162 STEREOCHEMISTRY 7.17 Epoxidation of (Z)-2-butene gives the meso(achiral)epoxide Oxygen transfer from the peroxy acid can occur at either face of the double bond, but the product formed is the same because the two mirror-image forms of the epoxide are superposable H-C CH3 CH3 CHA COOH CH COOH H-C H, C meso-2,3-Epoxybutane Epoxidation of (E)r-2-butene gives a racemic mixture of two enantiomeric epoxides 3C CHa COOH H CH, COC HC (2R, 3R)-2, 3-Epoxybutan eS, 3S)-2, 3-Epoxybutane 7.18 The observed product mixture(68% cis-1, 2-dimethylcyclohexane: 32%o trans-1, 2-dimethylcyclo- hexane) contains more of the less stable cis stereoisomer than the trans The relative stabilities of the products therefore play no role in determining the stereoselectivity of this reaction 7.19 The tartaric acids incorporate two equivalently substituted stereogenic centers. (+)-Tartaric acid, as noted in the text. is the 2R 3R stereoisomer. There will be two additional stereoisomers the enan- acid(2S, 3S)and an optically inactive COH COH lane of symmetry HO-H H H COH CO,H S)-Tartaric acid meso Tart optically active) 7.20 No. Pasteur separated an optically inactive racemic mixture into two optically active enantiomers. A meso form is achiral, is identical to its mirror image, and is incapable of being separated into opti- cally active forms Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website7.17 Epoxidation of (Z)-2-butene gives the meso (achiral) epoxide. Oxygen transfer from the peroxy acid can occur at either face of the double bond, but the product formed is the same because the two mirror-image forms of the epoxide are superposable. Epoxidation of (E)-2-butene gives a racemic mixture of two enantiomeric epoxides. 7.18 The observed product mixture (68% cis-1,2-dimethylcyclohexane: 32% trans-1,2-dimethylcyclo￾hexane) contains more of the less stable cis stereoisomer than the trans. The relative stabilities of the products therefore play no role in determining the stereoselectivity of this reaction. 7.19 The tartaric acids incorporate two equivalently substituted stereogenic centers. ()-Tartaric acid, as noted in the text, is the 2R,3R stereoisomer. There will be two additional stereoisomers, the enan￾tiomeric ()-tartaric acid (2S,3S) and an optically inactive meso form. 7.20 No. Pasteur separated an optically inactive racemic mixture into two optically active enantiomers. A meso form is achiral, is identical to its mirror image, and is incapable of being separated into opti￾cally active forms. (2S,3S)-Tartaric acid (optically active) (mp 170°C, []D 12°) meso-Tartaric acid (optically inactive) (mp 140°C) CO2H CO2H HO H OH H H H CO2H CO2H OH OH Plane of symmetry O CH3 CH3 H H S S (2S,3S)-2,3-Epoxybutane O H3C H3C H H R R (2R,3R)-2,3-Epoxybutane CH3COOH O CH3COOH O H CH3 H E H3C C C O CH3 CH3 H H S R meso-2,3-Epoxybutane O H3C H3C H H R S meso-2,3-Epoxybutane H3C CH3 H Z H C CH3COOH C O CH3COOH O 162 STEREOCHEMISTRY Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website