( -55 ) ( -61 ) = 90 %

10 CAPTE True. False. True specific rotation = [] = c l = ( 0.47º ) ( g / ml) (1.00 dm) = ()-limonene ()-limonene ()-limonene ()-limonene C C C 3 C 3 C 3 C The first compound has three chirality centers: chirality center three chirality centers two chirality centers This is apparent if we assign the configuration at C1 and C3 of the cyclohexane ring. In the first compound, the configuration at C1 is different than the configuration at C3. As a result, there are four different groups attached to the C2 position. That is, C1 and C3 represent two different groups: one with the configuration and the other with the configuration. In contrast, consider the configuration at C1 and C3 in the second compound. Both of these positions have the same configuration, and therefore, the C2 position in that compound does not have four different groups. Two of the groups are identical, so C2 is not a chirality center.

11 90 CAPTE enantiomers diastereomers enantiomers same compound enantiomers f) diastereomers g) same compound h) constitutional isomers i) diastereomers j) diastereomers k) same compound l) enantiomers % ee 95 % of the mixture is ()-carvone chiral chiral achiral achiral chiral f) achiral g) achiral h) chiral i) chiral j) achiral k) chiral l) chiral l) achiral m) chiral n) achiral o) achiral [] = c l = [] c l = (+24)( g / ml)(1.00 dm) = º optically inactive (meso) optically active optically active optically inactive optically active f) optically inactive (3-methylpentane has no chirality centers) g) optically inactive (meso) h) optically inactive 5.54.

12 CAPTE o. A racemic mixture is not optically active. Yes, because d and e are not enantiomers. They are diastereomers, which are not expected to exhibit equal and opposite rotations C 2 C 2 C methylpentane and 2-methylpentane are constitutional isomers. trans-1,2-dimethylcyclohexane and cis-1,2-dimethylcyclohexane are diastereomers The following two compounds are enantiomers because they are nonsuperimposable mirror images. You may find it helpful to construct molecular models to help visualize the mirror image relationship between these two compounds. C C C C C C This compound will be achiral This compound cannot be completely planar because steric hindrance prevents the two ring systems from rotating with respect to each other. The compound is locked in a particular conformation that is chiral. This ring system cannot be planar because of steric hindrance, and must therefore adopt a spiral shape (like a spiral staircas. The spiral can be right handed or left handed, and the relationship between these two forms is enantiomeric The compound is chiral because it is not superimposable on its mirror image. C 3 3 C This compound has a center of inversion, which is a form of reflection symmetry. As a result, this compound is superimposable on its mirror image and is therefore optically inactive.