Alkenes designating configuration

As with alkenes, the orientation around an asymmetic carbon can be only one of two choices. In three dimensions, clockwise and counterclocWise are the only two directions that are definite, and even that description requires a fixed reference point. To designate configuration, the lowest (fourth) priority group is always placed farthest away from the viewer (indicated by a dashed line), and the group priorities will follow 1 to 2 to 3 in either a clockwise or a counterclockwise direction. 

C. Systems for Designating Configuration in Alkenes The Cis, Trans System... 

Cycloaddition reactions with the Si(lOO) surface have been investigated for the purpose of designing microelectronics, nonlinear optical materials, sensors, and biologically active surfaces. The features of the [2+2] cycloadditions characteristic of the reactions in the pseudoexcitation band [133] predicts that [2+2] cycloadditions of electron-donating alkenes with Si(100)-2 x 1 surface could proceed with retention of configurations, in agreement with the observation [134]. Such stereospecific functionalizations of surfaces are of potential use for specific applications. 

Fumaric acid has the Econfiguration, and it can be designated as a trans alkene. Aconitic acid has the Z configuration, and it can be designated as a trans alkene (note that while the two COOH groups are cis to each other, the C5 parent chain is trans). 

Control of the regiochemistry of the attack has been achieved using specially designed substrates where the presence of a coordinating group induces a configuration of the complex that directs the attack of the nucleophile. For example, an alkene (that coordinates to palladium) induces a ring strain in the complex and directs the attack of the carbanion to C3 in the allylic moiety (Equation (47)).°... 

Cyclic alkenes with fewer than seven carbons in the ring, such as cyclopentene and cyclohexene, can exist only in the cis configuration because they do not have enough carbons to form a trans double bond. Therefore, it is not necessary to use the cis designation with their names. If the ring has seven or more carbons, however, then both cis and trans isomers are possible, so the configuration of the compound must be specified in its name. 

To assign the configuration of the alkene, we must first assign the priorities to the substituents attached to each carbon in the double bond. Once this is done, we can assign the E or Z designation. 

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