STEREOCHEMISTRY 161 7. 13 Examine the structural formula of each compound for equivalently substituted stereogenic centers The only one capable of existing in a meso form is 2, 4-dibromopentane B H-H H-Br H3 2, 4-Dibromopentane Fischer projection of meso-2.4-dibromopentane None of the other compounds has equivalently substituted stereogenic centers. No meso forms are possible for: CH_CHCHCH,CH3 CH,CHCHCH2CH3 CH_ CHCH, CHCH Br Br 4.Brom 7.14 There is a plane of symmetry in the cis stereoisomer of 1, 3-dimethylcyclohexane, and so it is an achiral substance--it is a meso form H3 Plane of symmetry passes through C-2 and C-5 and bisects the ring. The trans stereoisomer is chiral. It is not a meso form 7. 15 A molecule with three stereogenic centers has 2, or 8, stereoisomers. The eight combinations of R Stereogenic center Stereogenic center 123 123 Isomer 1 RRR Isomer 5 SSS Isomer 2 Isomer 6 SSR somer RSR Isomer 7 SRs Isomer 4 SRR Isomer 8 RSS 7.16 2-Hexuloses have three stereogenic centers. They are marked with asterisks in the structural HOCHCCHCHCHCHOH oHOH No meso forms are possible, and so there are a total of 2, or8, stereoisomeric 2-hexuloses Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website7.13 Examine the structural formula of each compound for equivalently substituted stereogenic centers. The only one capable of existing in a meso form is 2,4-dibromopentane. None of the other compounds has equivalently substituted stereogenic centers. No meso forms are possible for: 7.14 There is a plane of symmetry in the cis stereoisomer of 1,3-dimethylcyclohexane, and so it is an achiral substance—it is a meso form. The trans stereoisomer is chiral. It is not a meso form. 7.15 A molecule with three stereogenic centers has 23 , or 8, stereoisomers. The eight combinations of R and S stereogenic centers are: Stereogenic center Stereogenic center 1 2 3 1 2 3 Isomer 1 R R R Isomer 5 SSS Isomer 2 R R S Isomer 6 SSR Isomer 3 R S R Isomer 7 SRS Isomer 4 S R R Isomer 8 RSS 7.16 2-Hexuloses have three stereogenic centers. They are marked with asterisks in the structural formula. No meso forms are possible, and so there are a total of 23 , or 8, stereoisomeric 2-hexuloses. HOCH2CCHCHCHCH2OH OH O OH OH * * * CH3 H3C 1 3 5 4 2 6 Plane of symmetry passes through C-2 and C-5 and bisects the ring. 2,3-Dibromopentane CH3CHCHCH2CH3 Br Br 4-Bromo-2-pentanol CH3CHCH2CHCH3 OH Br 3-Bromo-2-pentanol CH3CHCHCH2CH3 OH Br 2,4-Dibromopentane Equivalently substituted stereogenic centers CH3CHCH2CHCH3 Br * Br * CH3 Br H CH3 H H H Br Fischer projection of meso-2,4-dibromopentane Plane of symmetry STEREOCHEMISTRY 161 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website