Stereoisomers_CHEM 236_Spring 20100

Stereoisomers_CHEM 236_Spring 20100 - CHEMISTRY 236...

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Unformatted text preview: CHEMISTRY 236 Stereoisomers Peter Beak Department of Chemistry UIUC Structural representations of molecules establish that compounds which have the same molecular formula can have different structures. It is the case that molecules which have the same kind of bonding can have different arrangements of atoms in space leading to stereoisomers. Stereoisomers The requirement that p atomic orbitals on adjacent carbon atoms which participate in the double bond overlap implies that twisting of the atoms would break the bond. The Tl: bond strength is considered to be 65 kcal/mol and high temperatures can be required to break such a bond thermally. As a result of the restricted rotation about the carbon-carbon double bond if there are different groups on each terminus on the double bond there will be structural isomers. Structural isomers have the same molecular formula and bonding but have different locations of the atoms in space. In this case, the isomers differ in geometry about a site of restricted rotation, and are identified as geometrical isomers. Geometrical isomers have different chemical and physical properties. An example shown is 1,2-dichloroethylene. E isomer Z isomer Any group other than hydrogen bonded to a carbon is considered a substituent. Groups which are on the same side of the bond of restricted rotation are termed Z isomers, or cis isomers. When the substituents are on opposite sides of the bond with the restricted rotation the compounds are E isomers or trans isomers. Such isomers can exist with any system which has a restricted rotation. For example, in the case of 1,2-dibromocyclopropane two isomers exist. Problem fl — Indicate for the following structures which compounds will exist as geometrical isomers. In cases for which you indicated there would be a geometrical isomers draw the structures of the isomers and label them. a) CH3CH= CHCHZ CH3 EH2 b) CH3CH2CH2CCH2CH3 c) CH2=CH—— CHz—CH= CHCl Br d) OCH3 There are other ways which two or more isomers which have the same constituent atoms and bonding can differ in the arrangement of atoms in space. Compounds which are nonsuperimposable mirror images are called enantioisomers or enantiomers. An example of two such isomers is bromochlorofluoromethane as shown below. CI F F C] C/ \C/ \ / \ Br/ H H Br mirror plane Enantiomers have identical chemical and physical properties unless they are in an environment which itself is non-identical with its mirror image. Such environments are termed chiral. Chiral molecules have the property of rotating the plane of plane polarized light. Enantiomers rotate the plane to equal extents but in opposite directions and such molecules are said to be “optically active”. In many, but not all enantiomeric carbon molecules, there is a carbon which is bonded to four different groups. Such a carbon is designated a stereogenic carbon. It is possible to have more than one stereogenic carbon in a molecule. In such a case isomers will exist which are not enantiomers. These are diastereoisomers. An example is shown below. enantiomers H H CH?“ 33"?“ F ——C-—CI{3 CHa—"C --Cl C1--<;-—-CI-13 E IEI H O E . diasterioisomcrs ‘3‘ diasteremsomers s E H H CH.,--— (:3--F F --('3--CH3 Cl——C —-ICH3 CH3——(:3--Cl i1 1"1 enantiomers There are additional rules for indicating the arrangements in space of different groups and for designating additional features of these types of isomerism. These will be introduced as needed. Study Problem 2 — Indicate which compounds of the following structures can exist as enantiomers. Draw the enantiomeric pairs for each case in which such isomers are possible. a). CI-I,CHOHCHO b) mammal-Cigar; e; as c: " 1 firm-1&1 ,2-d ichloroeyclobutane Problem i Answers In cases of a), c), and d) geometrical isomers exist. a) Z C) Cl M/ E d) Br Z OCH3 Problem .1 Answers a), c), and d) can exist as chiral isomers. a) flux /OH CPI/Keno 0) CH3CH2K~~ /0H (:11fo a M 2 Cl Br. E “0CH3 HR I, H /C\ OHC CH3 HR ,CHZCH3 / d) ...
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Stereoisomers_CHEM 236_Spring 20100 - CHEMISTRY 236...

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