l13 - CH 203 O R G A N I C C H E M I S T R Y I The E,Z...

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The E , Z system © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I The E , Z system cis ” and “ trans ” stereochemical descriptors in use with alkenes We saw in a previous lecture that the stereochemical descriptors “ cis ” and trans ” are employed to describe the relative stereochemistry of two sub- stituents on a ring. These terms are also used to indicate stereochemical re- lationship in alkenes. The correct use of these terms is rather tricky. The alkene must satisfy two requirements: (1) The carbon–carbon double bond must be stereogenic. (2) Both alkene carbons must have an equivalent substituent. A double bond is stereogenic if switching the groups at one alkene carbon re- sults in a structure different from the original. For example, switching the C2 hydrogens in 1-chloro-1-ethene does not result in a new structure so the double bond is not stereogenic: H H H Cl switch H H H Cl not different from the original structure Switching the groups at C1 groups doesn’t result in a new structure either: it’s merely the original structure flipped like a pancake: H H H Cl switch H H Cl H flip H H H Cl same as original structure The second requirement that must be fulfilled before “ cis ” or “ trans ” can be used is that both alkene carbons must have an equivalent substituent, that is, both alkene carbons must be bonded to hydrogen atoms, or to chlorine at- oms, or to methyl groups, etc.
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The E , Z system © Bruno I. Rubio 2 The “ cis ” or “ trans ” labels are appropriate to the 2,3-dichloro-2-pentene structure shown below. Switching the groups at one of the alkene carbon re- sults in a new structure (i.e., the double bonds are stereogenic) and both alkene carbons are bonded to equivalent substituents, namely, to two chlori- nes. Cl Cl H 3 C H 3 C " cis " or "t rans " OK In contrast, the “ cis ” and “ trans ” labels are not appropriate to the 1- chloro-1-fluoro-1-propene structure shown below because the alkene carbons do not bear an equivalent substituent: the substituents at C1 are completely different from those at C2. H 3 C F Cl H " cis " or " trans " not OK To determine whether a stereogenic carbon–carbon double bond is cis or trans , draw an imaginary line collinear with that carbon–carbon double bond. The structure that has the equivalent substituents on the same side of the imagi- nary line is the cis stereoisomer; the structure that has the equivalent sub- stituents on opposite sides of the imaginary line is the trans stereoisomer.
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l13 - CH 203 O R G A N I C C H E M I S T R Y I The E,Z...

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