164 STEREOCHEMISTRY (c) Both stereoisomers have two equivalently substituted stereogenic centers, and so we must be alert for the possibility of a meso stereoisomer. The structure at the left is chiral. The one at the right has a plane of symmetry and is the achiral meso stereoisomer CH ymmetry H,N-H H-1NH2 H-+NH H-1NH2 Chiral Meso: achiral (d) The first structure is achiral; it has a plane of symmetry Plane of symmetry passes through C-1, C-4, and C-7 The second structure cannot be superposed on its mirror image; it is chiral Cl Mirror image Reoriented mirror image 7. 25 There are four stereoisomers of 2, 3-pentanediol, represented by the Fischer projections shown. All CH H-OH H HO→H H-OH Ho-t-H HO H-OH CH,CH3 CH,CH3 CH, CH3 CH,CH3 There are three stereoisomers of 2, 4-pentanediol. The meso form is achiral; both threo forms are H3 H3 H-OH Ho-H H-OH HO→H meso-2. 4-Pentanediol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(c) Both stereoisomers have two equivalently substituted stereogenic centers, and so we must be alert for the possibility of a meso stereoisomer. The structure at the left is chiral. The one at the right has a plane of symmetry and is the achiral meso stereoisomer. (d) The first structure is achiral; it has a plane of symmetry. The second structure cannot be superposed on its mirror image; it is chiral. 7.25 There are four stereoisomers of 2,3-pentanediol, represented by the Fischer projections shown. All are chiral. There are three stereoisomers of 2,4-pentanediol. The meso form is achiral; both threo forms are chiral. meso-2,4-Pentanediol CH3 CH3 H H H OH OH H CH3 CH3 H H HO OH H H CH3 CH3 H H HO OH H H Enantiomeric threo isomers Enantiomeric erythro isomers CH2CH3 CH3 H H OH OH CH2CH3 CH3 HO HO H H Enantiomeric threo isomers CH2CH3 CH3 HO H OH H CH2CH3 CH3 HO H H OH Cl Cl Cl Reference structure Mirror image Reoriented mirror image Cl 1 4 7 Plane of symmetry passes through C-1, C-4, and C-7. H2N NH2 CH3 CH3 H H Chiral Plane of symmetry NH2 NH2 CH3 CH3 H H Meso: achiral 164 STEREOCHEMISTRY Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website