Trans-form

It is the simplest compound containing the -N = N- chromophore, but is of no practical importance since it lacks affinity for fibres. Normally it exists in the most stable trans form, but may be converted to a less stable, bright red as form, m.p. 71 -S C by ultra-violet irradiation. Cis to irans isomerization occurs rapidly on heating. 

CqHaOj. Colourless crystals m.p. lOS C. Obtained by boiling coumarin with sodium ethoxide. Irradiation of o-coumaric acid produces coumarinic acid. The stable form of the acid is the trans form. 

Although a compound such as (II) could theoretically exist in a number of geometrically isomeric forms, only one form is produced in this synthesis it is almost certainly the trans form throughout the chain. 

Sorbic acid could theoretically, by virtue of the two olefine linkages, exi.st in four geometrical I y-isomeric forms. The above synthesis gives only one form, which is undoubtedly the trans-trans form, analogous to the acu-diphenylpolyenes (p. 238) which are also normally produced solely in one form. 

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

It is interesting to note that altliough cis-lrans isomerism about the double bond is theoretically possible in cinnamic acid, the Perkin reaction gives rise only to the trans form, m.p. 133°, the cis form, m.p. 68° (termed aHo-cinnamic acid) being unstable and easily converted into the trails acid. 

Basic catalysts other than alkali acetates have been employed in the Perkin reaction thus salicylaldehyde condenses with acetic anhydride in the presence of triethylamine to yield coumarin (tlie lactone of the cis form of o-hydroxy-cinnamio acid) together with some of the acetyl derivative of the trans form (o-acetoxycoumaric acid) ... 

This interpretation is supported by literature studies on copper(II) complexes containing two -amino-acid ligands. For N-unsubstituted -amino-acid ligands, deductions as to position of the cis -trans geometrical equilibrium in solution are difficult as illustrated by the fact that for some -amino acids solid complexes have been isolated of both the ds and trans geometry. In contrast it seems as if copper(II) complexes containing two N-alkylated -amino-acid ligands crystallise exclusively in the trans form ". ... 

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... 

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. 

Bufa-1,3-diene is one of many examples of molecules in which torsional motion may convert a sfable isomer info anofher, less sfable, isomer. The more sfable isomer in fhis case is fhe s-trans form, shown in Figure 6.44(e), and fhe less sfable one is fhe s-cis form, ... 

Properties of PGT. Typical properties of PCT are shown in Tables 5 and 6 (154). Table 5 shows the standard grades and Table 6 the PR grades. This material is in effect a copolymer of terephthaUc acid with two diols, the cis and trans forms of cyclohexanedimethanol. These two isomers do not form a eutectic copolymer with a minimum melting point, but instead the melting point rises monotonically between the two values of the pure... 

Furylfuramide. a[(5-Nitro-2-furaiiyl)-2-methylene]-2-furaiiacetamide, withdrawn from the market in Japan in 1974 because of mutagenicity, is prepared by condensation of 5-nitro-2-furancarboxaldehyde with 2-furanacetic acid followed by chlorination and amination (30). The isomerization of cis to trans form of fudyfuramide has been shown to occur in the presence of a variety of biological reducing agents (31). 

Polymerization catalyzed by free radicals occurs with sorbic acid. The polymers (3) formed have high molecular weights with linear stmctures the trans form of the residual double bond is preserved (13). 

An excess of crotonaldehyde or aUphatic, ahcyhc, and aromatic hydrocarbons and their derivatives is used as a solvent to produce compounds of molecular weights of 1000—5000 (25—28). After removal of unreacted components and solvent, the adduct referred to as polyester is decomposed in acidic media or by pyrolysis (29—36). Proper operation of acidic decomposition can give high yields of pure /n j ,/n7 j -2,4-hexadienoic acid, whereas the pyrolysis gives a mixture of isomers that must be converted to the pure trans,trans form. The thermal decomposition is carried out in the presence of alkaU or amine catalysts. A simultaneous codistillation of the sorbic acid as it forms and the component used as the solvent can simplify the process scheme. The catalyst remains in the reaction batch. Suitable solvents and entraining agents include most inert Hquids that bod at 200—300°C, eg, aUphatic hydrocarbons. When the polyester is spHt thermally at 170—180°C and the sorbic acid is distilled direcdy with the solvent, production and purification can be combined in a single step. The solvent can be reused after removal of the sorbic acid (34). The isomeric mixture can be converted to the thermodynamically more stable trans,trans form in the presence of iodine, alkaU, or sulfuric or hydrochloric acid (37,38). 

Sorbic acid and its salts are highly refined to obtain the necessary purity for use in foods. The quaUty requirements are defined by the Food Chemicals Codex (Table 3). Codistillation or recrystaUization from water, alcohoHc solutions, or acetone is used to obtain sorbic acid and potassium sorbate of a purity that passes not only the Codex requirements but is sufficient for long-term storage. Measurement of the peroxide content and heat stabiUty can further determine the presence of low amounts of impurities. The presence of isomers, other than the trans,trans form, causes instabiUty and affects the melting point. 

