Isomerism geometric isomers

Effects of Isomerism Geometrical isomers of coordination compounds can exhibit different values of redox potential, as accounted for, in various cases, by simple djr orbital level splitting diagrams or by MO calculations. The dependence of the relative stability and redox behavior of the geometrical isomers on the electronic configuration of the metal is also... 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

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). 

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. ... 

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... 

Which of the following octahedral complexes show geometric isomerism If geometric isomers are possible, draw their structures. 

As we saw earlier, there are three structural isomers of the alkene C4H8. You may be surprised to learn that there are actually/owr different alkenes with this molecular formula. The extra compound arises because of a phenomenon called geometric isomerism. There are two different geometric isomers of the structure shown on the left, on page 597, under (1). 

Isomerization of lycopene in tomato oleoresin increased at 75 and 100°C, reaching the formation of eight unidentified lycopene geometrical isomers in tomato oleoresin stored at 100°C. The k of 0.2597/min was higher, whereas the (11.7 kcal/mol) was lower than that observed for lycopene standard heated in safflower oil at 75°C. 

After succeeding in the asymmetric reductive acylation of ketones, we ventured to see if enol acetates can be used as acyl donors and precursors of ketones at the same time through deacylation and keto-enol tautomerization (Scheme 8). The overall reaction thus corresponds to the asymmetric reduction of enol acetate. For example, 1-phenylvinyl acetate was transformed to (f )-l-phenylethyl acetate by CALB and diruthenium complex 1 in the presence of 2,6-dimethyl-4-heptanol with 89% yield and 98% ee. Molecular hydrogen (1 atm) was almost equally effective for the transformation. A broad range of enol acetates were prepared from ketones and were successfully transformed into their corresponding (7 )-acetates under 1 atm H2 (Table 19). From unsymmetrical aliphatic ketones, enol acetates were obtained as the mixtures of regio- and geometrical isomers. Notably, however, the efficiency of the process was little affected by the isomeric composition of the enol acetates. 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

The less stable isomers were obtained from the more stable ones by photoisomerization. (Z)-Fumaridine (113) when exposed to sunlight was isomerized to a separable 3 2 mixture of geometric isomers (5). The Z form of narceine imide (116) is unstable and in daylight equilibrates easily to a mixture of Z and E forms (123). 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

Two milestones in chemistry may be addressed to the geometric isomers of diamminedichloroplatinum(II). About 100 years ago Alfred Werner laid down the principles of modem coordination chemistry with the help of isomeric [PtCl2(NH3)2] [1]. About 70 years later Barnett Rosenberg and co-workers discovered that the cis isomer is antitumor active, whereas the trans isomer is not [2,3]. Today m-[PtCl2(NH3)2] (cisplatin) is routinely used as a drug, particularly effective against testicular and ovarian cancer [4],... 

Like the rule for optical isomers, if n represents different double bonds, each satisfying the requirements of geometric isomerism, the number of geometric isomers will be given by 2 . 

The hydrosilylation of l,4-bis(trimethylsilyl)but-3-en-l-yne (141) was beautifully controlled and four different isomeric products could be prepared independently with 93-96% selectivity by a proper choice of geometric isomers of 141 and transition metal catalysts [113]. One of the four products from the reaction of 141 with 132p was allene 142, which was obtained as a mixture (142 143 = 96 4) in 93% yield (Scheme 3.73). 

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