Trans—cis isomers

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

FIGURE 11.6 Ball-and-stick models of the cis and trans isomers of 2-butene. 

In any alkene, the double bond is rigid, which means there is no rotation around the double bond (see Explore Your World Modeling Cis-Trans Isomers ). As a result, the atoms or groups are attached to the carbon atoms in the double bond on one side or the other, which gives two different structures called geometric isomers or cis-trans isomers. 

For example, the formula for 1,2-dichloroethene can be drawn as two different molecules, which are cis-trans isomers. In the expanded structural formulas, the atoms bonded to the carbon atoms in the double bond have bond angles of 120°. When we draw 1,2-dichloroethene, we add the prefix cis or trans to denote whether the atoms bonded to the carbon atoms are on the same side or opposite sides of the molecule. In the cis isomer, the chlorine atoms are on the same side of the double bond. In the trans isomer, the chlorine atoms are on opposite sides of the double bond. Trans means across, as in transcontinental cis means on this side.  

As with any pair of cis-trans isomers, cw-2-butene and trans-2- ii ae are different compounds with different physical properties and chemical properties. In general, trans isomers are more stable than their cis counterparts because the groups that are bigger than hydrogen atoms on the double bond are farther apart. 

Build and compare handheld models of the two possible 2-butenes. 

The p orbitals of the pi bond of cis-2-butene are in the plane of the page, and the plane defined by the atoms attached to the carbons of the double bond is perpendicular to the page. To convert to the trans-isomer, one of the carbons of the double bond must be rotated about the axis of the double bond. 

Interconversion of as- and trans-2-butene by rotation about the double bond. 

For an alkene to exhibit cis-trans isomerism, the two groups on one end of the double bond must be different and the two groups on the other end of the double bond must be different. That is, in terms of the following structure, A must be different from B, and D must be different from E. When this is the case, both of the carbons of the double bond are said to be stereocenters. A stereocenter or stereogenic atom is defined as an atom at which the interchange of two groups produces a stereoisomer. 

Which of these compounds exhibit cis-trans isomerism Draw both cis-trans isomers when they exist. 

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Stereoisomerism at double bonds is indicated in SMILES by / and . The characters specify the relative direction of the connected atoms at a double bond and act as a frame. The characters frame the atoms of a double bond in a parallel or an opposite direction. It is therefore only reasonable to use them on both sides Figure 2-78). There are other valid representations of cis/trans isomers, because the characters can be written in different ways. Further details are listed in Section 2,3.3, in the Handbook or in Ref, [22]. 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

When two different substituents are attached to each carbon atom of the double bond, cis-trans isomers can exist. In the case of c T-2-butene (Fig. 1.11a), both methyl groups are on the same side of the double bond. The other isomer has the methyl groups on opposite sides and is designated as rran5--2-butene (Fig. l.llb). Their physical properties are quite different. Geometric isomerism can also exist in ring systems examples were cited in the previous discussion on conformational isomers. 

Selenourea [630-10-4] ]5k.e urea and thiomea can form channel inclusion compounds (87) with a variety of hydrocarbons. Though the difference in channel diameter between thiourea and selenourea is small, selenourea seems to be much more selective for the inclusion of certain guest molecules (eg, cis/trans isomers). 

The couplings of vicinal protons in 1,2-disubstituted alkenes lie in the range 6-12 Hz for cis protons (dihedral angle 0°) and 12-17 Hz for trans protons (dihedral angle 180°), thus also following the Karplus-Conroy equation. Typical examples are the alkene proton AB systems of coumarin (16a, cis) and tra 5-cinnamic acid (16b), and of the cis-trans isomers 17a and b of ethyl isopente-nyl ether, in addition to those in problems 3, 4, 8, 11, 13 and 38. 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Three different conformations are possible for monomeric chalcogen diimides (Eig. 10.1). Variable-temperature NMR spectra indicate that the cis,trans isomer of S(NR)2 is most stable in solution for small organic groups (R = Me, Bu). With bulkier organic substituents, small amounts of the trans,trans isomer exist in equilibrium with the cis,cis isomer. " The cis,cis isomer is observed in solutions of certain sulfur diimides with... 

