Trans isomers, of alkenes

It should be clear to you that there will be no cis-trans isomers of alkenes in which one end of the double bond carries identical groups. Thus we do not expect there to be cis-trans isomers of 1-butene or 2-methylpropene, and... 

ACD/H-NMR from Advanced Chemistry Development (ACD) Labs calculates H-NMR spectra under any basic frequency. The system uses 3D molecular structure minimization and Karplus relationships to predict proton-proton coupling constants. The software recognizes spectral differences among diastereotopic protons, cis-trans isomers of alkenes, syn-anti isomers of amides, oximes, hydrazones, and nitrosa-mines. The base data set includes more than 1,000,000 experimental chemical shifts and 250,000 experimental coupling constants. To quantify intramolecular interactions in new organic structures and to predict their chemical shifts, ACD/HNMR uses an algorithm based on intramolecular interaction parameters to quantify intramolecular interactions in new organic structures and to predict their chemical shifts. 

The reaction hexenes —> hexadienes was demonstrated without using radiotracers both on oxide and metal catalysts, Nil 1 and Ptj l Mixtures containing [ " CJ-hexene contributed to the clarification of the further reaction pathway. These studies showed that neither the hexene cyclohexane nor the hexene —> cyclohexene ring closure pathway took place.Table 2 indicates that radioactivity appeared in both the hexatriene and 1,3-cyclohexadiene fractions when their inactive form was admixed to radioactive hexene. The aromatisation of both inactive components was much more rapid than that of hexene, therefore their specific radioactivities showed very low absolute values, however, these were still higher than that of benzene produced mainly from these non-radioactive precursors. The true precursor of ring closure should have been cis-cis-1,3,5-hexatriene. Its ring closure takes place without any catalyst from 513 The stepwise dehydrogenation of open-chain hydrocarbons produces cis- and trans-isomers of alkenes and alkadienes. Any c s-c s-triene... 

Several five- and six-carbon non-carbohydrate derivatives have been converted into deoxy-sugars. An epoxidatlon-hydrolysls sequence has been utilized to convert the racemic trans-isomer of alkene (29) into an 4 1 mixture of DL-dlgltoxose (rlbo) and DL-oliose (lyxo) dimethyl acetals (30) and (31) (Scheme 8). The cls-isomer of alkene (29), however, yielded an -4 1 mixture of analogous DL-canarose (arabino) and DL-bolvlnose (xylo) derivatives. Similar treatment of the els- and trans-isomers of the related alkene (32) provided v4 l mixtures of DL-dlgltoxose... 

Cis-trans isomers of alkenes (Section 1.13B) have different physical properties. They have different melting points and boiling points, and often cis—trans isomers differ markedly in the magnitude of their dipole moments. Table 2.2 summarizes some of the physical... 

Cis and trans isomers of alkenes do not have the same stability. 

Now the question becomes How strong is the C(2)—C(3) overlap What consequences will this overlap have for the structure of conjugated dienes Overlap between 2p orbitals on adjacent atoms has profoimd consequences for the stmcture of alkenes, for example, and the same might be tme for dienes. It is the overlap between 2p orbitals that makes cis and trans isomers of alkenes possible (Rg. 12.19). 

Draw the condensed structural formulas and give the names for the cis-trans isomers of alkenes. 

Geometric or cis-trans isomers of alkenes occur when the carbon atoms in the double bond are connected to different atoms or groups. 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

In investigating the mechanism of addition to a double bond, perhaps the most useful type of information is the stereochemistry of the reaction. The two carbons of the double bond and the four atoms immediately attached to them are all in a plane (p. 8) there are thus three possibilities. Both Y and W may enter from the same side of the plane, in which case the addition is stereospecific and syn they may enter from opposite sides for stereospecific anti addition or the reaction may be nonstereospecific. In order to determine which of these possibilities is occurring in a given reaction, the following type of experiment is often done YW is added to the cis and trans isomers of an alkene of the form ABC=CBA. We may use the cis alkene as an example. If the addition is syn, the product will be the erythro dl pair, because each carbon has a 50% chance of being attacked by Y ... 

Infrared and Raman spectra. The infrared spectra of cis-trans isomers of unsymmetrical alkenes of the type R-CH=CH-R show characteristic differences... 

When either the cis or the trans isomer of an alkene is irradiated, a mixture of both isomers will be formed in a particular ratio, which is dependent on the wavelength of light used. For example if either trans-stilbene or cis-stilbene is irradiated at 313 nm, the final composition of the reaction mixture will consist of a mixture of 93% cis and 7% trans (Figure 8.2). 

Scheme 21) (92TL5597>. In cases where EjZ isomers of the product alkene are possible, the thermodynamically preferred isomer predominates <90SL479>. Electrochemical reduction of certain 1,3,2-dioxathiolane 2,2-dioxides also affords alkenes <84BCJ3160>. Reduction of the cis and trans isomers of 4,5-dimethyl-1,3,2-dioxathiolane 2-thione using Raney nickel afforded meso- and ( + )-2,3-butanediol, respectively <65JOC2696>. 

Alkenes which have no symmetry planes perpendicular to the plane of the double bond such as Pmr-butene-2 or propene can coordinate to platinum in two enantiomorphous ways (77) and (78). If an optically active ligand is also bound to platinum(H), then two diastereoisomers are found which can be separated by fractional crystallization657,658 or by HPLC.659 Both cis and trans isomers of complexes PtCl(N—0)(alkene) have beenprepared, where N—O is an anion derived from an amino add (equations 235a and 235b).660-664 Epimerization cannot occur by simple rotation of the alkene about its bond axis, but only by a mechanism involving cleavage of the platinum(II)-alkene bond. 

An important problem in many syntheses is to produce the desired isomer of a cis-trans pair of alkenes. The problem would not arise if it were possible to isomerize the undesired isomer to the desired isomer. In many cases such isomerizations can be carried out photochemically. A typical example is afforded by cis- and /raw.v-1,2-diphenylethene (stilbene) ... 

The triplet state of the alkene is most stable when the p orbitals, which make up the normal tt system of the double bond, are not parallel to one another (Figure 6-17). Therefore, if the energy-transfer process leads initially to a planar triplet, this is converted rapidly to the more stable nonplanar form. The excitation of either the cis or the trans isomer of the alkene appears to lead to a common triplet state, as shown in Figure 28-4. 

In addition to the alkene isomers that exist because of double-bond position, alkene isomers can also exist because of double-bond geometry. For instance, there are two geometrical, or cis-trans isomers, of 2-butene, which differ in their geometry about the double bond. The cis isomer has its two -CH3 groups on the same side of the double bond, and the trans isomer has its two -CH3 groups on opposite sides. Like other kinds of isomers we ve discussed, the individual cis and trans isomers of an alkene are different substances with different physical properties and different (although often similar) chemical behavior. cfs-2-Butene boils at 4°C, for example, but trans-2-butene boils at 0.9°C. 

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