Butadiene, the simplest conjugated diene, has been the subject of intensive theoretical and experimental studies to understand its physical and chemical properties. The conjugation of the double bonds makes it 15 kJ/mole (3.6 kcal/mol) (13) more thermodynamically stable than a molecule with two isolated single bonds. The r-trans isomer, often called the trans form, is more stable than the s-cis form at room temperature. Although there is a 20 kJ/mole (4.8 kcal/mol) rotational barrier (14,15), rapid equiUbrium allows reactions to take place with either the s-cis or r-trans form (16,17). 

In the stable trans-form the H atoms lie along the diagonal of the square. The energy of the cis-form, in which the atoms are positioned on one of the edges, is 3-5 kcal/mol higher than that of the trans-form [Smedarchina et al. 1989]. The transition state energies for trans-cis and... 

The PES found by Smedarchina et al. [1989] has two cis-form local minima, separated by four saddle-points from the main trans-form minima. The step-wise transfer (trans-cis, cis-trans) - because of endoergicity of the first stage - displays Arrhenius behavior even at T < T. . The concerted transfer of two hydrogen atoms was supposed to become prevalent at sufficiently low temperatures. However, because of too high a barrier for the concerted trans-trans transition, this... 

Figure 6.9 (a) Peptide units can adopt two different conformations, trans and cis. In the trans-form the C=0 and the N-H groups point in opposite directions whereas in the c/s-form they point in the same direction. For most peptides the trans-form is about 1000 times more stable than the c/s-form. (b) When the second residue in a peptide is proline the trans-form is only about four times more stable than the c/s-form. C/s-proline peptides are found in many proteins. 

One part of the molecule (dark blue and red) rotates 180° around a double bond between two carbon atoms (green). The geometry of the molecule is changed by this rotation from a trans form to a cis form. Carbon atoms are blue, hydrogen atoms gray and the oxygen atom red. 

This effect is also observed with some polymers. The trans form of a hydrocarbon chain requires an energy about 0.8 kcal/mole less than the gauche. The trans form leads to an extended molecule and in hydrocarbons this becomes more favoured as the temperature is lowered. Linear polyethylenes take up this conformation in the crystalline state. 

Note This result could also have been obtained at equation (7.44) by using z-trans-form number 7 in Table 7.1, but the solution demonstrates the use of partial fractions. 

The case of a, -unsaturated caAonyl compounds is analogous to that of 1,3-dienes, in that stereoelectronic factors favor coplanaiity of the C=C—C=0 system. The rotamers that are important are the s-trans and s-cis conformations. Microwave data indicate that the s-trans form is the only conformation present in detectable amounts in acrolein (2-propenal). The equilibrium distribution of s-trans and s-cis conformations of a,fi-unsatuiated ketones depends on the extent of van der Waals interaction between substituents. Methyl vinyl ketone has minimal unfavorable van der Waals repulsions between substituents and exists predominantly as the s-trans conformer ... 

The chemical structure of SBR is given in Fig. 4. Because butadiene has two carbon-carbon double bonds, 1,2 and 1,4 addition reactions can be produced. The 1,2 addition provides a pendant vinyl group on the copolymer chain, leading to an increase in Tg. The 1,4 addition may occur in cis or trans. In free radical emulsion polymerization, the cis to trans ratio can be varied by changing the temperature (at low temperature, the trans form is favoured), and about 20% of the vinyl pendant group remains in both isomers. In solution polymerization the pendant vinyl group can be varied from 10 to 90% by choosing the adequate solvent and catalyst system. 

Syntheses of members of the lobeline group have been effected by Wieland and Drishaus and by Seheuing and Winterhalder. JiorLobelane was prepared by the former authors by condensing 2 6-dimethylpyridine with benzaldehyde to 2 6-distyrylpyridine (IX), which was then reduced by sodium in alcohol, giving a mixture of meso-and trans- forms of 2 6-di-(3-phenylethylpiperidine (worlobelane). From this by crystallisation of the mixed hydrochlorides, meso-norlobelane was separated, which on A-methylation yielded lobelane (X) as the methiodide. 

The cis and trans forms of 1,2-dimethylcyclopropane are stereoisomers. Stereoisomers are isomers that have then- atoms bonded in the sane order—that is, they have the sane constitution, but they differ in the anangernent of atoms in space. Stereoisomers of the cis-trans type are sometimes refened to as geometric isomers. You learned in Section 2.18 that constitutional isomers could differ in stability. What about stereoisomers ... 

All of the optimizations are successful. The frequency jobs for the two forms where the H-C-C-H dihedral angle is 0° produce no imaginary frequencies, and the cis form is lower in energy than the trans form by about 0.63 kcal/mole. 

We must look further in order to locate the transition structure linking the cis and trans forms of 1-propene. Since we are looking for a normal mode which su esis... 

We will use a similar procedure to investigate the second reaction, where formaldehyde transforms into the trans form of hydroxycarbene ... 

We can easily identify both structures by the value of the dihedral angle. In the one on the left, the dihedral angle has increased to 118.3°, indicating that this side of the path is leading to the trans form. Indeed, if we look at ail of the points in the reaction path, we see that the dihedral angle steadily increases on this side of the transition structure, and steadily decreases on the opposite side. From the latter, we can conclude that the right structure is tending toward the cis form. Thus, we have confirmed that this transition structure does in fact connect the cis and trans isomers of hydroxycarbene. 

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