The geometries and relative energies of the different conformations of model chalcogen diimides E(NR)2 (E = S, Se R = H, Me, Bu and SiMe3) have been investigated by using ab initio and DET molecular orbital methods.The cis,trans conformation is predicted to be most stable with the exception of the parent molecules E(NH)2 and the unsymmetrical systems RNSNH, for which the cis,cis conformation is slightly more stable than the cis, trans isomer. 

The 1,2-diinethylcyclopropanes are members of a subclass of stereoisomers called cis-trans isomers. The prefixes cis- (Latin "on the same side") and tmns-(Latin "across") are used to distinguish between them. Cis-trans isomerism is a common occurrence in substituted cycloalkanes. 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

There are four cis-trans isomers of menthol (Problem 4.3/), including the one shown. Draw the other three. 

Figure 6.4 The requirement for cis-trans isomerism in alkenes. Compounds that have one of their carbons bonded to two identical groups can t exist as cis-trans isomers. Only when both carbons are bonded to two different groups are cis-trans isomers possible.

These two compounds are identical they are not cis-trans isomers. 

R Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order but with a different geometry. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers (both in alkenes and in cycloalkanes). Actually, cis-trans isomers are just another kind of diastereomers because they are non-mirror-image stereoisomers. 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple alkene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4-addition of the growing chain to a conjugated diene monomer. 

Tranexamic acid, a drug useful against blood clotting, is prepared commercially from p-methylbenzonitrile. formulate Lhe steps likely to be used in the synthesis. (Don t worry about cis-trans isomers heating to 300 °C interconverts the isomers.)... 

Cis-trans isomers (Sections 4.2, 6.4) Stereoisomers that differ in their stereochemistry about a double bond or ring. 

In the cis isomer, the two CH3 groups (or the two H atoms) are as close to one another as possible. In the trans isomer, the two identical groups are farther apart. The two forms exist because there is no free rotation about the carbon-to-carbon double bond. The situation is analogous to that with cis-trans isomers of square planar complexes (Chapter 15). In both cases, the difference in geometry is responsible for isomerism the atoms are bonded to each other in the same way. 

Alkane Hydrocarbon containing only single carbon-carbon bonds. The simplest example is methane, CK, 580 cis-trans isomer in, 598 isomers in, 580-582 nomenclature, 582t physical properties, 591t sources, 583-585 structural isomerism, 597 tests for, 602 uses, 583-585... 

Hogeveen138 measured apparent acidity constants of substituted /i-phenylthio-, /J-phenylsulfinyl- and / -phenylsulfonyl-acrylic acids (cis and trans) in 50% v/v ethanol. The p values for transmission through SCH=CH, SOCH=CH and S02CH=CH were 0.531, 0.389 and 0.320 respectively for the cis acids and 0.652, 0.282 and 0.331 for the trans acids. These results were discussed in considerable detail and compared with those of pertinent related systems. Little importance was attached to the small differences between p values for cis/trans isomers, and the relative transmissions were taken as the mean p values 0.59,0.34 and 0.33. The superior transmission of SCH=CH was attributed to greater polarizability. The values for pKJtrans) — pKJcis) of isomeric acids were also discussed. For the sulfonyl acids this is almost constant at 0.1 unit for the sulfinyl adds there is some variation about a mean value of 0.26 unit, and for the thio adds there is also some variation about a mean value of — 0.15 unit. The differing behavior of the three systems in this respect was explained in terms of hypothetical conformations and electrostatic interactions therein. 

The submitters report obtaining 1.29 (12%) of the c/.v-diastereoisomer and 8.39 g (80%) of the trans-diastereoisomer. The submitters recommend that the purified diastereomers be stored in the freezer (-18°C). Both pure isomers slowly equilibrate to a 21 79 mixture of cis/trans isomers upon standing at ambient temperature for several weeks. 

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