Isomeric relationship

Two molecules share an isomeric relationship if they have the same molecular formula. All molecules with the same molecular formula constitute a set of structural isomers and are to some degree similar. However, they may have different chemical constitutions, as indicated in Figure 1.2.1 for 1-butanol and five structural isomers. Any two of these molecules placed in the same row make a pair of constitutional isomers. For the purpose of property estimation, it is helpful to further classify the constitutional isomers according to type and position of the functional groups and branching of the isomers. In the dicussion that follows, we focus on two different types of isomeric sets positional isomers and branched isomers. 

In geometric isomerism, the isomeric relationship can be explained in terms of two dimensions—as shown by the relationship of the two isomers, maleic acid and fumaric acid ... 

How many products would be expected from the reaction between (2 ,4Z)-hexa-2,4-diene and 2-methoxycyclohexa-2,5-diene-1,4-dione What isomeric relationships do the resulting products have to one another ... 

The reaction of the tetrabutylammonium salt of cAMP with 4-(bromo-methyl)-2Jf-chromen-2-one yields two products which result from axial and equatorial attack of the phosphate group. What is the isomeric relationship between the two products Determine the configuration of all the chirality centres in both products. 

Scheme 1. Comparison of whole molecules to determine isomeric relationships. The question marks signify Superposition yes or no . The three tests are Syi —comparison by symmetry operations of the first kind (rotation, torsion) BG—comparison of bonding (connectivity) graphs vertex by vertex Syn—comparison by symmetry operations of the second kind (reflection).

Emil Fischer- was the first to synthesize and name the aminodeoxyketose shown below as I. Because of its isomeric relationship to D-glucosamine... 

This isomeric relationship is called tautomerism, which in this particular case is an example of the keto/enol variety. Now list those bonds that are different between the two compounds. 

The valence isomeric relationship in the cycloheptatriene/norcaradiene system has been known for over ninety years. ... 

What, then, are the various types of selec-tivities (or specificities) encountered in xeno-biotic metabolism What characterizes an enzyme from a catalytic viewpoint is first its chemospecificity, i.e., its specificity in terms of the type(s) of reaction it catalyzes. When two or more substrates are metabolized at different rates by a single enzyme under identical conditions, substrate selectivity is observed. In such a definition, the nature of the prod-uct( nd their isomeric relationship are not considered. Substrate selectivity is distinct from product selectivity, which is observed when two or more metabolites are formed at different rates by a single enzyme from a single substrate. Thus, substrate-selective reactions discriminate between different compounds, whereas product-selective reactions discriminate between different groups or positions in a given compound. 

By now you are well aware that molecules are three-dimensional objects. This chapter explores. some of the more subtle, but extremely critical consequences of this fact. If you have not done so yet, don t wait any longer to ohtain a set of models to aid you in visualizing the structures described in this chapter. For immy students, the isomeric relationships discussed here are the most difricult ones encountered in organic chemistry, and they are important later on in descriptions of several types of compounds and reactions. The implications for biological chemistry are especially significant. 

The 1-phosphate ester of this diol is said to occur in brain tissue and sea-urchin eggs. Strong evidence that cordycepic acid is really D-mannitol. Not an enantiomorph of d-quercitol other isomeric relationship is involved. 

For a discussion of stereochemical nomenclature, see Eliel, E. L. J. Chem. Educ. 1971, 48,163. A more detailed flow chart of isomeric relationships was given by Black, K. A. J. Chem. Educ. 1990, 67,141. 

Dimethyl maleate (9) and dimethyl fumarate (10) are examples of cis and trans isomers, called geometric isomers. Their names do not reflect this isomeric relationship, however, because the parent diacids from which they can be prepared were discovered and named before this stereochemical fact was known. Being diastereomers, these diesters would be expected to have different physical and chemical properties, and indeed they do. The difference in their physical states at room temperature is the most dramatic The maleate 9 is a liquid with a freezing point of about -19 °C, whereas the fumarate 10 is a solid with a melting point of 103-104 °C. 

Note that each of these two cations exists in a number of isomeric forms. The reader may find it a useful exercise to draw pictures of all of the forms and to enquire into the isomeric relationship between pairs. 

The two isomeric compounds below are naturally occurring insect pheromones. The isomer on the left attracts the male olive fruit fly the one on the right, the female, (a) What kind of isomeric relationship exists between these two structures (b) What functional group is contained in these molecules (c) Both of these compounds are hydrolyzed under aqueous acidic conditions. Draw the products. Are the product molecules from the two starting isomers the same or different ... 

Gautier and Hoffman developed the first syntheses of isocyanides in 1867 (Scheme 7.1). Gautier s work with isocyanides pioneered the discovery of the isomeric relationship between isocyanides and nitriles [9]. Hoffman s synthesis is currently used as a chemical test for primary amines. 

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