Isomers compounds

The X-ray crystal structure of rapamycin reveals the constitution of the major isomer, compound 1, although in solution at least four isomers are detectable. The most abundant isomers are 1 (major) and the isomeric seven-membered hemiketal (mixture of stereoisomers). For the purposes of this chapter, we will restrict ourselves to structure 1. 

Many complexes and coordination compounds exist as isomers, compounds that contain the same numbers of the same atoms but in different arrangements. For example, the ions shown in (13a) and (13b) differ only in the positions of the Cl ligands, but they are distinct species, because they have different physical and chemical properties. Isomerism is of more than academic interest for example, anticancer drugs based on complexes of platinum are active only if they are the correct isomer. The complex needs to have a particular shape to interact with DNA molecules. 

While Lavoisier had established a rational system for naming elements and compounds, Frankland developed the system that we use today for writing chemical formulas and for depicting the bonds between the atoms in molecules. As Frankland synthesized more and more isomers, compounds with the same formulas but different molecular structures, he found traditional formulas confusing they showed the types and numbers of elements but provided no clue as to how the atoms were arranged inside the molecule. To remedy the problem, Frankland depicted the atoms in functional groups and drew lines between them to indicate the bonds between the elements. 

The crude product isolated from the reaction mixture clearly shows the presence of mixtures of (Z,Z)- and (E,Z)-isomers. Compound 91 could be obtained under the same conditions from 25 in superior yields. 

For a given molecular formula there is often more than one way of joining the atoms together, whilst still satisfying the rules of valency. Such variants are called structural isomers or constitutional isomers - compounds with the same molecular formula but with a different arrangement of atoms. A simple example is provided by C4H10, which can be accommodated either by the straight-chained butane, or by the branched-chain isobutane (2-methylpropane). 

Structural Isomer compounds that have the same formula but have different atomic arrangement... 

The furopyridine isomers, 13-18 (shown in Figure 2), contain a six-membered pyridine ring and an oxygen atom in the five-membered furan ring. Among these isomers, compounds 13-16 have been most widely studied. 

Isomers. Compounds having the same empirical formulas but different molecular arrangement, as well as different chemical and physical properties, are known as isomers... 

A novel synthesis of branched-chain monosaccharides was based on the finding that 3-furoic acid readily undergoes Birch reduction, affording 2,3-dihydro-3-furoic acid. Treatment of its methyl ester 398 with methanol and acid gave,264 in quantitative yield, methyl tetrahy-dro-5-methoxy-3-furoate 399 as a mixture of the isomers. Compounds... 

One of the more interesting aspects of coordination chemistry is the existence of isomers, compounds that have the same formula but a different arrangement of their constituent atoms. Because their atoms are arranged differently, isomers are different compounds with different chemical reactivity and different physical properties (such as color, solubility, and melting point). Figure 20.16 shows a scheme for classifying... 

Many complexes exist as isomers, compounds that have the same formula but a different arrangement of the constituent atoms. Constitutional isomers, such as linkage isomers and ionization isomers, have different connections between their constituent atoms. Stereoisomers (diastereoisomers and enantiomers) have the same connections but a different arrangement of the atoms in space. The most common diastereoisomers are cis and trans isomers of square planar MA2B2 and octahe-... 

Isomers Compounds which have the same molecular formula but different structural arrangements of the atoms. 

Gal J. New single-isomer compounds on the hori 5MS Spectr, 2002 7(Suppl 1) 45-54. 

Based on the calculations at the B3LYP/6-311-I— -G level and experimentally observed vibrational spectra for the ring and chain isomers, compounds 2 and 3, respectively (see Equation 2), it was concluded that the compounds existed in the cyclic form. The crucial fact is the absence of a stretching band at 2206 cm-1 in the experimental spectra that could, according to calculation, be associated with an azido group <1999PS201>. The experimental and theoretically calculated IR and Raman spectral data for l,2,3,4-thiatriazol-5-thiol derivatives are placed in Table 7. 

Conversions in pentane proceed in an nonuniform manner at high temperatures. In addition to unidentified products exhibiting different 31P NMR AX patterns, as well as the main product dichlorophosphane and another unknown substance, which exhibits a 31P NMR shift of + 301 ppm, one obtains a red compound, of which elemental analysis and the molecular mass point to the trimer phosphathioketene. The number of isomer compounds of this composition is limited by the 31P NMR spectrum. The A2X system, of which the X triplet is split into a double doublet if detected in solution in chloroform, indicates two acyclic PC double bonds and one phosphorus atom as a ring member (117). A possible explanation is given in Fig. 22. This could be in agreement with the addition of a monomer to the dimer, forming the six-membered ring compound with the proposed structure. 

In the absence of a sufficiently good leaving group, stable adducts are formed. Thus 5-acetyl-2-(p-methoxyphenyl)thiopyrylium cation (58) reacts with ethanethiol in benzene to yield the 2H adduct 289 (75T3059). 2-Methylthio-4,6-diphenylthiopyrylium (159) ion reacts with sodium thio-phenoxide to give the 2H adduct 293 (86S916). Heating of the adduct 293 yields the Z-2,2 -bithiopyranylidene 294 with a minor amount of the E isomer. Compound 294 can also be directly obtained by reaction of cation 159 with thiophenol and triethylamine. 

Identifying of individual compounds in liquid products, especially branched molecules from polypropylene, is rather difficult, because of the cracking of polypropylene yields a great number of isomer compounds. The liquid product obtained by cracking of polyethylene consisted mostly of n-alkenes and n-aUcanes, which were evenly distributed by carbon number, whereas the cracking of polystyrene yielded blends of aromatic compounds, styrene, ethylbenzene, benzene, toluene [45, 54],... 

Isomers. Compounds having the same empirical formulas but different molecular arrangement, as well as different chemical and physical properties, are known as isomers For instance, ortho, meta and para-mononi- > trotoluenes all have the same empirical formula, C7H7N02, but the N02 group is attached to different carbons of the benzene ring... 

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