Ch 5 - Stereochemistry

Ch 5 - Stereochemistry - 5 S te reochem i stry...

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5 Stereochem istRy StereochemisTry is the study of the three-dimensional structure of molecuLes. No one can understand organic chemistRy, biochemistry± oR biology without using stereochem² istry. Biological systems aRe exquisitely seLective, and they often discrIminate between molecules with subtle stereochemical diFerences³ We have seen (Section 2´8) that iso² meRs are gRouped into two broad cLasses: constitutional isomeRs and stereoisomersµ ConsTiTuTional isomers (strucTural isomers) diFeR in theiR bonding sequence; their atoms are connected diFerently. STereoisomers have the same bonding sequence, but they di¶·eR in the orientation of theiR atoms in space¸ DiFeRences in spatial oRientation might seem unimpoRtant, but stereoisomERs often have Remarkably diFeRent physical± chemicaL, and bioLogicaL propertiesµ ¹oR exampLe, the cis and tRans ºsomeRs of butenedioic acid are a special type of steReoiso» meR caLled cis-trns isomeRs (oR geometRic isomeRs). Both compounds have the foRmu² La HOOC -CH = CH -COOH, but they diFeR in how these atoms are aRRanged in space. The cis isomeR is calLed maleic acid, and the tRans isomer is CAUEDfumaRic acid. ¹umaric acid is an essentiaL metaboLic intermediate in both plants and animaLs, but maleic acid is toxic and irritating to tissuesµ o Ii H C±OH " / C=C ² " HO³C H II o fumaric acid, mp 287°C essential meTaboLite o 0 II iI HO³C " ² C³OH H ² " H maleic acid± mp 138±C Toxic irriTaNT The discovery of steReochemistry was one of the most impoRtant bReak² thRoUgHs in the st¼uctuRaL theo¼y of oRgan½c chemistry. SteReochemistry explained why seveRal types of isomers exist, and it forced scientists to propose the tetrahedRal carbon atomµ ¾n this cHapter, we study the three´dimensional structures of molecuLes to understand theiR steReochemicaL relationships³ We compare the various types of stereoisomeRs and study ways to di¶·erentiate among stereoisomeRs³ ¿n future chap» teRs, we wilL see how steReochemistry pLays a majoR Role in the pRopeRties and ReacÀ tions of organic compounds. 5-1 Introduction A double bond in rhodopsin, a visu- al pIgment found In your eyes that enables you to see at night, is con- veRted fRom the cis Isomer to the trans isomer when light strikes the eye. As a result± a nerve impulse travels to the brain and you see the source of the light² / ²-opsIn I / / rhodopsi³ 167
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168 Chapter 5: Stereochemistry 5-2 Chirality Figure 5-1 Use of a mirRoR to test FoR chIRalIty. An object is chiRaL if its miRroR Image is d±²eRent fRom the oRIgInal object. FIgUre 5-2 Common chiRal objects³ Many objects come ´n "leµT-handed¶ and "R±ght·handed¶ veRs¸ons³ What is the dIfferen±e between your left hand and your right hand? They Look simi- Lar, yet a left²handed gLove does not Ft the right hand. The same pRin±Iple applies to your feet³ They look aLmost Identi±al´ yet the left shoe Fts paInfulLy on the right foot.
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This note was uploaded on 02/27/2010 for the course CHEM 140 taught by Professor Wade during the Spring '10 term at Whitman.

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Ch 5 - Stereochemistry - 5 S te reochem i stry...

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