Geometric isomers Compounds with

Geometric isomers Compounds with different arrangements of groups on the opposite sides of a bond with restricted rotation, such as a double bond for example, cis-trans isomers of certain alkenes. 

The simplest compounds, -pyrroline and -piperideine,donotexistin the monomeric form. Schdpf et al. (29S) described two geometric isomers of J -piperideine trimer and called them a- and -tripiperideines (182). An equilibrium exists between A -piperideine and both trimers which, therefore, react as typical aldehyde ammonia. The trimer rearranges at pH 9-10 in an almost quantitative yield to isotripiperideine (183) which, in turn, is in equilibrium with tetrahydroanabasine (184) and -piperideine. 

The isolation of several pairs of geometric isomers of 4-unsaturated-5-oxazolones has been described. Generally, only one isomer is obtained when an aldehyde reacts with hippuric acid in the presence of acetic anhydride. Occasionally, mixtures have been separated in base-catalyzed reactions. In acetic anhydride-sulfuric acid or in 100% sulfuric acid, a mixture is obtained, and it has been suggested that sulfuric acid inhibits mutarotation of the intermediate addition product 53, which is a mixture of diastereomers (see, e.g., compound... 

Ullmann condensation of the sodium salt of p-chlorothiophe-nol (31) with 2-iodobenzoic (32) acid gives 33. Cyclization by means of sulfuric acid affords the thioxanthone, 34. Reaction with the Grignard reagent from 3-dimethylaminopropyl chloride affords the tertiary carbinol (35). Dehydration by means of acetic anhydride affords chlorprothixene as a mixture of geometric isomers, 36. (Subsequent work showed the Z isomer-chlorine and amine on the same side—to be the more potent compound.) Chlorprothixene is said to cause less sedation than the phenothiazines. ... 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

Because a double bond between two carbons prevents the carbons from rotating, isomers involving the atoms bonded to the carbons are possible, as shown above with dichloroethylene. Such isomers are called geometrical isomers, in contrast to the structural isomers discussed previously. When the substiuent groups are on the same side of the molecule, the compound is designated the cis- isomer. When the substituent groups are on the opposite side, the compound is the trans- isomer. Like all isomers, cis- and trans-isomers have the same molecular formula, but differ in certain physical and chemical properties. For example, cw-l,2-dichloroethylene boils at 60°C whereas 1,2-dichloroethylene boils at 48°C. 

Problems Pear ester (43) is an important industrial flavouring compound with a pear-like taste and smell. Consider all possible Wittig disconnections and choose a reaction which should give the right geometrical isomer. 

There is no reported separation by Johnson and coworkers of geometric or optical isomers from their reaction mixtures. Connick, Pepperman, and coworkers (10,24) and Cook et al. (22) separated geometric isomers into their two diastereomeric racemates and noted differences in activity for the geometric isomers. Many of the reported activities of strigol-related compounds have been obtained with mixtures of isomers. 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

Evidence can be presented to show that rotation around a C-C single bond happens readily, but rotation around a C=C double bond does not. Consider the compound, CH2C1CH2C1. No matter how this compound is synthesized, there is only one compound that is made with that formula. However, when CHC1CHC1 is prepared, there are two different compounds made with that formula. We call these two compounds geometric isomers. One is labeled cis and the other is trans. They have different physical and chemical properties. If there were free rotation around a double bond, this could not happen. 

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