Isomers, geometrical

In geometrical isomers there exists a spatial arrangement of either atoms of funetional groups in the carbon-carbon double bond loeations, which has been duly expatiated earlier as under  

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Buiene, -butylene, CH3CH CHCH3-Prepared by heating 2-buianol with sulphuric acid. It occurs as two geometrical isomers. 

The carbon atoms of the double bond have a trigonal planar configuration and free rotation about the C—C bond is prevented by the n bond. The inability to rotate means that geometrical isomers can be produced, with substituents a and b, thus ... 

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... 

Stereoisomeric alkenes are sometimes referred to as geometric isomers... 

As in all double-bond situations, the adjacent chain sections can be either cis or trans-structures [XV] and [XVI], respectively-with respect to the double bond, producing the following geometrical isomers ... 

The value of many chemical products, from pesticides to pharmaceuticals to high performance polymers, is based on unique properties of a particular isomer from which the product is ultimately derived. Eor example, trisubstituted aromatics may have as many as 10 possible geometric isomers whose ratio ia the mixture is determined by equiHbrium. Often the purity requirement for the desired product iacludes an upper limit on the content of one or more of the other isomers. This separation problem is a compHcated one, but one ia which adsorptive separation processes offer the greatest chances for success. 

Astemi2ole (10) has further been modified into a series of 4-phenylcyclohexylamine compounds, resulting in the synthesis of cabastine, for example. Cabastine is a highly active compound and its geometric isomers are also active, demonstrating the stereoselectivity of histamine receptors toward chiral ligands. The > S, 4 R-levo antipode of cabastine was the most active, and therefore this isomer, levocabastine (13), has been chosen for further development. Because of high potency, levocabastine has been developed for topical appHcation such as eye drops and nasal spray. 

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). 

Other minor raw materials are used for specific needs. Eumaric acid [110-17-8] the geometric isomer of maleic acid, is selected to maximize thermal or corrosion performance and is the sole acid esterified with bisphenol A diol derivatives to obtain optimum polymer performance. CycloaUphatics such as hydrogenated bisphenol A (HBPA) and cyclohexanedimethanol (CHDM) are used in selective formulations for electrical apphcations. TetrahydrophthaUc anhydride [85-43-8] (THPA) can be used to improve resiUence and impart useful air-drying properties to polyester resins intended for coating or lining apphcations. 

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. 

Amin omethyl-3,5,5-trimethyl cyclohexyl amine (21), commonly called isophoronediamine (IPD) (51), is made by hydrocyanation of (17) (52), (53) followed by transformation of the ketone (19) to an imine (20) by dehydrative condensation of ammonia (54), then concomitant hydrogenation of the imine and nitrile functions at 15—16 MPa (- 2200 psi) system pressure and 120 °C using methanol diluent in addition to YL NH. Integrated imine formation and nitrile reduction by reductive amination of the ketone leads to alcohol by-product. There are two geometric isomers of IPD the major product is ds-(22) [71954-30-5] and the minor, tram-(25) [71954-29-5] (55). 

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). 

There are a myriad of possible isomers, including positional and geometrical isomers of the double bond(s) as well as stmctural isomers resulting from head-to-head or head-to-tad alignment of the reacting fatty acids. 

Polyisoprene (Synthetic). Polyisoprene has four possible chain unit geometric isomers cis- and /n j -l,4-polyisoprene, 1,2-vinyl, and 3,4-vinyl. 

Imidazolidine, 5-methyl-1,2-diphenyl-geometrical isomers, 5, 354 Imidazolidine, triketo-synthesis, 5, 466... 

Determination of C -C cai bonyl compounds, phthalic acid alkyl esters, mutagen MX and its geometric isomer E-MX, chlorophenols, organochlorine pesticides (OCPs) - a-, P-, J-, 5-HCH, DDE, DDD, DDT, total isomeric-specific content of polychlorinated byphenyls (PCBs) (tri-, tetra-, penta-, hexa-, hepta-, octachloroisomers) in SCAN and SIM mode was canned out. MDE of procedure for OCPs makes up 0.01-0.1 and PCBs - 0.17-0.38 ng/1 at enrichment factor K = (1.50-4.00)T0. ... 

The stereochemistiy of reactions involving substituted alkenyl free radicals indicates that radicals formed at trigonal centers rapidly undergo interconversion with the geometric isomer. Reactions proceeding through alkenyl radical intermediates usually give rise to the same mixture from both the E- and the Z-precursor ... 

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 ... 

The simplest compounds, -pyrroline and -piperideine,donotexistin the monomeric form. Schdpf et al. (29S) described two geometric isomers of J -piperideine trimer and called them a- and -tripiperideines (182). An equilibrium exists between A -piperideine and both trimers which, therefore, react as typical aldehyde ammonia. The trimer rearranges at pH 9-10 in an almost quantitative yield to isotripiperideine (183) which, in turn, is in equilibrium with tetrahydroanabasine (184) and -piperideine. 

Figure 15.47 Schematic representation of the three geometric isomers of N3S3CIF2O3. The three isomers of the monofluoro derivative are similar but with Cl and F interchanged.

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. 

Where unidentate ligands are present, the ability to effect the resolution of an octahedral complex (i.e. to separate 2 optical isomers) is proof that the 2 ligands are cis to each other. Resolution of [PtCl2(en)2] therefore shows it to be cis while of the 2 known geometrical isomers of [CrCl2en(NH3)2] the one which can be resolved must have the cis-cis structure since the trans form would give a superimposable, and therefore identical, mirror image ... 

The Ciamician-Dennstedt reaction is currently the only way to date to make calix[4]pyridine 20.(Scheme 8.3.6) Four sequential treatments of calix[4]pyrrole 19 with sodium trichloroacetate results in all four possible geometric isomers of 20 (all four... 

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