Physical properties, geometrical 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. 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. 

Table 3 fists cycloaliphatic diamines. Specific registry numbers are assigned to the optical isomers of /n t-l,2-cyclohexanediamine the cis isomer is achiral at ambient temperatures because of rapid interconversion of ring conformers. Commercial products ate most often marketed as geometric isomer mixtures, though large differences in symmetry may lead to such wide variations in physical properties that separations by classical unit operations are practicable, as in Du Font s fractional crystallisation of /n t-l,4-cyclohexanediamine (mp 72°C) from the low melting (5°C) cis—trans mixture. 

Cycloahphatic diamines which have reacted with diacids to form polyamides generate performance polymers whose physical properties are dependent on the diamine geometric isomers. (58,74). Proprietary transparent thermoplastic polyadipamides have been optimized by selecting the proper mixtures of PDCHA geometric isomers (32—34) for incorporation (75) ... 

Methylenedi(cyclohexyhsocyanate) (45) [5124-30-1] (MDCHl, Desmodur W) is the dominant derivative of MDCHA and is used in light-stable urethanes. Polyurethane physical properties are dependent on the diamine geometric isomer composition used for the derivative diisocyanate which reacts with diol (87). 

A similar type of isomerism occurs for [Ma3b3] octahedral complexes since each trio of donor atoms can occupy either adjacent positions at the comers of an octahedral face (/hcial) or positions around the meridian of the octahedron (meridional). (Fig. 19.12.) Geometrical isomers differ in a variety of physical properties, amongst which dipole moment and visible/ultraviolet spectra are often diagnostically important. 

Two or more species with different physical and chemical properties but the same formula are said to be isomers of one another. Complex ions can show many different kinds of isomerism, only one of which we will consider. Geometric isomers are ones that differ only in the spatial orientation of ligands around the central metal atom. Geometric isomerism is found in square planar and octahedral complexes. It cannot occur in tetrahedral complexes where all four positions are equivalent... 

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. 

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. 

Diastereoisomers are stereoisomers which do NOT have a mirror image of one another. Figure 11.20 shows the diastereoisomers of 2-butene (alkenes such as this are sometimes called geometric isomers and are a consequence of the prohibition of rotation about double bonds). If a vertical mirror was placed between the two structures in Fig. 11.20 they would not reflect onto one another. If the functionality is on the same side then the isomer is the cis-form, if on the opposite side then it is the trans- form. The chemical properties are very similar because the functional groups are identical. However, as they have different shapes their physical properties are different. Interconversion requires breaking and remaking bonds so these isomers are also stable under normal conditions. 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

Returning to our example, you will find that there are two geometric isomers of but-2-ene. One is referred to as the cis isomer and the other is the trans isomer. It is important to remember that geometric isomers are different compounds and have distinct physical properties. For example, ds-but-2-ene melts at -139°C, whereas trans-but-2-ene melts at -105°C. 

Double bonds are not freely rotatable (see p.4). If double-bonded atoms have different substituents, there are two possible orientations for these groups. In fumaric acid, an intermediate of the tricarboxylic acid cycle (see p. 136), the carboxy groups lie on different sides of the double bond (trans or E position). In its isomer maleic acid, which is not produced in metabolic processes, the carboxy groups lie on the same side of the bond (cis or Z position). Cis-trans isomers (geometric isomers) have different chemical and physical properties—e.g., their melting points (Fp.) and pl

The configuration with the bromines adjacent is called cis (from the Latin derivative for on this side ), whereas that with bromines opposite is called trans (which means on the other side ). The two configurations are different substances with unique chemical and physical properties. They are described as geometric isomers. (See Figure 6-l4.)... 

Diasteroisomers, also known as geometric isomers, have different relative orientations of their metal-ligand bonds. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other. Enantiomers have identical chemical and physical properties except for their ability to rotate the plane of polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of zero. 

The importance of being able to determine the geometrical isomers of TGs in partially hydrogenated fats can be ascribed to their effect on the physical behavior of the fats, such as their polymorphic behavior and melting properties. 

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. 

Geometric isomers have different physical properties. For the MCAT, it is important to know that cis molecules have a dipole moment while trans molecules do r-... 

In the open-ring isomer, free rotation is possible between the ethene moiety and the aryl group. Therefore, the open-ring isomer is non-planar and the it-electrons are localized in the two aryl groups. On the other hand, the closed-ring isomer has a bond-alternative polyene structure and the jt-electrons are delocalized throughout the molecule. These geometric and electronic structural differences result in some differences in their physical properties. For example, the closed-... 

Differences in spatial orientation might seem unimportant, but stereoisomers often have remarkably different physical, chemical, and biological properties. For example, the cis and trans isomers of butenedioic acid are a special type of stereoisomers called cis-trans isomers (or geometric isomers). Both compounds have the formula HOOC—CH=CH—COOH, but they differ in how these atoms are arranged in space. The cis isomer is called maleic acid, and the trans isomer is called fumaric acid. Fumaric acid is an essential metabolic intermediate in both plants and animals, but maleic acid is toxic and irritating to tissues. 

Diastereomers that are not geometric isomers also have different physical properties. The two diastereomers of 2,3-dibromosuccinic acid have melting points that differ by nearly 100 °C ... 

Two enantiomers are chemically identical because they are mirror images of one another. Other types of stereoisomers may be chemically (and physically) quite different. These two alkenes, for example, are geometrical isomers (or cis-trans isomers). Their physical chemical properties are different, as you would expect, since they are quite different in shape. 

These two compounds, ( )-and (E)-dimethyl but-2-enedioate, are commonly known as dimethyl maleate and dimethyl fumarate. They provide a tel ling exam pie of how different the physical properties of geometrical isomers... 

In this chapter we shall talk about reactions similar to the ones on the previous page and we shall be interested in how to control the geometry of double bonds. Geometrical isomers of alkenes are different compounds with different physical, chemical, and biological properties. They are often hard to separate by chromatography or distillation, so it is important that chemists have methods for making them as single isomers. 

A compound that has two or more alternate structures but the same molecular formula. Isomers differ in physical properties from each other. There are many different types of isomerism, e.g. structural, optical, functional group and geometrical isomerism. The term is most widely used in organic chemistry. 

Geometric (cis-trans) isomers are stereoisomers because they differ only in the spatial arrangement of the groups. They are diastereomers and have different physical properties (m.p., b.p., etc.). 

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