Trans and cis

Me0>2P(0)0CMe = CHC02Me. A trade name for a highly active systemic insecticide both CIS and trans isomers are active. 

The terms cis and trans signify, respectively, that the two hydrogen atoms are on the same side or on the opposite side with respect to the perpendicular plane of the double bond. 

The goal of this project is to determine the enthalpies of formation of cis- and trans-2-butene and to calculate the enthalpy of isomerization between them. 

Sketch the molecules on graph paper to help in determining the atomic coordinates. This is the first use of molecular geometry, a property that will become increasingly important as we go on. At this stage, the geometries are approximate the difference, for example, between cis and trans isomer s is ignored. 

Crotonic acid exists in cis and trans forms (compare maleie and fumaric acids) CH3—C—H CH3—C—H... 

Both cis- and irans- 1,2-dibromoethylenes react with metal acetylides, but with different reactivity. For the competitive reaction of cis- and trans- fl-dibromoethylenes with the zinc acetylide 314 shows that the irans isomer is more reactive than the cis isomer[228,229]. It was also found by a competitive reaction with the zinc acetylide 317 that the irans vinyl monobromide 315 is more reactive than the cis isomer 316[230]. 

Exclusive of compounds with double bonds four hydrocarbons are constitutional isomers of cis and trans 1 2 dimethylcyclopropane Identify these compounds... 

The CIS and trans forms of 1 2 dimethylcyclopropane are stereoisomers Stereoisomers are isomers that have their atoms bonded m the same order—that is they have the same constitution but they differ m the arrangement of atoms m space Stereoiso mers of the cis-trans type are sometimes referred to as geometric isomers You learned m Section 2 18 that constitutional isomers could differ m stability What about stereoisomers We can measure the energy difference between as and trans 1 2 dimethylcyclo propane by comparing their heats of combustion As illustrated m Figure 3 20 the two compounds are isomers and so the difference m their heats of combustion is a direct measure of the difference m their energies Because the heat of combustion of trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) less than that of its cis stereoisomer it follows that trans 1 2 dimethylcyclopropane is 5 kJ/mol (12 kcal/mol) more stable than as 1 2 dimethylcyclopropane... 

We 11 begin with cis and trans 1 4 dimethylcyclohexane A conventional method uses wedge and dash descriptions to represent cis and trans stereoisomers m cyclic systems... 

Wedge and dash drawings fail to show conformation and it s important to remember that the rings of cis and trans 1 2 dimethylcyclohexane exist m a chair conformation This... 

Hydnde shifts often occur during the dehydration of primary alcohols Thus although 1 butene would be expected to be the only alkene formed on dehydration of 1 butanol It IS m fact only a minor product The major product is a mixture of cis and trans 2 butene... 

Dehydrohalogenation of cycloalkyl halides lead exclusively to cis cycloaUcenes when the ring has fewer than ten carbons As the ring becomes larger it can accommo date either a cis or a trans double bond and large nng cycloalkyl halides give mixtures of CIS and trans cycloalkenes... 

Further insight into the E2 mechanism comes from stereochemical studies One such experiment compares the rates of elimination of the cis and trans isomers of 4 tert butyl cyclohexyl bromide... 

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common m organic chemistry and are examples of steric effects on reactivity Previously we saw steric effects on structure and stability m the case of CIS and trans stereoisomers and m the preference for equatorial substituents on cyclo hexane rings... 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

A good thing to remember is that the cis and trans isomers of a particular compound are diastereomers of each other... 

A reaction that introduces a second chirality center into a starting material that already has one need not produce equal quantities of two possible diastereomers Con sider catalytic hydrogenation of 2 methyl(methylene)cyclohexane As you might expect both CIS and trans 1 2 dimethylcyclohexane are formed... 

The CIS and trans isomers of 4 methylcyclohexanol are stereoisomers but they are not related as an ob ject and its mirror image they are diastereomers... 

You will find It revealing to construct a splitting diagram similar to that of Figure 13 20 for the case in which the CIS and trans H—C=C—H coupling con stants are equal Under those circumstances the four line pattern simplifies to a triplet as It should for a proton equally coupled to two vici nal protons... 

Both CIS and trans 1 4 dimethylcyclohexanol are formed in the hydrolysis of either as or trans 1 4 dimethylcyclohexyl bromide... 

H2C=CHCH(CH2)4CH3 as well as the cis and trans stereoisomers of BrCH2CH=CH(CH2)4CH3 Br... 

The bicyclic compound decahydronaphthalene, or bicyclo[4.4.0]decane, has two fused six-mem-bered rings. It exists in cis and trans forms (see Fig. 1.10), as determined by the configurations at the bridgehead carbon atoms. Both cis- and rran -decahydronaphthaiene can be constructed with two chair conformations. 

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