Organic Chemistry

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Organic Chemistry - Ch 11 557 Daley & DaleyHHCH 3OHCH 3OHHOHOCH 3CH 3HHApparent pair of enantiomersRotate 180oin the planeof the pageHHCH 3OHCH 3OHIdentical to thestructure on the left.These two structures are the same because there is an internal planeof symmetry in the Fischer projection. This internal plane ofsymmetry reflects one half onto the other half. The molecule has twostereogenic centers: one is (R) and the other is (S). It is almost as if themolecule were a racemic mixture within itself.HHCH 3OHCH 3OHInternal planeof symmetryA meso compoundcontains stereogeniccenters and an internalplane of symmetry.Compounds, such as 2,3-butanediol, which exhibit an internalplane of symmetry and are symmetrical, even though they havestereocenters, are called meso compounds. Meso compounds aresymmetric structures with the two halves of the molecule havingidentical substituents, but opposite configurations. The symmetricdiastereomer is called the meso diastereomer. A meso compound is anexample of a compound with stereogenic centers, but does not exhibitoptical activity. The compounds that are diastereomers of the mesocompound and are enantiomers of each other are called the (±) or dlpair.Exercise 11.7For the following compounds mark any stereocenters and determinewhether they are (R) or (S). Mark any meso compounds. Determinewhich are optically active.a)HCH 3Clb)CH 3BrCH 3HCH 3www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 558 Daley & Daleyc)d)H 3 CCH 3OHOHe)CH 3f)BrHHHOHOOHOHHHClCH 3Sample Solutionb) Redraw the structure as a Fischer projection using your models tohelp you see the transformation.CH 3CH 3HHPlane of symmetryThis compound does have an internal plane of symmetry making it ameso compound with no optical activity. The top stereocenter is (S)and bottom one is (R).An erythro moleculehas two stereocenterswith similar groupsnext to each other inthe eclipsedconformation.A threo molecule hastwo stereocenters withtwo pairs of the similargroups on oppositesides of the molecule.When naming a molecule with two stereocenters, consider eachcenter separately and assign each stereocenter the appropriate (R) or(S) designation along with the appropriate carbon number. Forexample, the name of the compound in Exercise 11.7d is (1R,2R)-1,2-cyclopentanediol. Another common way of naming molecules with twostereocenters is to use the terms erythro and threo. The termserythro and threo are derived from the simple sugar molecules 2,3,4-trihydroxybutanal that are commonly known as erythrose andthreose.www.ochem4free.com 5 July 2005
Page 1 and 2: Richard F. Daley and Sally J. Daley
Page 3 and 4: Organic Chemistry - Ch 11 532 Daley
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Organic Chemistry

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